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NORTH DAKOTA CLOUD MODIFICATION PROJECT
OPERATIONSMANUAL
A Program Designed for the
Seeding of Convective Cloudswith Glaciogenic Nuclei to
Increase Rainfall and
Diminish Hail Damage on theNorthern Great Plains
North Dakota Atmospheric Resource Board
State Water Commission
900 East Boulevard AvenueBismarck, ND 58505-0850
voice: (701) 328-2788
fax: (701) 328-4749
May 1993, May 2005Revised May 2010
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NORTH DAKOTA CLOUDMODIFICATION PROJECT
OPERATIONS
MANUAL
RevisedMay 2010
NORTH DAKOTAATMOSPHERIC RESOURCE BOARD
State Water Commission900 East Boulevard AvenueBismarck, ND 58505-0850
voice: (701) 328-2788fax: (701) 328-4749
Darin Langerud, DirectorMark Schneider, Chief Meteorologist
Kelli Schroeder, Business ManagerDan Brothers, Meteorologist
Matt Ham, Meteorologist
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North Dakota Cloud
Modification Project
OPERATIONS MANUALMay 2010 Revision
TABLE OF CONTENTS
1. INTRODUCTION ...................................................................................1
1.1 Operations History ......................................................................1
1.2 Research ......................................................................................2
2. PROJECT ORGANIZATION.................................................................3
2.1 Organizational Structure .............................................................32.2 Cloud Seeding Operations........................................................... 4
2.3 Project Equipment Locations ......................................................4
2.4 NDCMP Operations Area ...........................................................5
2.5 Statement of Equal Opportunity.................................................. 5
3. EVALUATION .......................................................................................6
3.1 Rainfall........................................................................................6
3.2 Crop Hail Damage....................................................................... 7
3.3 Other Studies ...............................................................................9
3.4 Urban Effects ..............................................................................10
4. RESPONSIBILITIES............................................................................ 124.1 Radar Meteorologists ................................................................ 12
4.2 Intern Meteorologists ................................................................ 12
4.3 Pilot - Meteorologist Joint Responsibilities .............................. 13
4.4 Pilots-In-Command................................................................... 14
4.5 Intern Copilots........................................................................... 14
4.6 Safety..14
4.7 General Responsibilities............................................................ 15
5. SEEDING CONCEPTUAL MODEL.................................................... 18
5.1 Seeding for Rainfall Increase .................................................... 18
5.2 Hail Suppression and the Conceptual Model ............................ 18
6. IDENTIFICATION OF SEEDING OPPORTUNITIES ....................... 23
6.1 Targets Suitable for Rainfall Increase Seeding......................... 23
6.2 Hail Suppression Opportunities ................................................ 24
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TABLE OF CONTENTS,
continued
7. DELIVERY TECHNIQUES ................................................................. 25
7.1 Rain Augmentation Seeding Procedures................................... 25
7.1.1 Seeding in the Absence of Shear .................................. 26
7.1.2 Forward-Shear Seeding ................................................ 26
7.1.3 Reverse-Shear Seeding ................................................. 27
7.1.4 Embedded Cumulus...................................................... 27
7.2 Hail Suppression Seeding Techniques ...................................... 28
7.2.1 Squall Lines .................................................................. 28
7.2.2 Multicell Storm Techniques ......................................... 31
7.2.3 Supercell Seeding Techniques...................................... 32
7.3 Considerations for the Deployment of Aircraft......................... 33
7.4 Targeting Upwind of the Project and Broadcast Seeding ......... 35
7.5 Aircraft Sharing Between Districts ........................................... 36
8. DECISION MAKING ........................................................................... 388.1 Daily Strategy............................................................................ 38
8.2 Conditions Control Decision Ladder ........................................ 39
8.3 Seeding Strategy........................................................................ 39
8.4 Rainfall Increase........................................................................ 40
8.5 Hail Suppression ....................................................................... 40
8.6 Seeding Suspension Criteria ..................................................... 41
9. SEEDING EQUIPMENT ...................................................................... 45
9.1 Efficiency of Seeding Agents.................................................... 45
9.2 Hazardous Materials.................................................................. 46
9.3 Handling, Use and Storage of Flares......................................... 47
9.4 Preparation and Handling of Acetone Solution......................... 47
9.5 Handling, Use and Storage of Dry Ice ...................................... 48
9.6 Seeding Equipment Maintenance and Troubleshooting............ 49
10. PROJECT DOCUMENTATION .......................................................... 52
10.1 Aircraft Flight Forms ................................................................ 52
10.2 Flight Track Maps ..................................................................... 54
10.3 Chemical Inventory Form ......................................................... 54
10.4 NDCMP Records Distribution and Quality Control ................. 55
10.5 Archival of NDCMP Records ................................................... 55
APPENDICES.................................................................................................. 58I. Mixing Chemical....................................................................... 58
II. Seeding Opportunities and Methods ......................................... 59
III. Burn In Place Flare Usage......................................................... 60
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NDCMP Operations Manual 1
1. INTRODUCTION
The North Dakota Atmospheric Resource
Board (ARB) has prepared this operations manual for
the North Dakota Cloud Modification Project
(NDCMP). The manual draws extensively upon
previous operations manuals dated February, 1980 and
May, 1984. This manual contains information most
likely to be needed on a day-to-day basis by project
personnel. [The radar meteorologists will also receive a
Radar Manual, which includes information needed to
operate and interpret the radar and IRIS/TITAN software
programs. The PICs and ICPs will receive a PARS
manual that includes instructions necessary for operating
the Palm devices.]
The Operations Manual is both informative and
directive in nature. Information given represents asummary of detailed laboratory and field work, and the
reader must make use of the references listed to obtain
more complete information. The procedures described
are required for use by all project personnel. It is thus
important for each project person to become as familiar
with this manual as possible.
Teamwork is essential for quality operation.
This teamwork extends from the cooperation required
between meteorologists and pilots, to the relationship
between the Atmospheric Resource Board and the cloud
seeding contractor. Open communication among field
personnel, county weather modification authoritymembers and ARB office staff is essential.
Project personnel having ideas as to how to
improve any aspect of the NDCMPare strongly urged to
communicate such ideas to Bismarck staff. Continuous
quality improvement is a primary objective of the project
and only through the sharing of constructive ideas can
this be realized.
Because of the NDCMPs 24/7 around-the-
clock operational nature, project personnel shouldnt
hesitate to contact the ARB office or staff at home after
hours to discuss any real or potential problem that might
affect the project or the performance of their job.The Atmospheric Resource Board
acknowledges with deep appreciation the efforts of those
individuals who contributed to the earlier versions of this
manual: Merlin C. Williams, Dr. Pierre St. Amand,
Wilbur E. Brewer, Robert D. Elliott, Chester Wisner,
Jackson Pellet, John A. Donnan, Dr. D. Ray Booker, Leo
F. Ritter, Martin Schock, Jim Miller, John Thompson,
Mark Schultz, R. Lynn Rose, James Scarlett, Bruce Boe,
and Brenda Hove, among others.
This version reflects the present knowledge of
storms, storm structure and cloud physics, as interpreted
by the Executive Director of the Board. Additional
operational insights have been provided by North
Dakota Atmospheric Resource Board and State Water
Commission Staff and by Hans Ahlness and the staff of
Weather Modification, Inc., Fargo, North Dakota.
1.1 Operations History
Operational cloud seeding got its start in North
Dakota in the 1950's, when ground-based seeding
activities began in the western part of the state. By the
late 1950's, hail was recognized as the greatest weather-related threat to small grain crops; many growers
suffered significant hail damage or total losses in back-
to-back years.
The beginnings of what is today'sNorth Dakota
Cloud Modification Projectcame about when Bowman
County farmer-rancher Wilbur Brewer joined forces
with pilot neighbors, Bill Fisher and Bill Mazaros, to
form Weather Modification, Incorporated (WMI), the
state's first all-airborne commercial cloud seeding
company. Seeding first for just a few townships, then
later entire counties, the program expanded and spread
eastward throughout much of North Dakota. Theprogram at that time was entirely locally sponsored.
In the early years, silver iodide was the seeding
agent released into the updrafts of mature storms
perceived to pose a hail threat. Though little was known
about how the hail suppression effect came about within
the clouds, the results were positive (Butchbaker 1973).
In 1975, support from the State of North Dakota was
sought and the North Dakota Weather Modification
Board was created as a division of the Aeronautics
Commission. In 1976, state cost sharing was available
for the program for the first time and a total of 17
counties participated in the NDCMP.Within the next few years, participation
diminished sharply in the eastern portion of the state but
remained in the west. The eastern half of North Dakota
receives significantly more rainfall, which lessens the
desire for more moisture and hence the program. It was
noted by Dr. Archie Kahan, while director of the United
States Bureau of Reclamation's cloud seeding research
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Introduction2
program, that "Interest in weather modification is
soluble in rain water." In Dr. Kahan's words lies a basic
truth about humanity, that being when one has an
abundant supply of an essential commodity, they lose
interest in acquiring more-- at least until they run short
again.
As cost-sharing dollars were pared from the
board's budget, additional western North Dakota
counties dropped out of the program. This was likely a
contributing factor for the reduction in participation in
eastern North Dakota as well.
A resurgence of interest has recently developed
after several very positive evaluations of the NDCMP
(see Chapter Three) have been published. Williams
County joined the program on a full-time basis after a
four-year trial program was concluded in 2000. The
issue of whether to continue the program in WilliamsCounty passed by 80 to 20 percent in the November
2000 general election. Preliminary interest has also been
indicated by other western North Dakota counties,
although it is too early to tell if that interest will translate
to participation.
1.2 Research
In 1980, a federally funded research program
was undertaken to develop an understanding of the
physical processes involved in hail and precipitationformation and how such processes might be best
modified beneficially. That program, known as the
Federal-State Cooperative Program in Atmospheric
Modification Research, was funded through the National
Oceanic and Atmospheric Administration (NOAA), an
agency within the Department of Commerce.
Roughly half a million federal dollars per year
were pooled with the available state resources to collect
and analyze thunderstorm data. Major field efforts were
mounted every three to four years; analysis efforts filled
the years between field efforts. Numerous citations of
technical publications resulting from this researchprogram appear in latter chapters of this manual.
The ARB has gained national recognition
through the Atmospheric Modification Research
Program and in the process has built strong ties with the
John D. Odegard School for Aerospace Sciencesat the
University of North Dakota (UND), the Institute of
Atmospheric Sciences at the South Dakota School of
Mines and Technology (SDSMT), the National Center
for Atmospheric Research (NCAR), and several NOAA
research laboratories. Ties and interactions with the
National Weather Service have also been strengthened.
In 2001, another federal research program was
approved with the mission of advancing the science of
weather modification. The Weather Damage
Modification Program was a collaborative effort
involving a number of western states conducting cloud
seeding operations.
Beginning in 2006, a seeding experiment called
POLCAST, or Polarimetric Cloud Analysis and Seeding
Test, was conducted to study whether hygroscopic
seeding could increase rainfall and reduce hail formation
in North Dakota. POLCAST was a cooperative
experiment involving the ARB, UND, WMI, and Ice
Crystal Engineering. For additional experiment cases,another season of POLCAST was conducted in 2008 and
is planned for 2010.
REFERENCES
Butchbaker, A.F., 1973: Results of the Bowman-Slope
Hail Suppression Program. Journal of Weather
Modification, 5.
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NDCMP Operations Manual 3
2. PROJECT ORGANIZATION
2.1 Organizational Structure
The overall organizational structure of the
North Dakota Atmospheric Resource Board (ARB) is
shown in Fig. 1. The ARB is by law a division of the
North Dakota State Water Commission (SWC).
However, because it (the ARB) has its own law,
administrative rules, and regulations, it is different than
the other SWC divisions. While the Governor chairs the
SWC, the ARB chairperson is elected from the ARB
membership.
The Atmospheric Resource Board itself
consists of ten members. Seven of these are governor-
appointed representatives from multi-county districtsaround the state. The other three members are: the
Director of the Aeronautics Commission, the State
Engineer, and a representative from the State
Department of Health and Consolidated Laboratories.
Project personnel are provided with membership lists for
the ARB and each County Authority during the pre-project ground school each spring. County residents
have direct input to the ARB through their countys
Weather Modification Authority. County Weather
Modification Authority members are appointed by their
county commission after the Authority has been created
by popular election or petition.
Operations Advisory Committees (OAC) are
comprised of an Authority member and county
commissioner from each county in each target area and
may also include a county commissioner from each
adjacent non-target county.
OACs provide ARB with input regarding thestatus of cloud seeding operations during the program.
Figure 1: Organizational structure of the North Dakota Atmospheric Resource Board.
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Project Organization4
Note that the ARB Director also receives
counsel from a number of advisors in universities,
government and private industry.
2.2 Cloud Seeding Operations
During the NDCMP field season the County
Authorities and the OACs have a great deal of
interaction with the Director concerning operations (see
Fig. 2). Seeding may be suspended entirely if conditions
become too wet. Seeding may also be momentarily
suspended if weather conditions meet or exceed safety
criteria established by the ARB. Safety criteria are
periodically reviewed by a panel of experts, which
makes recommendations to the ARB through a thoroughoverview of NDCMP operations (Orville et. al, 2003).
2.3 Project Equipment Locations
Figure 3 illustrates typical locations of aircraft
and radars involved in the NDCMP. Each radar is
equipped with Global Positioning System (GPS) real-
time aircraft tracking which increases operations
effectiveness and safety. Radar sites are also providedwith Internet access and GR Analyst, allowing radar
meteorologists to monitor weather conditions in and
around the operations area. In the case of a radar outage
at either of the Bowman or Stanley sites, GR Analyst
allows for limited back-up radar coverage for direction
of seeding operations.
Support facilities at the Bismarck ARB office
also include Internet access for collecting weather
information from a vast array of Internet web sites.
Satellite and radar data, surface observations, upper-air
observations and numerical model data are collected for
the purpose of making daily, district specific, weatherforecasts. In addition to ASOS and AWOS Systems at
Bowman and Stanley Airports, both radar field sites
employ a basic weather station that displays and records
high and low temperatures, dew point, wind direction,
speed, gusts, rainfall and atmospheric pressure.
Figure 2: Infrastructure of NDCMP operations.
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NDCMP Operations Manual 5
2.4 NDCMP Operations Area
Participation in the NDCMP has always been a
local decision. Grass-roots support of the program has
been the base of support for the program since the earlydays of cloud seeding in North Dakota. Historically,
county taxes have been the primary funding mechanism
for the program, with the state providing assistance
through cost-sharing. Participation in the NDCMP has
ebbed and flowed over the last fifty-plus years.
Participation peaked in the mid-1970s when the state
first implemented 50 percent cost sharing with
participating counties. Shortly thereafter, when state
funding was pared back, many counties were unable to
bridge the funding gap and dropped out of the program.
In 2000, the program grew as Williams County joined
the NDCMP, more than offsetting the loss of part of
Slope County.
The NDCMP is currently split into two
districts; District I in the southwest, and District II in the
northwest. The total project area covers 10,425 square
miles, or nearly 6.7 million acres, almost 15% of thestates area.
2.5 Statement of Equal
Opportunity
The Atmospheric
Resource Board does not
discriminate on the basis of race,
color, national origin, gender,
religion, age or disability in
employment or the provision of
services.
REFERENCES
Orville, H.D., D.A. Banaszewski,
J.A. Heimbach, Jr., L. Osborne,
P.L. Smith, and W.L. Woodley,
2003: A Review of the North
Dakota Cloud Modification
Program. Report of a Review
Panel to the North Dakota
Atmospheric Resource Board,
March, 2003. 29p.
Figure 3:NDCMP operations districts, aircraft locations, and radar
coverage.
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Evaluation6
3. EVALUATION
Since the creation of the Atmospheric Resource
Board (originally the Weather Modification Board) in
1975, a number of evaluations have been made of theeffects of the program. These evaluations have been
based on a variety of different data sources. Among
them are: Atmospheric Resource Board Cooperative
Observer Network (ARBCON) growing season
precipitation data, National Weather Service (NWS)
rainfall data, crop hail insurance data compiled by the
National Crop Insurance Service (NCIS), and crop yield
and price data compiled by North Dakota State
University (NDSU).
Computer-based numerical models have also
played an important role. Economic models have
provided estimates of the economic benefits of theprogram, while cloud models have produced increased
insights into the complex processes within the clouds
that ultimately govern hail and rain production.
All of these evaluations have been independent,
as they've been conducted by agencies and universities
unaffiliated with the ARB. [It is here noted that one
might be subjected to severe criticism if one engaged in
self-evaluation.] In addition, the evaluation by outside
agencies allows the most qualified persons to do the
evaluations, improving the chances for an unbiased,
scientific approach.
As of this date, all evaluations of the programhave shown positive or neutral results. No suggestions
have been found of negative impacts, either within or
outside target areas. Overviews of the most significant
evaluations follow. For the more interested reader, the
reference to each is provided at the end of the chapter.
3.1 Rainfall
The first rainfall evaluation in North Dakota
was appropriately named the North Dakota Pilot Project
(NDPP). Conducted in McKenzie County from 1969-72(Mountrail and Ward Counties participated in 1972), the
NDPP was a randomized experiment, allowing for the
best possible statistical analysis of the results . As
explained by Dennis et al. (1975), experimental protocol
set up eight-day blocks in advance of each project
season where six days were randomly designated seed
days and two were no-seed days. Following the four-
year project, data from 67 rain gauges in McKenzie
County were subjected to a variety of statistical tests to
determine the seeding effects. Analysis of the data
revealed strong evidence that silver iodide seeding of
towering summertime clouds led to an increase in thefrequency of rainfall events, an increase in the average
rainfall per rainfall event, and an increase in the total
rainfall in the seeded area. Further, the total potential
rainfall increase for the area was estimated at one inch
per growing season.
Two recent rainfall evaluations have been
conducted on the NDCMP; one looking at ARBCON
rainfall data (Wise, 2005) and the other at NWS climatic
rain gauge data (Smith et al., 2004). The ARBCON
rainfall analysis, which takes advantage of hundreds of
rain gauge locations in western ND, indicated that
between the years 1977-2003, the seeded areas and
those areas slightly downwind received about 4.2% to
9.2% more rainfall than the upwind control areas where
1The significance level is an indicator of the degree of confidence, which
may be accorded the result of a statistical test of two populations
(seeded vs unseeded, for example). The lower the numericalsignificance, the greater the confidence. For example, a significance
level of 0.08 indicates a 92% chance that the two populations are reallydifferent, i.e, that the observed effect is real.
Figure 4: Target (counties), control (solid line),
and downwind (hatched) areas for the 1982 project
year (from Johnson, 1985).
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NDCMP Operations Manual 7
no seeding
occurred. Not only positive news for
the
seeded areas, but it also addresses the concern of some
that seeding upwind will reduce downwind precipitation,
or "steal someone's rain. Conversely, there is no
evidence that cloud seeding contributes to largeincreases in precipitation at long distances downwind of
seeded areas. Previous studies of the NDCMP and other
states' seeding programs indicate that downwind effects
diminish with increasing distance from the target area.
The Smith et al. (2004) study found no statistical
difference in rainfall between the NDCMP counties and
an upwind control area. The author concluded, however,
that the sparse coverage of NWS rain gauges in both
areas and the 0.1 confidence interval would not allow
the analysis to determine differences in precipitation
over the two areas at a level of ten percent or less, the
typical range of effect identified by several other rainfallstudies.
Other evaluations of rainfall in and near the
NDCMP include one by Eddy and Cooter (1979) and by
Johnson (1985), which examined NWS rainfall data for
seven years, 1976-82. Johnson defined three areas for
each storm event: the multi-county target areas (Target),
the regions downwind of the Targets (Downwind), and
the areas neither in the target nor downwind (Control)
(see Figure 4).
Figure 5 illustrates the rain gauge coverage
used in a typical season.
Some of the more interesting findings of this analysis
showed:
1. Evidence of an overall increase in precipitation
downwind of the target (relative to the control area),
especially notable in July (15% increase, significant at
the 0.08 level1).
2. Evidence of increase in the target areas on days
with heavier rains (14% increase significant at the 0.13
level).
Other findings showed additional increases both
in and downwind of the target areas, correlated at various
(generally lower) confidence levels with upper air
features. The report concluded that, "It appears that
seeding for hail suppression in the mode employed in
North Dakota likely increases rainfall."
3.2
Crop Hail Damage
Several evaluations have been completed which
help define the efficacy of the North Dakota hail
suppression efforts; one of the earliest by Miller et al.
(1975), which indicated positive impacts of seeding on
hail during the NDPP. Others employed years of crop
hail insurance data in an attempt to determine the degree
of effectiveness of the hail suppression effort (Smith et
al.,1987, 1997). Smith et al. (1987) defined a control
area comprised of upwind eastern Montana counties
adjacent to North Dakota. Hail insurance claims
recorded for a lengthy pre-project periodfrom this control area were compared to
those for the modern-day NDCMP target
to first establish the historical
relationship. Then the losses in both
areas accrued during the NDCMP were
compared to establish the magnitude of
the change. The loss ratiois the ratio of
the amount of claims paid to the amount
of insurance sold. Thus, a high loss ratio
indicates a high hail loss. When the loss
ratios for the target were plotted versus
those for the upwind (west) control areafor each year, a significant difference
emerged between the historical years and
those of the NDCMP (Fig. 6). Sixty-one
years are compared; ten for the NDCMP
years (circles), and 51 years for the
"historical" period which preceded the
NDCMP. In Fig. 6, a single (solid)
regression line is plotted for all years,
Figure 5: National Weather Service rain gauge locations used in a
typical seasons evaluation by Johnson (1985).
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Evaluation8
Figure 8: Economic impacts of the NDCMP and
potential statewide impacts in dollars.
seeded and unseeded. When separate lines are plotted
for the seeded and historical years however, a striking
difference is observed. The difference between the slopes
of the Historical and NDCMP regressions can be
interpreted as the change induced by seeding. In thisevaluation, the NDCMP averaged 43.5% less crop
hail damage when compared to the upwind control
area. The statistical confidence level for this test was
.002, indicating that "...the target area loss ratio values
during these [NDCMP] years were considerably lower
than would have been predicted by the historical and
control area regression lines". The authors of the report
conclude with the observation that, "Therefore, the
sponsors of the NDCMP would seem to be fully
justified in continuing their support of seeding
operations."
Since the completion of the original Smith et al.
report, an additional three years' data from the NDCMP
(1986-88) have been added to the original database. In
an identical analysis (Smith et al., 1997), the expanded
13-year NDCMP database showed a 45% decrease in
losses in the target area with a statistical significance of
.025. These findings were published in the Journal of
Applied Meteorology in 1997.
The findings of the 1987 Smith et al. study
were used to develop a detailed, county-by-county
economic assessment of the effects of crop-hail damage
mitigation efforts by cloud seeding (Johnson et al.
1989). Results put the ten-year average annual savings
for the target area at $3.75 million. However, that figure
did not include any forage crops (hay) or other less
common crops (at that time) such as sugar beets,
potatoes, beans, canola, lentils, and so on. Statewide, it
was estimated about $97 million could be saved each
year by hail suppression cloud seeding, according to
Johnson, et al.
Likewise, results from the 1997 Smith et al.
study were used, in addition to rainfall results from
Johnson (1985), to calculate the economic impacts the
program was having in the target counties and thepotential benefits to the rest of North Dakota. The report
was conducted by Sell and Leistritz (1998) and later
updated by Bangsund and Leistritz (2009) and used 5
and 10 percent increased rainfall scenarios. Results
indicated that the NDCMP has an annual direct
economic impact of $19.7 million in the target counties
(using the 10 percent rainfall scenario). Further, the
total business activity resulting from that increase was
estimated at $60.5 million annually. If applied
Figure 7: Direct impact and total business activity
increase per planted acre for NDCMP counties (10
percent rainfall scenario).
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NDCMP Operations Manual 9
statewide, the report estimated cloud seeding could yield
a direct impact of $134.5 million and an increase in total
business activity of $414.2 million annually! The $8.3
million increase in state tax revenues would be more
than the estimated annual cost of the project.Economic impacts for the NDCMP target
counties are shown in Figure 7. Based on those figures
and estimated project costs, the benefit to cost ratio for
total business activity is 78 to 1 for NDCMP counties!
Breakdowns of the five and ten percent scenarios for
increased rainfall benefits to the NDCMP and for a
potential statewide program are shown in Figures 8 and
9.
3.3 Other Studies
Though evaluations that attempt to statisticallyestablish and/or quantify precipitation or hailfall are
useful, the "bottom line" for the county-sponsored
NDCMP is really what is obtained in terms of increased
crop yields. When all is said and done, it doesn't really
matter how the crop production is increased, or how the
objective of less hail is achieved, it is only important that
benefits are realized and that they in fact are due to the
cloud seeding program-- not chance. In an attempt to
define this "bottom line", wheat yield data for western
North Dakota were analyzed for the growing seasons
from 1935 through 1988 (Smith et al.1992). Figures 10
and 11 best illustrate the findings of this study. Thehistorical target-control relationship is shown in Fig. 10,
while the NDCMP years are shown in Fig. 11. The
difference? While the historical period revealed no
significant differences in yields, about 6% more wheat
was harvested in the target areas during the NDCMP
years.
Note that during drought years (1980, 1988),
little difference is observed between the target and
control. This comes about because droughts in North
Dakota are generally characterized by a shortage of
clouds having sufficient lifetimes and/or liquid water
Figure 10: Historical relationship between
NDCMP target areas and adjacent control
areas.
Figure 11: Wheat yields during NDCMP project
years. Except for droughts, most years exhibit a
significant increase within the target areas.
Figure 9: Five and ten percent economic
benefit scenarios for the NDCMP.
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Evaluation10
contents to produce much rain-- and these factors also
significantly limit seedability. While the ultimate
conclusion is that present cloud seeding technology
cannot break a drought, the larger implication for the
wheat yield study by Smith et al.is that one would notexpect to observe a significant difference in yields
during dry years. If a difference between target and
control is consistently not observed during such years,
the conclusion that the 6% otherwise observed is
attributable to cloud seeding is reinforced.
Thus, the bottom line seems to be more
production in the target area, without any decrease being
observed in the control areas. This is important, for it
supports observations that suggest rainfall is not simply
being redistributed. It appears that more rain is falling
over a significant fraction of the state.
3.4 Urban Effects
To date, the effects of the seeding program on
urban areas have not been quantified. This is an area
that certainly deserves more attention from the
evaluation standpoint, but some statements may be made
in this regard.
First, the North Dakota economy remains
largely agriculture-based. When harvest is bountiful, the
rural dollars are spent in the cities. This translates into
sales of automobiles, trucks, farm implements,
appliances, and so forth. Considerable additional taxrevenue is also generated for the state and counties,
which eases pressure in the urban areas. Many have
noted that, "As goes agriculture, so goes North Dakota."
However, the above impacts are all secondary.
The primary impact on urban areas comes in the same
form as it does to rural areasless, or smaller hail and
therefore less hail damage. Less hail damage means
fewer roofs to be replaced, fewer dents to be repaired on
motor vehicles, and fewer gardens turned to mush. The
damage caused by a single severe hailstorm passing over
an automobile dealership can be on the order of many
millions of dollars.Increased rainfall also has a positive impact by
reducing the amount of water (from city water supplies)
that must be used on lawns and gardens.
REFERENCES
Bangsund, D., and F.L. Leistritz, 2009: Economic
Impacts of Cloud Seeding on Agricultural Crops in
North Dakota. February, 2009. 37p.
Dennis, A.S., J.R. Miller, Jr., D.E. Cain, and R.L.
Schwaller, 1975: Evaluation by Monte Carlo tests of
effects of cloud seeding on growing season rainfall in
North Dakota. Journal of Applied Meteorology, 14,
959-969.
Eddy, A., and E. Cooter, 1979: The Evaluation of
Operational Cloud Seeding in North Dakota: Some
Preliminary Findings. Final report to the North Dakota
Weather Modification Board. Amos Eddy, Inc.,Norman,
Oklahoma. 43p.
Johnson, H.L, 1985: An Evaluation of the North Dakota
Cloud Modification Project. A final report to the North
Dakota Weather Modification Board, June, 1985. 35p.
Johnson, J.E., R.C. Coon, and J.W. Enz, 1989:
Economic Benefits of Crop-Hail Reduction Efforts in
North Dakota. Agricultural Economics Report No. 247,
Department of Agricultural Economics, North Dakota
State University, Fargo. 26p.
Miller, J.R., Jr., Boyd, E.I., Schleusener, R.A., andDennis, A.S., 1975: Hail suppression data from western
North Dakota, 1969-1972. Journal of Applied
Meteorology, 14, 755-762.
Sell, R.S., and F.L. Leistritz, 1998: Economic Impact of
Reducing Hail and Enhancing Rainfall in North Dakota.
December, 1998. 29p.
Smith, P.L. Jr., J.R. Miller, Jr., and Paul W. Mielke, Jr.,
1987: An Exploratory Study of Crop-Hail Insurance
Data for Evidence of Seeding Effects in North Dakota.
Consortium for Atmospheric Resource Development,Rapid City, South Dakota. 21p.
Smith, P.L. Jr., L.R. Johnson, D.L. Priegnitz, and P.W.
Mielke, Jr., 1992: A Target-Control Analysis of Wheat
Yield Data for the North Dakota Cloud Modification
Project Region. Journal of Weather Modification, 24,
98-105.
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NDCMP Operations Manual 11
Smith, P.L. Jr., L.R. Johnson, D.L. Priegnitz, B.A. Boe,
and P.W. Mielke, Jr., 1997: An Exploratory Analysis of
Crop Hail Insurance Data for Evidence of Cloud Seeding
Effects in North Dakota. Journal of Applied
Meteorology, 36, 463-473.
Smith, P.L. Jr., P.W. Mielke Jr., and F.J. Kopp, 2004:
Exploratory Analysis of Climatic Raingage Data for
Evidence of Effects of the North Dakota Cloud
Modification Project on Rainfall in the Target Area.
Institute of Atmospheric Sciences, South Dakota School
of Mines and Technology, Rapid City, SD. 11p.
Wise, E.A., 2005: Precipitation Evaluation of the North
Dakota Cloud Modification Project (NDCMP). A thesis
submitted to the graduate faculty of the University ofNorth Dakota. 63p.
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NDCMP Operations Manual 13
4.3 Pilot-Meteorologist Joint Responsibilities
Note that radar meteorologists' responsibilities
listed in Table 1 include weather watch, handling
weather briefings, and pilot communications. Pilotcommunication is critical to overall program
performance. Meteorologists must be able to assimilate
information from a variety of sources to keep the pilots
well informed. Meteorologists must also understand the
aircraft requirements for safe and timely operations.
Periodic visual observations by all field staff, is
essential.
The basic seeding hypothesis of the NDCMP
depends upon seeding young clouds, which may noteven be producing a radar echo. Visual observations of
the sky are to be made often to detect clouds before they
become too mature to be effectively seeded.
The frequency of observations required will
TABLE 1
Radar Meteorologist Responsibilities
Direction of Operations
Launching seeding aircraft for seeding and reconnaissance.
Prescribing seeding methods and rates.
Recall of aircraft if weather conditions might present an aircraft safety concern.
Operation of Linux-based computer (IRIS/TITAN) for archival and display of radar echoes.
Radio communication with aircraft.
Tracking storms, monitoring of intensity (severe weather potential).
Start-up and shut-down of radar auxiliary power unit when power outages threaten/strike.
Flying with aircraft when severe weather is not imminent nor forecast.
Tracking aircraft (GPS-based system).Visual observations prior to and during aircraft operations.
Monitor aircraft fuel and chemical status, to ensure that storms are seeded continuously.
In short, the radar meteorologist must make the real-time observations and decisions that run
the show and must plan for and anticipate storm development.
Meteorological Record Keeping
Entries in Radar Log.Archiving weather data on a daily basis.
Logging of all hail and significant rainfall reports received from project personnel and local citizens.
Routine Field Site Administration
Assignment of aircraft status: standby or alert.
Pilot Briefings and updates.Visual observations prior to and during aircraft operations.
Supervision of intern meteorologists.
Informing the ARB office of district chemical status.
Weather Watch when no aircraft are on Alert Status.
Equipment maintenance, such as can be done by persons other than electronics technicians.
Assist technician with radar calibration.
Operation of radar when in surveillance mode (prior to operations).
Severe WeatherSuspension of seeding if tornado, funnel cloud, or flash flood potential is observed.
Notification of the National Weather Service when severe weather is observed.
Interaction with local weather modification authorities and law enforcement agencies (civil defense).
Administrative Record KeepingEntries in Radar Log.
Completion of Operations Summary reports.
Mailing of weekly forms, reports.
Verify that radar is operating at expected performance levels.
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Responsibilities14
vary. In some cases, it will be important to make a
visual observation every 5 to 10 minutes! Don't forget
to look directly overhead. The radar is seldom pointed
there, and a storm can quickly develop and go
undetected until it is well beyond the threshold of
seedability-- in the extreme case, already hailing.
Once operations are underway, the flight crew
must in a sense serve as the radar meteorologist's eyes,
conveying descriptions of cloud character, areas of
development, new growth, rainfall, and so forth. The
radar conveys a picture of what already is, not what is
to be, and therefore cannot readily depict nuances of
storm behavior apparent to the seasoned Pilot-In-
Command (PIC). In return, the radar meteorologist
must provide the flight crews current storm echo
information, including cloud tops, motion, position
relative to target, estimated time before exiting thetarget, and so on.
4.4 Pilots-In-Command
It is important that the PIC's and ICP's keep the
district meteorologist(s) well informed. Any problems
such as illness or potential aircraft problems should be
communicated to the district meteorologist(s) as soon as
possible so that schedule/staffing changes might be
considered.
The PIC's work for the contractor, but havedirect responsibilities to the ARB. In general, the PIC's
are responsible for ensuring their aircraft and seeding
equipment are in good working order. They should be
ready to launch within 15 minutes if on standby, and
immediately if on alert status. Each PIC and ICP is
required to be reachable by telephone or pager at all
times. Pilots are responsible for keeping accurate
records of their chemical inventory and flight data. The
intern may assist, but is not required to do inventory;
consequently the responsibility lies with the PIC. The
inventory is expected to be completed for each week by
noon on Monday.One means by which communication between
aircraft and radar is expedited is through the use of
"code" descriptors for precipitation. There are five
"codes" used to describe convective rainfall, all of which
are perfectly acceptable to use on flight forms, radar
logs, and other project documentation. They are:
Code Zero: No precipitation observed below the base of
the subject cloud.
Code One: Precipitation beginning from cloud base, but
not reaching the ground (also called virga).
Code Two: Precipitation just beginning to reach the
ground, but very easy to see through. No obscuration of
surface features beyond the precipitation is observed.
Code Three: Significant precipitation observed, partial
obscuration resulting. Features beyond precipitation
shaft may be difficult to discern.
Code Four: Heavy precipitation resulting in complete
obscuration beyond rain shaft, at least portions of it.
4.5 Intern Copilots
The first purpose for Intern Co-pilots (ICPs) is
to be trained on the principles of cloud modificationprocedures and become knowledgeable of safe aircraft
operations around thunderstorms. Interns will also be
extremely conscientious in their record keeping. All
NDCMP records are public information. The PIC is
responsible for seeing that this information is collected
in a manner that accurately portrays the events of each
flight. Please refer to the PARS Manual and Chapter 10
of this manual for further information.
It is understood that the PIC will provide the
ICP opportunities to fly the aircraft from the left seat and
gain seeding experience. The intern may become a PIC
in later years so this experience is important. Since thePIC has ultimate responsibility for the operation of the
aircraft, decisions about the intern having left seat time
must remain entirely at the PICs discretion. In the past,
the quality of the ICP's and the training they received at
the University of North Dakota has been consistently
very good.
Weather Watch is a responsibility of all
project personnel, including interns. At points distant
from the radar, pilots become the eyes of the
meteorologists. Again, many seeding opportunities,
including those for hail suppression, must be detected by
visual observation. Pilots should never assume themeteorologists know about a cloud or storm, but should
always call just to make sure.
4.6 Safety
The NDCMP has an excellent safety record.
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NDCMP Operations Manual 15
Safety comes first when making any decision. Stunt
flying with project aircraft is prohibited and will result in
termination from the program. The Director need only
write a letter to the contractor or the University
removing any PIC or ICP from the program. Whensafety becomes a concern during the NDCMP, it takes
precedence over all operations and activities. If any
NDCMP staff member feels that a situation or decision
would be unsafe, they should contact Bismarck ARB
Office personnel no matter what time of day it is. ARB
staff are on-call 24/7 during the project to assist with
questions or concerns.
4.7 General Responsibilities
All project personnel are considered to berepresentatives of the Board, the contractor, and the
County Authorities. As such, they are expected to be
courteous and helpful to people that visit field sites.
Keep in mind that even antagonistic people will often
calm down a great deal if you are patient and polite.
Most of the local people who contact you will just be
curious, or supporters of the program. Local people pay
for the program and should always be treated with
respect. Dont try to answer questions about
unfamiliar subjects. Refer those questions to the
radar meteorologists or the ARB office. If you need
materials describing the program, the ARB will
supply them on request.
A host of informational materials are availablewhich address natural precipitation processes as well as
the physics, effects, limitations, and benefits of cloud
seeding. Copies will be made available to field
personnel upon request.
Although flight hours may be limited based
upon the districts budgets, pilots and meteorologists
should not be hesitant to launch reconnaissance
missions if there is concern about severe weather
potential. In general, the best policy is better safe than
sorry.
Research conducted by the South Dakota
School of Mines and Technology (Smith et al., 1985)concludes that; of the storms treated for hail damage
reduction in 1981, only one-half to two-thirds were
seeded on-time (Fig. 12). A more strict definition of
on-time, (seeding feeders before or as a radar echo
forms) yields an even lower percentage.
Significant strides have been made since the
study was released in 1985. Upon re-evaluation using
2002 hail events and comparable criterion, ARB staff
found that 95 percent of hail flights fell between what
Figure 12: Reproduction of timeliness graph of initial seeding events for hail suppression in 1981
(from Smith et. al. 1985) using a 45dBz threshold (74 instances of hail suppression).
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Responsibilities16
was designated early and late in the Smith study (Fig.
13). Furthermore, a full 86 percent fall in the category
of being either early or on time (within the fifteen
minute standby time limit). An updated study of 2007-
09 seeded events (Fig. 14) shows an encouraging
timeliness trend. Although there were only 74 total
events evaluated for the three operational years, 88
percent of them were in either the early or on time
classification. When it comes to operations, however,
timeliness can always be improved on.
Timeliness issues stem from several factors.
Perhaps the single largest factor is the inability of field
personnel to consistently anticipate the transition of
subject clouds from cumulus humilis or cumulus
mediocris to cumulus congestusor cumulonimbus. A
second factor is the reluctance of radar meteorologists to
launch aircraft until they have a suitable target cloud in
hand. A third factor may be a perceived lack of funding,
which supports the use of numerous flight hours for
reconnaissance purposes. The final factor is that pilots
are sometimes less prepared than they should be, or are
not easily contacted by the radar meteorologists, so
ground delays contribute to the tardiness.
Consequently, reconnaissance time and adequate
attention to weather watch are vital to the success of
the project. Flight crews should note that
Figure 13: Timeliness graph of initial seeding events for hail suppression in 2002 (by ARB staff) using
a 45 dBZ threshold 5000 ft above O
C level (129 instances of hail suppression).
Figure 14: Timeliness graph of initial seeding events for hail suppression in years 2007-2009 (by ARB
staff) using 45 dBZ threshold 5000 ft above 0
C level.
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NDCMP Operations Manual 17
reconnaissance time does not include return flight time
aftera seeding mission, or chemical deliveries. Flight
hours not spent seeding or in reconnaissance should be
designated as other. An exception to this is
maintenance time which is billed to the contractor, notthe Board (see Chapter 10).
From time to time, difficulties may develop in
the field that cannot be resolved by the parties involved.
Examples of such problems include personality clashes,
recurrent equipment problems, and the failure of project
personnel to perform as expected. In all cases, it is
imperative that such problem be reported to the ARB
office so that corrective action can be taken. This allows
a recovery plan to be put in motion that enables the
situation to be corrected. Equipment can be modified or
replaced, personal differences can be resolved (perhaps
even through reassignment of person(s) involved), andpersonnel performing below expectations can be
contacted to determine the nature of the problem. It has
been found that in organizations and businesses as a
whole, 85% of all problems are related more to
processes than people. This means that a problem is far
more likely to arise as the result of a process that has
been poorly designed than because someone isnt doing
their job. Before corrective action can be taken,
however, the problem must be made known. Therefore,
it is the responsibility of all project personnel to identify
problems of any type so that the source of the problem
can be modified/corrected, and a recovery plan put intoaction.
REFERENCES
Smith, P.L., J.R. Miller, Jr., A.A. Doneaud, J.H. Hirsh,
D.L. Priegnitz, P.E. Price, K.J. Tyler and H.D. Orville,
1985: Research to Develop Evaluation Techniques for
Operational Convective Cloud Modification Projects.
Institute of Atmospheric Sciences, South Dakota School
of Mines and Technology, Rapid City, SD 57701,
Report SDSMT/IAS/R-85/02, 94p.
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Seeding Conceptual Model18
5. SEEDING CONCEPTUAL MODEL
The seeding conceptual model for the
NDCMP has been developed, reviewed, and refined
over a period of years. A number of respected
atmospheric scientists have been involved in the
process. The following subsection covers the seeding
conceptual model in its most up-to-date form and is
largely based upon previous work of the NDCMP
Hypothesis Description and Assessment Committee
(1984) and the NDCMP Operations and Safeguards
Committee (2003).
Seeding in the NDCMP is conducted in two
modes: within updrafts at cloud base and by direct
injection during aircraft penetration at or near cloud
top. All seeding is done by aircraft. The former mode
is most often used, as three out of every four projectaircraft are not equipped for repeated cloud
penetrations and do not possess the performance
required for cloud top work. Seeding agents are
largely glaciogenic (silver iodide complexes from
solution and flares also have some hygroscopic traits)
as observations of northern Great Plains cumuliform
clouds indicate that most precipitation results from
processes involving the formation of ice.
5.1. Seeding for Rainfall Increase
In cloud base seeding silver iodide complexes
are produced either by the combustion of
acetone-based solutions or by the burning of
silver-iodate flares (or recently, other formulas)
attached to racks on the trailing edges of the wings.
The nuclei thus produced are ingested by the target
clouds, transported upward to the regions containing
supercooled liquid water and mixed through a
significant portion of the cloud volume, where
nucleation occurs. If treatment is timely the seeding
agent should reach the supercooled portions of the
cloud at about the time the cloud top is growingthrough the -10C level. Nucleation in these seeded
clouds is believed to occur on average about 5 to 10C
warmer than most natural nucleation. Given typical
cloud growth rates this affords a "head start" in
precipitation development on the order of 3 to 5 min.
In direct-injection seeding either dry ice
pellets (frozen CO2) or ejectable silver iodate flares
are used. Again, clouds growing through the -10C
level are targeted. The seeding agents are placed into
the supercooled cloud where nucleation is desired, so
the updrafts in these cases are relied upon only to
provide a continuing source of condensate, not to
transport the seeding agent upward from cloud base.
This delivery technique thus requires less anticipation
on the part of those directing the seeding and may
have a more immediate effect.
In both cases, the intent is to glaciate portions
of the cloud, initiating ice development minutes earlier
than would naturally have been the case. For smaller
or more isolated convective towers, glaciogenic
seeding may accelerate hydrometeor growth
sufficiently to allow the cloud to produce
precipitation-sized hydrometeors during its short
lifetime (microphysical effects) while addingbuoyancy, which may stimulate updrafts and prolong
the cloud lifetime as well (dynamic effects). Both
effects contribute to increased precipitation
production.
5.2. Hail Suppression and the Conceptual Model
As previously noted, the NDCMP is
dual-purpose. However, hail suppression is the
primary goal of the project. The hail suppression
concepts employed in North Dakota are based uponthe following:
Natural Development:
! The main updraft of a mature thunderstorm,
which is responsible for a large fraction of the
storm's total mass flux, also supports the larger
hydrometeors (hailstones) as they grow. Primary
hailstorm updrafts are usually tilted and
frequently possess speeds between 30 and 40 m
s-1
, which greatly facilitate the production of
large hail (Nelson 1983).
! Feeder clouds, which flank mature thunderstorm
cells, develop significant quantities of
supercooled liquid water (SLW). Cloud droplet
spectra are usually continental in nature, and
precipitation development through
collision-coalescence processes is uncommon
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NDCMP Operations Manual 19
within these typically short-lived clouds.
Furthermore, significant ice development most
often does not occur until cloud top temperatures
of -15 to -20C are reached.
! Because of the slow development of natural
(ice-phase) precipitation in typical feeder clouds,
precipitation-sized hydrometeors usually have
only very recently developed at the time of
merger with the mature cell. Cloud bases of such
feeder clouds are typically still rain-free.
Significant concentrations of millimeter and
larger graupel (potential hail embryos) remain
aloft within the feeder cloud.
! Graupel particles generated within the feeder
clouds of the flanking line then become hailembryos after either a) being transported to or
ingested by the mature updraft, or b) remaining
within the feeder cloud updraft as it matures and
becomes dominant. In either case, the initially
small embryos are transported into the
supercooled upper reaches of the storm by the
updrafts. Ensuing hydrometeor growth is rapid;
hailstones become too large to have any chance
of melting completely during fallout, and
damaging hail reaches the ground.
Seeded Development:
! Treatment with glaciogenic nuclei of supercooled
clouds within the flanking line as they grow
through the -10C level will initiate significant
ice development minutes before it would
otherwise occur (see Dennis and Koscielski
1972).
! Significant glaciation of the treated feeder cloud
often results (see Stith et al.1990). The release
of the latent heat of fusion during glaciationresulting from seeding adds buoyancy to the
feeder cloud. Much of the supercooled liquid
cloud water is converted to ice by either riming
or deposition. Less supercooled liquid water
remains in treated clouds than in untreated
clouds. Precipitation mass within the feeder
cloud is greatly enhanced.
! Updrafts in the treated feeder clouds remain
significantly weaker than those of the mature
cell. The developing hydrometeors (graupel)
soon gain significant terminal velocities. (Fig.
15).
! Precipitation begins. A developing precipitation
shaft exists where there otherwise would have
been a rain-free cloud base [early rainout].
Because the hydrometeors are yet small
(millimeters in diameter), melting occurs well
before most particles reach the surface. In some
cases, especially in cooler spring and autumn
environments, unmelted particles do reach the
ground, but such particles are usually small.
!
Merger with mature cell occurs.
! Because less SLW remains in the feeder cloud1,
less freezing occurs within the main cell.
Seeding accelerates the glaciation of the feeder
cloud, releasing the latent heat outsidethe mature
cell, depriving the main updraft of a source of
energy. Thus, the buoyancy and strength of the
mature updraft are reduced1 [energy transfer].
See again Fig. 15.
! A significant population of precipitating
hydrometeors is transported into or ingested bythe mature cell as merger occurs. This results in
additional mass loading, further slowing the
mature updraft [updraft loading.]
! Hydrometeors produced by seeding continue to
grow in the moisture-rich environment and
compete with naturally occurring
hydrometeors. This increased competition for
supercooled liquid water necessitates natural
hydrometeors in the seeded cloud growing
smaller than they would naturally [beneficial
competition].
! The large populations of precipitating ice-phase
hydrometeors present in the feeder clouds at
merger continue to deplete the supercooled liquid
1when compared to similar natural clouds.
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Seeding Conceptual Model20
water resident within the mature updraft as
rainout continues [trajectory lowering].
! Many multicell thunderstorms feed on moist
boundary layer air being drawn in from the
southern or eastern quadrants. The precipitation
shaft [where the rain-free base previouslyexisted] may partially restrict or block such
inflow, reducing the "fuel supply" to the mature
updraft [fuel starvation].
! Total supercooled liquid water is diminished in
the mature updraft. The mature updraft is
weakened by mass loading, reduced latent heat
release within it, and possible fuel starvation.
The environment is less favorable for the growth
of hail.
!
The rain shaft of the storm is broadened by earlyrainout. Some areas that would otherwise not
have received measurable precipitation now do
as a result of seeding. Some areas that would
have received locally intense precipitation
receive less intense precipitation.
Ice particles originating in the primary
updraft (perhaps as a direct result of artificial
nucleation) dont generally grow sufficiently during
their brief residence within the main updraft to
develop appreciable terminal velocities. Such
particles remain resident within the primary updraft
for only a few minutes, remain small and ultimatelyexit the updraft as they are transported into the upper
reaches of the storm anvil (Ryan 1974). Thus, seeding
the primary updraft of a mature storm has little effect
on the storm, and will not effectively diminish hail
development.
Figure 15 illustrates most aspects of the
conceptual model. In natural clouds, nucleation of
first ice often does not begin until temperatures on the
order of -15C or lower are attained by the developing
clouds. In a seeded cloud nucleation often begins at
temperatures between -5C and -8C. Thus, the ice
initiated at colder temperatures is afforded less timeresident in the flanking line (compared to ice
originating at warmer temperatures) and is
significantly smaller when merger with the mature cell
occurs. Subsequently these ice hydrometeors are
easily supported by the stronger, SLW-rich updrafts of
the mature cell, and they rapidly grow into hailstones.
When earlier ice development occurs the latent heat of
Figure 15: The NDCMP hail suppression conceptual model, generalized. For detail, see text.
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NDCMP Operations Manual 21
freezing is liberated, adding buoyancy (heat) to theyoung updraft, strengthening it. Because the latent
heat is released in the smaller turrets, rather than the
larger towers and the mature updraft, the latter are in
essence deprived of the energy. [Note: this assumes
that the total latent heat release is unchanged in the
course of the storms lifetime. This may not be the
case as additional heat release within smaller clouds
may or may not offset the decrease in latent heat
release in the mature updrafts. The conceptual model
is also supported by observations of Alberta hailstorms
(Krauss and Marwitz 1984).
All the factors discussed in the model areinteractive, that is each process affects another. For
example, the earlier release of latent heat will intensify
the updraft in the seeded flanking line cell, while
diminishing the mature "main" updraft. Large
hydrometeors already resident in the mature updraft
may then precipitate, resulting in increased
evaporative cooling, divergent outflow and subsequent
"cut-off" of the warm, moisture-laden air, which fuelsthe main updraft. Likewise, increased populations of
small ice-phase hydrometeors (as result from seeding)
might lead to increased hail production in an
environment without competition (among growing
hailstones) for available supercooled liquid water.
However, the processes that build the potential hail
embryos also deprive the mature updraft of some
energy and SLW, producing an environment less
favorable for the generation of hail. Because the
interactions are so numerous and variable the outcome
of a particular seeding action might seem to be very
much in doubt. However, the evaluations reviewed inChapter 3 suggest that the outcome on the whole is
very positive.
Presently, the full scale of these interactions
is being best addressed through numerical (computer)
cloud models (see Fig. 16, from Orville et al. 1991).
While such cloud models, until very recently,
suggested that increased rainfall must be accompanied
Figure 16: Various outcomes of glaciogenic (ice-phase) cloud seeding (Orville et al. 1991).
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NDCMP Operations Manual 23
6. IDENTIFICATION OF SEEDING
OPPORTUNITIES
Identifying seeding opportunities and properly
delivering seeding materials go hand-in-hand. Pilots andmeteorologists should carefully review this chapter as
well as Chapters 4 and 5. Differences between seeding
to achieve hail suppression and to gain increases in
rainfall are subtle. For the purposes of the NDCMP,
seeding for hail suppression shall be considered to be
the treatment of any cloud which possesses radar
signatures indicative of hail, or clouds which without
treatment, the radar meteorologist believes to be
capable of hail production. Under this definition, even
cumulus congestus would qualifyproviding the speed
and circumstances of their development (unstable
atmosphere, high surface dew points, significantdirectional wind shear, etc.) cause the radar
meteorologist to believe the cloud will, without
treatment, eventually produce hail. From this definition
it logically follows that seeding for rainfall increases
shall be considered to be the treatment of clouds not
considered to be hail threats, but believed capable of
producing additional precipitation if stimulated by the
addition of glaciogenic nuclei. In the completion of
flight documentation the statement of mission objectives
is required. Radar meteorologists will inform flight
crews, while missions are underway, of the seeding, i.e.,
hail suppression or rainfall increase.
The following sections discuss in some detail
the vigilance and cloud traits that together optimize
recognition of targets of opportunity and minimize
response times.
6.1 Targets Suitable for Rainfall Increase
Seeding
The primary candidates for rainfall
augmentation treatment are towering cumulus which are
either not making the transition to cumulonimbus, or are
very slow in doing so. Preliminary identification of dayssuitable for rainfall augmentation operations must come
from the strategy-planning portion of the day; the daily
forecast will detail the likelihood of suitable clouds
developing.
Rainfall enhancement seeding will generally
not be conducted at night due to safety considerations,
primarily limited visibility. [Since very few isolated
towering cumulus clouds develop at night, this is seldom
a limiting factor, but remember, target clouds are too
small to produce a radar echo, but must be clearly seen
to ensure that the seeding agent is properly targeted.]
Relatively isolated towers well illuminated by moonlightafford the only opportunity for nocturnal rainfall
augmentation seeding.
Forecast parameters are secondary in
recognizing any type of seeding opportunities. At best,
the forecast can only serve to alert field personnel to the
possibilities of seeding opportunities later in the day.
Regardless of the forecast, any time clouds are forming
which appear to be suitable, at least one nearby aircraft
should be placed on alert and if necessary, launched.
[Failure to recognize and respond to suitable
opportunities may result in telephone calls from
residents of the sponsoring counties, many of whom arewell acquainted with this aspect of the program.] Some
general characteristics of good rain enhancement
candidate clouds are crisp, cauliflower-like tops and
sides and firm, flat, rain-free bases. Some shear (as
evidenced by the cloud leaning with height) is
acceptable. Too much shear is present if the cloud tops
are being swept away from the base.
The reconnaissance aircraft should verify that
the cloud base is below 12,000 ft msl. The cloud top
must be colder than 0C and preferably around 5C. If
there is some doubt, it may be necessary for the
reconnaissance aircraft to climb to cloud top altitude.
[Note: the decision to launch seeding aircraft solely for
rain enhancement operations should be made prior to the
formation of a radar echo, when possible. If an echo has
formed, that cloud is too mature, however, there will
likely be many more clouds to grow through that stage in
the impending minutes and hours. The best rainfall
augmentation cases are generally characterized by
numerous suitable cumulus congestus scattered over the
target area. This being the case, a single echo should not
usually be sufficient to deter operations.]
Because rainfall increase operations work with
small non-precipitating clouds, it is not known at the
time of treatment whether rainfall will result or not.This uncertainty arises from the fact that the cloud
lifetimes are short (~30 min) and the target cloud must
be treated while it is only a few minutes old.
Treatment at cloud top by direct injection is
preferred for several reasons. Treatment at cloud base
requires greater time for the agent to reach the
supercooled cloud top. Penetration seeding at cloud top
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Opportunity Recognition24
affords the flight crew a taste of the cloud updraft and
SLW, while base seeding only allows the updraft to be
tested. And finally, any cloud base aircraft that circles a
prospective target cloud several times in search of
updraft is easily observed. If an updraft is found and thecloud treated, this is no problem. However, it has been
noted on numerous occasions that if the cloud base
seeder doesnt find an updraft and departs in search of
another cloud, uninformed persons on the ground may
misconstrue this as seeding. The impression left is a
devastating one, for the departure of the would-be cloud
base seeder is due to a lack of an updraft, which
guarantees that the cloud is no longer viabledying.
Observers on the ground have been known to attribute
the departure of the aircraft and subsequent cloud
collapse to seeding, which must be avoided. Thus,
operations to stimulate rainfall from non-precipitatingclouds are best conducted from cloud top. Cloud top
treatment is more rapid and the flight crew has a more
complete picture of the cloud character by virtue of
having flown through it.
Finally, rain enhancement seeding may be
temporarily suspended if a County Weather
Modification Authority or the Director indicates
conditions are too wet, or there is no immediate desire
for additional rainfall (e.g. during harvest). See Section
8 for additional details on seeding suspension criteria.
6.2 Hail Suppression Opportunities
As in rain enhancement cases, the morning
forecast provides initial clues that hail-bearing clouds
may form. The presence of abundant low-level moisture,
strong wind shear, a deep unstable layer, or a strong
trigger mechanism such as a short-wave trough, frontal
boundary, or dry line, all contribute to severe weather
situations.
This is especially true when these features are
present in combination. The decision trees shown in
Chapter 8 describe these parameters somewhat more
quantitatively. Again, the atmosphere may change
substantially during the day and so the morning forecastis only a planning tool.
Accurate and frequent visual observations
and quick reactions are vital to a well conducted hail
suppression mission in situations where clouds are
forming and growing. This is especially true in cases of
a very unstable atmosphere, where clouds can become
hail-bearing thunderstorms in a matter of minutes (Boe
and Johnson 1990). The visual appearance of a cloud
that will become a hail threat is initially similar to that of
a rain enhancement candidate. The only difference is
that the hailer-to-be will continue to grow, probably
rapidly, and soon extend above 30,000 ft msl or so.Ideally, seeding of the new growth should again begin
prior to the formation of the first 20 dBZ radar echo.
In situations where thunderstorms have
already formed and are moving into the district, the radar
must serve as the primary tool for identifying hail. The
height of radar-measured cloud top and reflectivity
structure of the storm are the main keys in detecting the
presence of hail.
The height of the 45 dBZ contour is a
criterion tested in the Swiss hail suppression program
(Waldvogel, et al. 1979). The Swiss research indicated
that all hailers had 45 dBZ contours that exceeded30,000 ft msl. There was a False Alarm Rate (FAR) of
50%, largely because some strong rain storms also met
the criterion. However, it is much preferable to make an
error and assume that a heavy rainstorm is going to
produce hail than to mistakenly believe that a hailer is
only going to produce heavy rain.
Although not proven for North Dakota,
experience suggests that the Swiss criterion is
reasonable. The physical reasoning behind it is simply
that high radar reflectivities suggest significant liquid
exists at temperatures cold enough for large hail growth.
Any cell having the 45 dBZ contour extending more
than 1.4 km (5 Kft) above the 0C level(as determined
by the most recent appropriate sounding), WILL BE
CONSIDERED A HAILER (i.e. the planes should
already be in the air).
REFERENCES
Boe, B.A., and H.L. Johnson, 1990: Destabilization
antecedent to a tornadic northern High Plains
mesoscale convective system. Preprints, Sixteenth
Conference on Severe Local Storms, American
Meteorological Society. Kananaskis Park, Alberta,Canada, 538-541
Waldvogel, A., B. Federer, and P. Grimm, 1979:
Criteria for the detection of hail cells. Journal of
Applied Meteorology, 25, 1521-1525.
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NDCMP Operations Manual 25
7. DELIVERY TECHNIQUES
One of the first things a student pilot is told
in ground school is that thunderstorms are dangerous,and that they should be avoided by at least 20 miles if
at all possible. This statement, examined in light of
what the average pilot knows about such storms, is
true. However, a little knowledge can go a long way
towards mitigating the hazard (Bertorelli 1993). As
far as operational cloud seeding goes, the early
identification of suitable clouds, the timely delivery
of the treatment material and SAFETY are the three
critically essential tasks. Since the cloud top aircraft
are used to measure temperature, supercooled liquid
water content, ice content and updrafts in potential
target clouds before seeding, they are ready to seed if adecision to do so is made. Thus, the identification of
suitable clouds and the initiation of seeding must be
considered as one continuous process.
Two modes may be used to deliver the
seeding material into the updraft of suitable target
clouds. These are:
! Nuclei can be released directly in the updraft
by flying through the top of the growing
turrets, or
! Nuclei can be released in the areas of updraft
(inflow) near the base of the cloud.
In both modes the pilot must be familiar with the rate
of climb, power settings and airspeed characteristics of
her/his aircraft.
For most seeding situations, cloud top
seeding is preferred. However, it is not always
possible to use this mode. Aircraft performance
limitations and/or the inability to identify the most
suitable seeding locations are the most common
problems. If cumuliform tops are higher than 25,000
feet, the cloud may be too turbulent for safe aircraft
penetration (clouds this size may be producing ice
naturally anyway). In night operations or when
intervening cloudiness hides the regions of new cloud
growth, it may not be possible to identify the desired
target areas.
Cloud base seeding is used in cases where
well-developed updrafts can be identified, or when in-
cloud seeding by penetration near cloud top is not
practical. This mode is generally the only mode used
at night unless unusual conditions (moonlight, for
example) provide the visibility required for cloud-top
seeding.
A disadvantage of below-base seeding is that
the pilot must infer the rate of cloud growth from the
strength of the updraft and the appearance of the cloud
base, and cannot know the degree of glaciation (if any)
at cloud top. Subcloud seeding does not permit
frequent visual observation of the cloud top.
Visual characteristics of cloud base provide
clues to the location of the inflowing (updraft) air.
Monitoring the evolution of radar echoes can also
provide hints about the updraft structure of the cloud.
In addition, knowledge of the environmental
conditions in which the cloud evolves is important in
locating updrafts. (Refer to other chapters of theOperations Manual for discussions of these topics).
Locating an updraft is generally not difficult.
However, in the case of large multi-cellular clouds,
multiple updrafts and multiple hail shafts are not
uncommon (Nelson 1987). It then becomes necessary
to ensure that the proper inflow area(s) are treated. It
is sometimes necessary to investigate the larger-scale
storm structure prior to initiating seeding.
Positioning of the aircraft with respect to the
target cloud must be done carefully, as a joint effort of
the meteorologist and the pilot. The meteorologist
must use the radar information and his/her knowledgeof cloud processes to assist the pilot in locating the
general area of seeding. The pilot then selects the
specific locations.
7.1 Rain Augmentation Seeding Procedures
Speed shear in the wind field may have a
significant effect on the results of seeding. Slightly
different techniques should be used for seeding under
different conditions of vertical (speed) shear. Any
possible variation of wind speed with altitude mayoccur and the pilot must be aware of the shear
conditions and be prepared to take advantage of them.
Cloud top seeding with CO2 pellets or
ejectable flares is used on clouds with top
temperatures between -5C and -15C, and is best used
on cauliflower cumulus whose tops are between -5C
and -10C. Dry ice is the preferred initial method of
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NDCMP Operations Manual 27
upwind of the cloud. Seeding is done in the upwind-
most turret that reaches or exceeds the height at which
the nucleant can be expected to function. Penetration
is from the side, across the shear; the seeding agent is
dropped in a recognizable updraft. The X marks the
seeding position relative to the cloud updraft. Again,
the seeding agent(s) is released in the edge of the
upwind-most turret that is between the -5C and -10C
levels. It is desirable to seed where total vertical cloud
depth is greatest. Penetration of isolated turrets can be
in any direction, but on larger clouds, prudence
dictates entry at right angles to the wind shear or
penetration in a turn into the upwind portion only. If
one studies the natural evolution of clouds in shear
environments, it will be observed that new turrets
appear and evolve progressively upwind, at intervals
of roughly ten minutes. If a given tower is seeded justas it reaches maturity, the next pass should be made,
not on the original tower, but on the next one upwind.
With ideal timing, each new tower is caught just as
it reaches seeding level (at temperature between -5C
and -10C). Successful seeding of leaning clouds
requires a slightly larger cloud than in the case of
vertical towers.
Seeding at base is also done upwind, toward
the rear portion of the storm, as shown in Fig. 18.
Inflow in the range of 500 fpm should be selected for
rain enhancement. If it is difficult to maintain less
than 500 fpm updrafts, this can be an indication thatthe system may become more than a rain storm.
Seeding rates may then be adjusted accordingly.
7.1.3 Reverse-Shear Seeding
When the wind speed decreases with
increasing height, the upper part of the cloud moves
downwind more slowly than the bottom of the cloud
and appears to lean into the wind. The occurrence of
reverse-shear clouds can be foreseen from pilot,
balloon or rawinsonde observations, forecasts orrecognized by noting smoke near the ground, waves
on water, or cloud drift. Seeding should be done on
the downwind side of the cloud so that coalescence of
old cloud and new growth is a maximum. As with
other cumulus congestus treated for rainfall increase,
seeding should be done by penetration (Fig. 19).
Seeding should be done on the downwind-most rising
tower that reaches the -5C temperature level at which
the nucleant becomes effective at generating ice.
Similarly, base seeding should be conducted
downwind (opposite of the shear) or in advance of the
cumulus.
7.1.4 Embedded Cumulus
Having determined that the embedded
cumulus clouds of interest are seedable (firm,
unglaciated tower tops, etc.) the pilot should seed thetower by penetration from the top using ejectable
flares or CO2 at a relatively low rate. Only one
ejectable should ever be released per tower. If the
tower grows following the first treatment, repeat
seeding at 20 min intervals until significant glaciation
is evident or growth stabilizes, then cease operations
on that tower. If no growth follows the initial
treatment the tower should not be seeded again.
When seeding embedded convection it is
imperative that the aircraft operate primarily in the
clear air above the stratus deck. Continuous
operations within a supercooled stratus deck may
lead to the rapid accumulation of airframe icing and
may pose a severe safety threat.
If the aircraft cannot climb above the stratus
deck seeding may be conducted be