MICROBURSTS Nolan T. Atkins Lyndon State College Prepared for COMET Mesoscale Analysis and Prediction Course 2002 (COMAP 2002) 13 June 2002

MICROBURSTS

Nolan T. Atkins Lyndon State College

Prepared for COMET Mesoscale Analysis and Prediction Course 2002

(COMAP 2002)

13 June 2002

OUTLINE

1. Introduction – Early Discovery2. Climatology3. Forcing Mechanisms4. Microburst Conceptual Models5. Wet Versus Dry Microbursts6. Detection7. Forecasting

INTRODUCTION – EARLY DISCOVERY

Aerial damage surveys by Fujita of 3 April 1974 super outbreak revealed unusual “starburst” surface wind damage pattern

315 fatalities, 5484 injuries

15% of damage paths were caused by outburst winds

“Starburst” wind damage pattern

Figure from Fujita 1985


“Starburst” wind damage pattern in corn field


“Starburst” damage pattern was very much different than swirling damage left behind in wake of tornado

Idea of “down burst” was conceived

Much like “pointing the nozzle of a garden hose downward”


On 24 June 1975, Eastern Airlines Flight 66 (Boeing 727) crashed while attempting to land at New York’s JFK Intl airport

112 fatalities, 12 injuries

Cause of crash was unknown, though thunderstorms were observed in the area

In an attempt to unravel the mystery behind the crash, Captain Homer Mouden (from the Flight Safety Foundation at the time) approached Fujita and asked him to investigate reasons for the crash


After analyzing only flight data recorders, pilot reports and an airport anemometer, Fujita hypothesized that Flight 66 flew through a low-level diverging wind field – downburst

First suggestion that a “starburst” wind pattern may be a cause for airline crashes



Fujita’s concept of a downburst, a strong downdraft which induces an outburst of damaging winds on or near the ground, was met with some skepticism

Many meteorologists at the time, believed that the downdraft should be relatively weak by the time it reaches the ground

Resolution of Fujita’s downburst theory ultimately led to the creation of the Northern Illinois Meteorological Research on Downbursts (NIMROD) field program employing NCAR Doppler radars

INTRODUCTION – EARLY DISCOVERYRadial velocities from first radar-detected downburst

Figure from Wilson 2001

On 29 May, 1978, the first radar-detected downburst was observed by the NCAR CP-3 Doppler radar by Fujita and Jim Wilson

The existence of the downburst had been verified.

Since then, a flurry of observational, applied and theoretical work surrounding the downburst has been pursued

Climatology

Severe thunderstorm wind gusts, 1955-1983.

From Kelly et al. (1985)

A national climatological summary of downbursts, unfortunately, does not exist

Kelly et al. (1985) have produced a climatology of damaging wind gusts. Based on 75,626 severe

thunderstorm reports from 1955-1983.

Does NOT distinguish damage created from different convective modes (for example, RIJ associated with a bow echo)

Three categories of wind gusts were created

Gust speed

Annual number

Percent

Damaging unknown 1114 70

Strong 25.8-33.5 m/s

375 23

Violent > 33.5 m/s

113 7

Total 1602

Climatology


Damaging wind gusts: Primarily a summer time

phenomena

Climatology


Damaging wind gusts: Most events occur during

late afternoon However, a non negligible

number of events occur between midnight and noon

Climatology

From Wakimoto (2002)

Geographical Distribution of damaging wind gusts:

Two major frequency axes:

1. Southern MN – IA – IL – IN – OH (NW flow events)

2. NW IA – Kansas City, MO – KS – OK – TX

3. Possibly a third from eastern TX – AL – up to New England

High probability of a population bias in data

Climatology

From Fujita (1981)

Kelly et al. results are similar to those by Fujita (1981) for the year 1979

Climatology

From Wakimoto (2002)

Data from Downburst field programs:

1. Northern Illinois Meteorological Research on Downburst (NIMROD) – 1978

2. Joint Airport Weather Studies (JAWS) – 1982

3. FAA/Lincoln Lab Operational Weather Studies (FLOWS) – 1985/86

4. Microburst and Severe Thunderstorm (MIST) project - 1986

Climatology

Figures from Wakimoto (1985)

186 microbursts during JAWS over 86 days

Diurnal variation similar to Kelly et al. (1985)

Climatology

Figures from Atkins and Wakimoto (1991)

62 microbursts during MIST over 61 days Diurnal variation similar to Kelly et al.

(1985) Data from field programs suggest

downbursts occur frequently

Forcing Mechanisms – Updrafts and Downdrafts

UPDRAFT DOWNDRAFTAscends supersaturated descends largely subsaturatedrc+rr+ri negate updraft rc+rr+ri enhance downdraftLatent heat release enhances UD evap cooling/sub/melt enhances DDMicrophysical details not that important microphysics can be very importantEntrainment is detrimental mid-level entrainment can enhance

DD, low-level entrainment can be

detrimental

Forcing Mechanisms

Q: What physical processes are responsible for generating strong, low-level downdrafts?

The answer can be found in the vertical momentum equation:

ircop

v

vo

v rrrp

p

c

cg

z

p

dt

wd

1

I II III IV

I – Vertical gradient of perturbation pressure

II – Thermal buoyancy (parcel theory)

III – perturbation pressure buoyancy

IV – Condensate loading of cloud, rain and ice water

Forcing Mechanisms

z

p

1I – Vertical gradient of perturbation pressure

In weakly sheared environments promoting the formation of ordinary cells, the vertical perturbation pressure gradient force tends to be weak

This force becomes more important in more strongly sheared environmentsExample: occlusion downdraft within supercell thunderstorms

Forcing Mechanisms

II – Thermal buoyancy

Well-understood process in convective downdrafts – is the most important forcing mechanism for most convective downdrafts

Created by the evaporation, melting and sublimation of cloud and precipitation particles within a sub saturated parcel of air

In weakly precipitating downdrafts: The downdraft can simply be though as the competing processes of

negative buoyancy generation through condensate phase changes and adiabatic compressional warming

Note the use of the virtual potential temperature Downdraft intensity has been shown to increase within higher relative

humidity environments at low levels by increasing the qv difference

between the sub saturated downdraft parcel and the environment (e.g., Srivastava 1985; Proctor 1989)

vo

vg

Forcing Mechanisms Yes, observational and modeling studies (e.g., Kamburova and Ludlam 1966;

Leary and Houze 1979; Srivastava 1985; Proctor 1989) have shown that the downdraft often descends sub saturated. Cooling due to condensate phase changes does not completely

compensate for adiabatic compressional warming

This may be true even with heavier precipitation events: Byers and Braham (1949) noted “humidity dips” associated with Florida

and Ohio thunderstorm downdrafts

Thus, microphysical details, while not as important for updrafts, appear to be quite important for generating stronger downdrafts:

Numerical calculations (e.g., Kamburova and Ludlam 1966; Srivastava 1985, 87; Proctor 1989) suggest that the maintenance and intensity of a downdraft by falling precipitation is a function of:

Precipitation type (i.e., rain, snow, hail or graupel)

Precipitation size

Precipitation intensity and duration

Forcing Mechanisms

III – Perturbation pressure buoyancy

This term is ignored in Parcel Theory

Has been shown to be relatively weak in comparison to the thermal buoyancy and vertical perturbation pressure gradient terms within convective storms (Schlesinger 1980)

Perturbation pressure buoyancy term has been shown to have appreciable magnitudes where the updraft penetrates the tropopause

op

v

p

p

c

cg

Forcing Mechanisms

IV – Condensate Loading

Long been recognized as an important process for the initiation and maintenance of downdrafts (e.g., Brooks 1922)

Compared to thermal buoyancy, this term is often of secondary importance for downdraft maintenance and intensity (but not always).

It is, however, important for downdraft initiation

irc rrrg

Forcing Mechanisms

Entrainment

Entrainment has long been recognized as an important process affecting the strength of updrafts within convective storms Weakens the updraft by mixing environmental air into buoyant

parcels Largely explains why Parcel Theory over estimates the maximum

vertical velocity expected for a surface-based ascending parcel, i.e.,

CAPEW 2max

For downdrafts, it is generally thought that entrainment of dry environmental air promotes downdraft initiation and maintenance by increased evaporation, melting and sublimation of cloud and precipitation particles within sub saturated downdraft parcels of air.

However………..

Forcing Mechanisms

Entrainment

Numerical simulations by Srivastava (1985) and Proctor (1989) suggest that entrainment can be detrimental to downdraft strength!

Srivastava’s Model configuration:

1-D, time-dependent model of evaporatively driven downdraft

Initial downdraft at top of model domain specified by P, T, RH, W, DSD

Environmental

RH = 70%

From Srivastava (1985)

Forcing Mechanisms

Entrainment

Resolution of these two conflicting ideas may be related to where and when entrainment is occurring:

Entrainment may be beneficial for downdraft initiation and subsequent maintenance say near cloud base.

Entrainment may be detrimental for downdraft maintenance at low levels since the virtual potential temperature difference between the sub saturated negatively buoyant downdraft parcel and the environment will decrease, particularly if the mixing ratio of the environment is larger than that of the downdraft parcel.

Microburst Conceptual Models

Fujita defined a downburst as a strong downdraft which induces an outburst of damaging, highly divergent winds on or near the ground.

The scale of the downburst varies from less than 1 km to 10s of km.

Thus, he subdivided downbursts into macrobursts and microbursts according to their horizontal scale of damaging winds:

Macroburst: A large downburst with its outburst winds extending in excess of 4 km in horizontal dimension. An intense macroburst often causes widespread, tornado-like damage. Damaging winds, lasting 5 to 30 minutes, could be as high as 60 m/s.

Microburst: A small downburst with its outburst, damaging winds extending only 4 km or less. In spite of its small horizontal scale, an intense microburst could induce damaging winds as high at 75 m/s.


The F2 Andrews Air Force Base Microburst on 1 August 1983



One of the earliest conceptual models was put forth by who else…., yes, Fujita (1985). The midair microburst may or may not reach the ground At touchdown, the microburst is characterized by a shaft of strong

downward velocity at its center and strong divergence. Soon thereafter, an outburst of strong, accelerating winds within a rotor

circulation spreads outward. The strongest winds are generally found in the base of the rotor

circulation and can have a significant impact on aviation operations


Microburst Conceptual Models Numerical Simulations of a microburst and associated rotors

Figure from Proctor et al. (1988)

Figure from Orf et al.

(1996)

Microburst Conceptual Models Observations of a microburst and associated rotor

Figure from

Kessinger et al. (1988) Also see Wilson et al. (1984)

Presumably, the rotor is generated through tilting of vertical vorticity and/or baroclinically along the leading edge of the outflow

As the outflow and rotor spreads out, the rotor circulation is enhanced through vortex stretching

Microburst Conceptual Models 3-Dimensional conceptual model of a microburst (Fujita, 1985)

Notice the intense small-scale (< 4 km; misocyclone) rotation associated with the microburst

This rotation is a relatively common feature associated with microbursts

Figure from Fujita (1985)

Some studies suggest the rotation enhances microburst strength (e.g., Rinehart el al. 1995; Fujita 1985; Wakimoto 1985)

Other studies suggest that the rotation weakens the microburst (e.g., Kessinger et al. 1988; Proctor 1989)

Microbursts – Wet and Dry A large number of studies have shown that microburst winds are

associated with a continuum of rain rates, ranging from heavy precipitation from deep cumulonimbi to virga shafts from altocumuli or high-based cumulonimbi.

There is no positive correlation between downburst winds and surface precipitation rates

Accordingly, microbursts are subdivided into wet/high reflectivity and dry/low reflectivity events and are defined as follows (Fujita and Wakimoto 1981; Wilson et al. 1984; Fujita 1985):

Dry/low reflectivity microburst: A microburst associated with < 0.25 mm of rain or a radar echo < 35 dBZ in intensity

Wet/high-reflectivity microburst: A microburst associated with > 0.25 mm of rain or a radar echo > 35 dBZ in intensity

Dry Microbursts - Observations

Photographs taken by B. Waranauskas, from Fujita (1985) of virga and curl of dust associated with the rotor circulation with a dry microburst

Example of altocumuli producing dry microbursts Photograph taken by B. Smith (from Wakimoto 1985)

Produced from innocuous pendent virga shafts from weakly precipitating altocumulus


figure from Hjelmfelt (1988)

Dual-Doppler radar observations of a dry-microburst outflow (also see Wilson et al. 1984)

Figure from

Fujita (1985)

Figure from

Fujita (1985)


Figure from

Wakimoto et al. (1994)

Dry Microbursts - Environment Deep, dry-adiabatic, well-mixed boundary layer.

High cloud bases – 500 mb

Dry sub cloud layer (3-5 g/kg) with mid-level moisture present

Figure from

Wakimoto (1985)

(Also see Krumm 1954; Wilson et al. 1984; McCarthy and Serafin 1984; Fujita 1985; Mahoney and Rodi 1987; Hjelmfelt 1988)

Dry Microbursts - Environment Dry microbursts are largely driven by negative thermal buoyancy created by

the evaporation, melting and sublimation of precipitation

When a deep, dry adiabatic layer is present, only light precipitation is required to generate strong downdrafts…., why?

Based on a figure from Wakimoto (1985)

Compressional warming can not counteract negative buoyancy created by precipitation phase changes

Parcel accelerates to the ground

Note that surface parcel temperature may not be much different than environment, may actually be warmer! (Fujita 85; Srivastava 85; Proctor 89)

Dry Microbursts - Environment With a slightly more stable layer just below cloud base, for example, it may

not possible to generate a strong downdraft.

Thus, deep, dry-adiabatic sub cloud layers are crucial for producing strong dry microbursts

Based on a figure from Wakimoto (1985)

Numerical simulations also suggest that low-level environmental moisture helps produce stronger downdrafts by

increasing the qv difference between the sub saturated parcel and environment (e.g., Srivastava 1985; Proctor 1989)

Dry Microbursts – Microphysical Considerations

In addition to the environmental profiles of temperature and moisture, dry microburst strength has been shown to be a function of:

Precipitation intensity, size, and phase

In particular, sublimation from snowflakes has been shown to very very effective at generating strong dry microbursts (Proctor 1989; Wakimoto 1994). Why?

Numerous low-density snowflakes readily sublimate

Large latent heat due to sublimation

Sublimation cooling (also melting) occurs quickly at relatively high altitudes (Srivastava 1987) – allowing the downdraft parcels to accelerate through a deep dry-adiabatic layer.

Dry Microbursts – Microphysical Considerations

Some visual evidence of the sublimation process was presented by Wakimoto et al. (1994)

Figure from Wakimoto et al. (1994)

Wet Microbursts - Observations Produced by deep cumulonimbus with

warm cloud bases in more humid environments

Figure from Atkins and Wakimoto (1991). Photo taken by K. Knupp

Figure from Fujita (1985) Photo copyrighted and taken by Mike Smith

Wet Microbursts - Observations

Figure from Atkins and Wakimoto (1991).

Figures from Kingsmill and Wakimoto (1991)

Wet Microbursts - Observations

Figure from Atkins and Wakimoto (1991)

Wet Microbursts - Environments Relative to dry microbursts, wet events form in more stable

environments

Accordingly, it is more difficult for negative thermal buoyancy to counteract compressional warming

Thus, more precipitation is required to enhance negative thermal buoyancy production and increase precipitation loading

Figure from Srivastava (1985)

Wet Microbursts - Environments

Notice that for lapse rates > 8.5 ºC km-1 , both wet and dry microbursts are observed to occur

However, when the lapse rate is < 8.0 ºC km-1 , only wet microbursts occur

Virtually no microbursts occur when the lapse rate was less than 7.0 ºC km-1 .

Figure from Wakimoto (2002), based on figure from Srivastava (1985)

Wet Microbursts - Environments

Numerical simulations by Srivastava (1985) and Proctor (1989) are consistent with the observations by Srivastava (1985) that suggest progressively larger amounts of precipitation are required to form microbursts in increasingly more stable environments

Figure from Wakimoto and Bring (1988)

Wet Microbursts – Microphysical Considerations

Similar to dry microbursts, the ice phase has been shown numerically (Srivastava 1987; Proctor 1989) and observationally (Wakimoto and Bringi 1988) to be important

Hail in particular, provides cooling throughout the entire depth of the downdraft extent – very important at low levels below cloud base!

Figure from Proctor (1989)

Wet Microbursts – Microphysical Considerations

Unlike dry microbursts, precipitation loading can be important for the initiation and initial maintenance of the wet microburst at higher levels

Notice that within the wet microburst, parcels can be warmer than the surrounding environment! (also see Wei et al. 1998 and Igau et al. 1999 for tropical downdrafts)

Below cloud base in the dry-adiabatic, well-mixed layer, thermal buoyancy becomes very important

Microburst Detection

Wilson et al. (1984) showed that Doppler radar could detect events at close range. Events during JAWS showed:

Typical downdraft is 1 km wide

Spread out horizontally below a height of 1km AGL

Median time from initial divergence at the surface to maximum differential velocity across microburst is 5 minutes

Height of maximum differential velocity is about 75 m AGL

Median velocity differential was 22 m/s over an average distance of 3.1 km

They are short-lived, low-level, small-scale events.

Microburst Detection

Roberts and Wilson (1989) suggest that the following radar attributes can be used to detect microburst development:

Descending reflectivity cores

Increasing radial convergence within cloud

Rotation

reflectivity notches

These typically appeared 2-6 minutes prior to initial surface outflow

Their results suggest 0-10 minute microburst nowcasts are possible

Microburst Detection - Examples

Descending reflectivity cores

Figure from Wakimoto (2002), original figures from Kingsmill and Wakimoto (1991)


Increasing radial convergence within cloud

Figure from Fujita (1985)


Rotation

Figure from Roberts and Wilson (1989)


Reflectivity notch

Figure from Roberts and Wilson (1989)

Other automatic detection schemes and algorithms are discussed in Dance and Potts (2002)

Figure from Wakimoto (1985), also see Krumm (1954), Beebe (1955) and Caracena et al. (1983)

Microburst Forecasting

When the environmental wind shear is relatively weak, the vertical profile of temperature and moisture can be used to assess microburst potential (Johns and Doswell 1992)

Dry Microbursts:

Deep dry-adiabatic sub-cloud layer to mid levels

Moist mid tropospheric layer, dry low-levels

Marginal updraft instability

Updraft sounding indices can not be used to forecast microburst potential or severity

Figure from Atkins and Wakimoto (1991) Also see Caracena and Maier (1987)

Microburst ForecastingWet Microbursts:

Moist low levels up to 3-5 km, dry mid levels

Dry adiabatic sub-cloud layer 1.5 km deep

Weak capping inversion

Figure from Atkins and Wakimoto (1991)

Microburst Forecastingqe difference from surface to qemin (Dqe) of 20 K or so appears to be a

characteristic of wet microburst producing environment

Dqe values less than 13 K produced thunderstorms, but no wet microbursts

The Cape Canaveral Air Station have developed the MDPI = Dqe/30.

(Wheeler and Roeder 1998). MDPI > is interpreted as high wet microburst probability, issued only when thunderstorm activity is forecast > 60%

Microburst Forecasting While sounding indices for predicting updraft strength work reasonably well,

the same can not be said for predicting peak downdraft strengths with sounding indices:

Downdraft sensitivity to microphysics

Largely sub saturated descent

Nonlinear relationship between maximum downdraft vertical velocity and outflow speeds (it’s not 1:1!!).

That said, previous investigators have developed potential microburst strength indices that can be easily calculated with routinely collected sounding data.

Microburst Forecasting Proctor (1989) put forth the following “wet microburst potential intensity”

index:

5

5.03/)(5.1)1(0

2mvvmm HQkmQHH

I

Where: Hm is the height of the melting level g is the mean lapse rate from the ground to the melting level go = 5.5 ºC/km Qv is the mixing ratio

If g<go, then I < 0 I is larger if:

Hm is large g is large Moist at 1 km and dry at the melting level

Worked well for modeled microbursts, but not for observed events

Microburst Forecasting McCann (1994) modified Proctor’s index in the following way:

Where: WI = Wind Index (WINDEX) Hm is the height of the melting level G is the mean lapse rate from the ground to the melting level QL is the mean mixing ratio of lowest 1km QM is the mixing ratio at the melting level RQ = QL/12 but is set to 1 if QL/12 > 1.

WI is larger if: Hm is large G is large (note G2 dependence) Moist at low levels and dry at the melting level

How well does WINDEX work?

5.02 2305 MLQm QQRHWI


Figure from McCann (1994)

24 August 1993 2000 UTC 2200 UTC

Notice the outflow boundary moving into an area with high WINDEX values

Microburst damage in vicinity of DFW was observed on this day

Microburst forecasting is intimately related to convective initiation forecasting – monitoring low-level convergence boundaries


Recently, Geerts (2001) has modified the WINDEX to account for other processes that help to generate strong wind gusts such as the downward transfer of horizontal momentum:

He created the GUSTEX to include this process:

GU = aWI + 0.5U500

Where a is a constant (he set it to 0.6)

WI = WINDEX

U500 is the 500 hPA wind velocity

For Australian wind gust events, he showed a better correlation between GUSTEX and observed gust speed than with WINDEX and observed gust speed.


Ellrod (1989) and Ellrod et al. (2000) have shown the value of using GOES satellite data form microburst forecasting.

Ellrod et al. (2000) tested the following indices derived from satellite data:

1. WINDEX

2. DMI = G(700-500hPa) + (T-Td)700 – (T-Td)500 (Ellrod and Nelson 1998); DMI > 6 for dry microbursts to occur

3. Dqe

Products are creating hourly and have been shown to provide “information useful in the preparation of short-range weather forecasts and advisories”.

Conclusions First discovered by Fujita in mid 70s while surveying tornado damage

Immediately realized their significance in creating damage at the surface (up to F3) and in impacting aviation operations

No comprehensive microburst climatology exists

Data from field programs suggest they are a relatively common occurrence – summertime phenomena, most common mid-late afternoon

Primary forcing mechanism is negative thermal buoyancy generated by evaporation, melting and sublimation of cloud and precipitation particles

Precipitation loading is also important, particularly with wet microbursts

Microphysics are very important for the downdraft that quite often descends subsaturated

Entrainment can be beneficial or detrimental depending upon where/when it occurs

Microbursts events are associated with a continuum of rain rates and are thus subdivided into “wet” and “dry” events

Conclusions, cont. Dry microbursts occur within deep, dry-adiabatic subcloud layers and

originate from innocuous virga shafts associated with altocumulus

Formed from negative thermal buoyancy – ice phase is important!

Wet microburst occur within more stable, humid environments and originate from deep cumulonimbus

Formed from negative thermal buoyancy and precip loading – again, ice phase is important!

Detection is challenging, they are short lived, low-level, small-scale in nature

There are useful radar attributes that can detect their occurrence 2-6 minutes before damaging winds are observed at the surface

In weakly sheared environments, soundings can be used to forecast their occurrence.

Downburst indices are problematic, though recent studies have shown they are of some utility in predicting downburst potential and intensity

Documents

MICROBURSTS Nolan T. Atkins Lyndon State College Prepared for COMET Mesoscale Analysis and Prediction Course 2002 (COMAP 2002) 13 June 2002