Convective Weather Maps

8/12/2019 Convective Weather Maps

1/29

Guide to using

Convective Weather Maps

Oscar van der Velde

www.lightningwizard.com

last modified: August 27th, 2007

Reproduction of this document or parts of it is allowed with permission. his document ma! "e updated at an! time.

#icture ta$en %a! &'th2007, &70( )*, in southwesterl! direction from oulouse, +rance
http://www.lightningwizard.com/http://www.lightningwizard.com/


2/29

Introduction

have finall! written some e-planation for !ou a"out the parameters plotted in the

maps and how to use them. he maps are presented on

http:www.lightningwizard.commapsandhttp:lightningwizard.estofe-.org, thelatter is the server hosting the maps.

%an! parameters are "ased on concepts of /parcel theor!/ which descri"es what

happens to a parcel1 of air when "rought to a different pressure, relative to their new

surroundings. %! purpose is not to e-plain all details of ph!sical meteorolog! !ou will

find it in an! meteorolog! te-t"oo$ and also in the ver! good %et3d modules on the

we"4, "ut how to appl! the parameters plotted in the *onvective 5eather %aps to

forecasting.

started pla!ing around with 6rA8 in +e"ruar! 2002 with data from the AV9 modelfrom the 9ational *enter for 3nvironmental #rediction 9*3#4 which had a grid

spacing of &-& degree. 9ow the maps are run from the 6+8 model at 0. degree grid.

5hile have alwa!s "een interested in forecasting thunderstorms, almost no model

output was availa"le for 3urope with useful parameters for this purpose, indicating

different measures of insta"ilit!, vertical wind shear, or low;level convergence. t is

ver! important to have enough parameters availa"le to construct a conceptual image of

the timing and t!pe of thunderstorms that are thought to occur. his is wh! the 8torm

#rediction *enter in the )nited 8tates has a wide variet! of parameters availa"le for

this purpose see their %eso;anal!sis section4. %an! parameters were tested on

soundings e.g. pu"lications of Rasmussen,


3/29

severe weather ma! result without warning.

hope this document will inspire ever!one to ta$e a deeper loo$ at the ph!sics of

severe storms, their forecasting, how to recognize them on radar, and what to do with

this information for !our purpose the latter two are "e!ond the scope of this

document4.

he three "asic ingredients for severe deep convection are insta"ilit!, lift, and vertical

wind shear. n man! situations, without a good source of lift e.g. a front, trough, sea

"reeze convergence, dr!line, forced flow over mountains4 a parcel which has

conditional insta"ilit! will have trou"le to rise out of the "oundar! la!er and form a

persisting convective storm. 5ithout vertical shear change of direction and speed of

the horizontal wind with height4 a storm will have trou"le to live longer than a"out >

minutes and die, instead of organizing itself into a cluster or %*8, or develop a rotating

updraft, with all conse?uences for severe weather potential.

Note: references will be added later.For a quick and entertaining crash-course in operational severe storm meteorology, highly

recommend !et"d: http:##www.meted.ucar.edu#topics$convective.php
http://www.meted.ucar.edu/topics_convective.phphttp://www.meted.ucar.edu/topics_convective.php


4/29

The Maps

he e-amples "elow are from &0 @ul! 2007, for &. his da! featured a cold front over

eastern 3urope along which severe thunderstorms developed, with a tornado at &(>

)* in southwestern Romania, large hail over central Romania in the evening. %oreevents li$el! happened along the front, "ut have not reached the 385. 8upercells

were visi"le also on 8er"ian radar. A t!pical spout situation was present over

9etherlands into 6erman! with several complete spouts and funnels having "een

reported during late morning and afternoon, and a few cases of up to 2 cm hail over

6erman! later towardsin evening. At the time of the satellite image, the storms over

western )$raine from the earl! afternoon have produced a large cirrus shield alread!.

he purpose here is onl! to show e-amples of the parameter fields, not to discuss the

case in detail refer to the forecast archive of the 3uropean 8torm +orecast 3-periment,

www.estofe-.org4. A different case ma! "e selected in a later version of the document.
http://www.estofex.org/http://www.estofex.org/


5/29


6/29

1. MLCAPE (and MSL Pressure, !! hPa Geopotentia" #eights$

his map offers a usual view of common heights for a ?uic$ overview of pressure

s!stems, with the addition of mi-ed;la!er *A#3 *onvectivel! Availa"le #otential

3nerg!4.

*A#3 is the potential energ! a parcel has when it is lifted to its level of free convection

and "ecomes warmer than its surroundings, e-periencing upward "uo!anc!. he

potential energ! can "e converted to $inetic energ! reflected in upward motion. An

vertical speed could in principle "e calculated from this, "ut parcel theor! is not perfect

and does not account for things li$e precipitation drag or d!namic pressure

contri"utions of vertical shear. Bowever, higher *A#3 t!picall! involves stronger

storms with a higher chance of large hail and other severe weather. hat said, note that

*A#3 is usuall! of lesser importance than the vertical shear environment for tornadoes,while the pro"a"ilit! of large hail increases with *A#3, given at least moderate shear

values around 00;&000 @$g are sufficient4.

*ontri"utors to *A#3 are steep temperature lapse rates from low to mid levels and a

warm and humid "oundar! la!er. he colder the mid levels are compared to the parcel,

and the higher the parcel e-periences upward "uo!anc! high e?uili"rium level4, the

larger *A#3 in general. Bowever, warm, dr! la!ers at low levels ma! function as a cap


7/29

that prevent "oundar! la!er parcels from reaching the level of free convection, and ma!

prevent storms from developing see C+*;C*C map4.

he *A#3 used in these maps is calculated for a parcel with mi-ing ratio and potential

temperature averaged from the 0;& $m la!er, "ecause it reflects the process of mi-ing in

the "oundar! la!er. 9ote that the pro"lem of 6+8 overestimating low level dewpoints

and hence *A#34 in conditions of wea$ winds and strong insolation in the summer

half !ear is somewhat mitigated "! not including the 2;meter level in the calculation.

+inall!, "e aware that *A#3 is ver! sensitive to small differences in the moisture and

temperature profiles, as well as the calculation and used parcel. t is therefore fairl!

useless to spea$ for e-ample of /D @$g *A#3/ or even /E00 @$g/. f the maps indicate

&000 @$g *A#3, "e prepared to find in soundings mostl! 00;&00 @$g, a wide margin

of at least 0F.

%. &'ega Advection o) *!! hPa Geostrophic +orticit - the !!/0!! hPa Ther'a"Wind +ectors, *!! hPa #eight

his map uses the ren"erth method for estimating the resulting vertical motion

induced "! differential vorticit! advection and temperature advection, giving a


8/29

?ualitative picture of geostrophic vertical motions. t is different from model output

vertical motions which are influenced also "! convection itself4. )se to get a sense of

large scale lift and su"sidence. *ellular convection over sea often is a"le to maintain

itself even under su"sidence, if low level lapse rates are strong enough, "ut comma

clouds for e-ample would need geostrophic lift usuall! a vorticit! ma-imum4. t is

recommended to dou"le;chec$ the e-istence of ascentdescent with other geostrophic

vertical motion parameters for critical use.

0. Mid/tropospheric Potentia" +orticit (!!/*!! hPa$

)sed to highlight atmospheric processes in a different wa!. #V is a conserved ?uantit!for adia"atic processes, e?uivalent to momentum. t can "e used to trace airmasses. he

tropopause is usuall! associated with 2 #V units, with lower #V "elow. 8trong vertical

motions can stir up the tropopause, such that high #V air enters the troposphere and is

"rought downwards. he presence of a strong #V anomal! in midlevels or lower

indicates either strong postfrontal su"sidence or a "u""le of mid;level cold air with

steep lapse rates and high vorticit!. wonGt go deepl! into #V theor!, "ut for practical


9/29

use: strong upward motions can "e e-pected ahead of a #V ma-imum, in other words,

mid;level lapse rates will steepen and mid;level vorticit! generating upward motion in

the direction a #V ma-imum moves. 3speciall! the dar$ "lue and "e!ond needs

attention. he patterns seen in this map often correspond with the dar$ "ands in

satellite water vapour images intrusions of dr! air4.

. Tho'pson inde2, Convective Precipitation, 3!! hPa #eight

he hompson thunderstorm inde- is an ancient inde-, from the times that ever!

calculation had to "e done "e hand from soundings. he inde- consists of = nde-

minus the Cifted nde-. he latter is simpl! the difference "etween the temperature aparcel has at 00 h#a and its surrounding air, so a result of parcel theor!. he = nde- is

D0 H dD0 ; 700 ; d7004 ;00 and thus a sum with no meaning, including a lapse

rate, a low level dewpoint and a mid level relative humidit!. he fi-ed levels ma$e this

ph!sicall! mean something different on high plains than at sea level.


10/29

serves to indirectl! include a source of lift, as usuall! it is more humid in the mid levels

around fronts. strongl! suggest using parcel theor! parameters though.

5hat to use the map for is mainl! a view of standard output 6+8 convective

precipitation... which actuall! is often fairl! relia"le although it ma! overreact in case

the evapo;transpiration pro"lem wea$ wind, strong insolation4 shows up. t ma!

underestimate potential for storms in areas with deep dr! "oundar! la!ers.

. E4ui"i-riu' Leve" Te'perature ('ost unsta-"e parce"$

A ver! useful map in cases of near;neutral environments of ver! low *A#3, read:

mostl! in the winter half !ear. *onvective cells need to have updrafts reachingsufficientl! into the mi-ed;phase temperature region usuall! ;&0 to ;'0 degrees

*elsius4, where ice particles in the cloud co;e-ist with li?uid water droplets, in order for

the non;inductive charging process to "e effective. he e?uili"rium level is where the

parcel will "e at the same temperature as the environment after its free convection. t

will e-perience an increasingl! negative "uo!anc! force as it ascends further and will

slow down. his often corresponds with levels near the tropopause, "ut ma! also "e an


11/29

inversion lower in the troposhere. he map indicates the temperature, not the height.

hunder ma! "e possi"le with 3C temperatures lower than ;&0 degrees, and "ecomes

li$el! especiall! "e!ond ;'0 degrees. n winter time the corresponding heights of the

cloud tops is lower and the moisture content lower as well, with wea$er updrafts so the

electrification process is less effective.

n the summer over large areas parcels can reach ver! cold temperatures and other

indicators ma! "e more useful to loo$ at. Bowever, for the calculation the /"est la!er/ is

used i.e. the level with the highest theta;e parcel "elow (00 h#a4, and this map is useful

in identif!ing elevated convection when %C parcel methods do not show potential.

Attention: there is currentl! no map to chec$ the C+* of an elevated parcel.

*. Li)ting Condensation Leve", L5C/LCL di))erence

he height of the C*C of a 0;& $m mi-ed parcel is plotted as "ac$ground. his C*C is

similar to the *onvective *ondensation Cevel, i.e. the cloud "ase height that

cumuliform clouds ma! have. t relates strongl! to the relative humidit! of the

"oundar! la!er, so ver! low heights ma! associate with low clouds or fog during the


12/29

night and in "ad cases persist during the da! and "loc$ solar heating re?uired for

storms4.

Bigh C*C heights can enhance down"urst winds "ecause the downdraft air will "e

colder relative to the surrounding air, the negative "uo!canc! accelerating downward

speeds. Bigh C*Cs I2000 m4 ma! also indicate more difficult! for "eginning

convection to sustain itself, due to entrainment in the dr! environment. Cow C*C

heights under &000 meters4 are favoura"le for tornadoes, as was found "! 8#*, the

reasons of which have not "een full! e-plained, "ut involve downdraft;updraft

"uo!anc! processes.

he C+* Cevel of +ree *onvection4 is the level "elow which a 0;& $m mi-ed parcel

when lifted is colder than its environment, and normall! wants to return to where it

came from. A ver! strong source of low;level lift ma! push a parcel to the C+*, so that

it "ecomes warmer lighter4 than the surrounding air and e-perience an upward force.

%ore common is that the capping warm la!er is adia"aticall! lifted and removed, or

that heating and mi-ing from "elow will !ield a higher C*C and a lower C+* theconvective temperature concept4.

n the form of vectors, the difference "etween the cloud "ase and the Cevel of +ree

*onvection is drawn. 9o vector means no %C*A#3 present. 8mall vectors indicate

small C+*;C*C differences, so that there is almost no e-tra heating or forcing re?uired

for initiation of convection. Conger vectors re?uire more, and thic$ vectors ma! indicate

too much capping inhi"iting the formation of thunderstorms. Along the dr!line in the

)8A 6reat #lains, the gradient ma! "e so steep that onl! a few points with small C+*;

C*C are visi"le on the grid points of the model. At night, the C+*;C*C difference ma!

increase again, "ut usuall! alread! developed storms will persist for some time,depending on moisture and storm;relative inflow a"ove the "oundar! la!er. n general,

the lower the C+*;C*C difference, the easier less forcing re?uired4 and earlier storms

develop. he same goes for lower C*Cs "ecause entrainment is less of a pro"lem.

9ote that "ecause the model adJusts its environment wea$ens lapse rates, lowers C*C4

as result of convection, the C+*;C*C difference ma! "ecome larger and ma! give a

counter;intuitive KcappedL impression where there is alread! convection. *hec$ this "!

loo$ing at the convective precipitation map.


13/29

3. !/0 6' MLCAPE, Spout inde2

0;' $m %C*A#3 low;level *A#34 uses the 0;& $m mi-ed la!er parcel, "ut represents

the %C*A#3 present not all the wa! to the 3C, "ut onl! in the lowest three $ilometers

a"ove the surface. his indicates whether a parcel is a"le to accelerate rapidl! a"ove the

C+*. A low C+* and temperatures dropping rapidl! with height in the 0;' $m la!er

ma$e for a upward acceleration in this la!er, which is important especiall! for

tornadogenesis. he t!pe of generall! wea$ tornadoes +0;+&4 $nown as GspoutsG

landspouts, waterspouts4 happen "! stretching of vorticit! with a vertical a-is into an

updraft. his process is enhanced "! vertical acceleration the same mechanism as the

whirl when draining water from a "athtu"4. #rere?uisite is a source of vertical vorticit!and convergence, such as wind shift lines. n addition it seems important that low;level

winds are not too strong, otherwise tur"ulence ma! distur" this process. 8teep near;

surface lapse rates will also help ne-t map4. %idupper level cold pools and wea$

troughs are notorious for out"rea$s of spouts. he green e-perimental composite inde-

incorporates these factors, "ut it is not cali"rated or tested, and ma! not alwa!s "e

useful.


14/29

8imilarl!, tornadoes can "e generated "! tilting of low;level vorticit! with a horizontal

a-is strong low;level shear4 into the vertical "! a strong updraft and ma! also profit

from stronger 0;' $m *A#3.

7. Te'perature Lapse 8ate !/!! ' AGL

he temperature difference "etween the surface not 2 meters4 and 00 m a"ove

ground. his map contains useful information a"out the relative temperature of the air

compared to the surface over which it flows. he dr!;adia"atic lapse rate is a"out &0;&&

=elvin decrease per $ilometer, while ;( degrees decrease per $ilometer is moist;

adia"atic. Cower values indicate inversions. Values higher than && =$m indicatesuperadia"atic conditions which necessaril! impl! tur"ulent mi-ing as surface parcels

have alread! positive "uo!anc! with the minimal lift. his is favoura"le for vertical

vorte- stretching such as dust devils and spouts.

One ma! often easil! infer which process is responsi"le for steep or inverted lapse rates.

Carge "odies of water do not change temperature ver! ?uic$l!, so ver! steep lapse rates

will mostl! "e the result of advection of relativel! cold air over the surface. 8imilarl!,


15/29

inverted lapse rates indicate strong warm air advection over the water surface. Cand, on

the other hand, responds ?uic$l! to radiative processes. *ontrasts "etween land and

adJacent water surfaces ma! induce mesoscale circulations li$e landsea "reeze. Capse

rates increase ?uic$l! during the afternoon when the sun shines, while in the evening a

ground inversion forms. his ma$es it possi"le to evaluate if the model produces

cloudiness that ma! inhi"it heating of the "oundar! la!er during the da!, or reflect of

long wave radiation to earth at night I> =$m over land4, ma$ing it a useful map if also

if !ou need to $now possi"ilities for clear s$ies at night for astronomical o"servations or

sprites.

. Te'perature Lapse 8ate %!!!/!!! ' AGL

his somewhat ar"itraril! chosen la!er for mid;level lapse rates is often used to identif!

an important contri"utor to *A#3, independentl! of moisture availa"ilit!. n maritime

polar airmasses "ehind cold fronts it generall! indicates values over ( =$m. %ore

e?uatorward it often is capa"le of defining the edge of deep convection rather well,

where su"sidence esta"lishes an inversion. 8hallow convection ma! still occur in these


16/29

regions.

3levated and dr! regions such as the 8panish #lateau and the 8ahara often create a

deep dr! la!er with steep lapse rates, that can "e advected awa! into western 3urope

e.g. 8panish #lume4. On the 6reat #lains in the )8A, ver! steep lapse rates can "e seen

developing over the Roc$! mountains and western Bigh #lains and "eing transported

eastward over a ver! moist airmass, creating Gloaded gunG soundings. Ver! steep lapse

rates I7 =$m4 in this la!er in warm airmasses are capa"le to create a GfatG *A#3,

allowing for rapid upward acceleration, and is often associated with large hail and

more indirectl! with severe down"urst winds. 9eutral lapse rates ;( =$m4 indicate

less e-citing conditions, often found in saturated frontal regions. 9ote that 2000 and

>000 m temperatures are advected "! different winds, so the lapse rate itself does not

alwa!s advect nicel! and can Just pop up and disappear out of nowhere.


17/29

1!. 3!! hPa Theta/e, Strea'"ines (convergence and divergence$

11. !/1 6' Theta/e, 1! ' Strea'"ines (convergence and divergence$


18/29

heta;e is the 3?uivalent #otential emperature. t is determined on a 8$ew; diagram

"! lifting a parcel to its C*C, then removing adia"aticall! all moisture from it "!

following the moist;adia"at upward and read its potential temperature at &000 h#a via

the dr!;adia"at. Actuall! it is e?uivalent to the 5et;"ul" #otential emperature theta;

w or 5


19/29

1%. !/1 6' average Mi2ing 8atio, !/1 6' average 9ind strea'"ines ('oisture

advection$

%i-ing ratio is another word for a"solute moisture content and is e-pressed in grams of

water vapour per $ilogram of dr! air. A directl! related parameter is the dewpoint

temperature. Bowever a dewpoint temperature cannot "e mi-ed verticall!. %i-ing

ratio is conserved for vertical motions until condensation occurs. his parameter is

easil! compared with o"served soundings "! ta$ing the average over the lowest

$ilometer on a 8$ew; diagram, useful to see if the model is on trac$ with its moisture

predictions, after all it is the source of the *A#3 calculations. he streamlines show

colours that tell where there is advection of moister or drier air, stressing gradients thatare advected perpendicular to the wind.

his map displa!s the dr!line in the )nited 8tates much "etter than heta;e.


20/29

10. :e"ta Theta/E, Convective Gust, Co"d Poo" Strength (T%' /Tdo9ndra)t$

he parameters in this map are somewhat e-perimental. elta;theta;e thic$ lines, if

present4 is the difference "etween the "oundar! la!er theta;e the moist adia"at used

for *A#34 and the lowest theta;e found in the mid levels under >00 h#a4. he drier and

colder the mid levels, and the more warmermore unsta"le the "oundar! la!er parcel,

the stronger updrafts and downdrafts and hence the chance of severe convective gusts.

3ven micro"ursts e-treme local down"ursts4 are possi"le especiall! with values a"ove

20 = At$ins and 5a$imoto, &EE&4.

he convective gust speed in shaded colours is simpl! the pressure;weighted average

of surface to 700 h#a winds, and is intended to give an indication what to e-pect whena downdraft digs down through a la!er of high winds, "ringing the momentum down

to the surface. t ma! alread! "e ver! wind!, "ut normall! over land the ratio "etween

gust speed and &0 minute average winds does not e-ceed &.7 or so &.> over sea4 with

some margin. A gust significantl! enhanced "! deep convection could well !ield higher

gust factors one can often use 8M9O# or %3AR to determine this4.

*old #ool 8trength is a parameter that ta$es the lowest theta;e from the mid;levels and


21/29

"rings it to the surface, where it is compared to the 2m temperature. One ma! interpret

this as the worst temperature drop that can "e e-perienced from thunderstorm outflow

if the model did not miss colder theta;e levels4. n practice this ma! often "e less

dramatic. #h!sicall! it corresponds with the negative "uo!anc! of the downdraft into

the "oundar! la!er. A relativel! cold downdraft will propagate awa! from the

thunderstorm with a higher storm;relative speed stronger gusts4. t ma! re?uire strong

low level storm;relative winds to prevent the storm from "eing cut off from its moisture

source. Values higher than &0 degrees are a good signal for strong gusts. Cow values

indicate an almost neutral profile. At night and when convection has alread! produced

precipitation in the model this parameter ma! not "e representative.

1. !/* 6' Shear, !/1 6' Shear, Signi)icant Tornado Para'eter

ispla!ed in $nots ma! change this to ms4, the length of the vector difference "ul$

vertical shear vector4 of the winds at ( $m and & $m a"ove ground level with the &0 m

wind. hese are often called Gdeep la!er shearG and Glow level shearG, respectivel!. he

chosen levels originate from those included in American studies, and their relation to


22/29

severe weather is well documented. he wa! these are plotted reflects the commonl!

cited GthresholdG levels, although there is some margin. eep la!er shear around 20 $ts

&0 ms, wea$ to moderate4 is often sufficient to sustain redevelopment of new cells at

outflow "oundaries ne-t to older cells, and support multicell storms and mesoscale

convective s!stems %*84, the latter especiall! when sufficient d!namic forcing is

present. %ore shear will cause a gradual transition from discrete stepwise4 renewing

cell growth to more stead!;state storms, with the downdraft less interfering with the

updraft so that cells can live longer. '0 $ts & ms4 or more will usuall! lead to prett!

well organised storms with wea$l! supercellular characteristics, and capa"le of

producing large hail. )suall! >0 $ts 20 ms4 is ta$en as threshold value for supercells,

meaning that the storm is a"le to develop and sustain a rotating updraft. 8upercells are

ver! capa"le of producing large hail I2 cm4, severe downdrafts and tornadoes.

6enerall!, the product of *A#3 and 0;( $m shear correlates well with increasing

pro"a"ilit! of the full spectrum of severe weather from thunderstorms.

Cow level shear over 20;2 $ts &0;& ms4 is favoura"le for tornadogenesis, as itrepresents horizontal vorticit! that can "e tilted into the vertical "! strong updrafts.

Additionall!, an %*8 in a high 0;& $m shear environment ma! tend to produce "owing

segments which are capa"le of causing concentrated damaging winds.

8ignificant ornado #arameter is a composite inde- "ased on deep la!er and low level

shear, *A#3, *9 and C*C height. t highlights regions where these ingredients for

tornadoes come together most, although it does not tell which necessar! ingredient ma!

"e lac$ing most. *omposite indices cannot replace a detailed anal!sis, "ut serve well as

an alert to the forecaster.


23/29

1. !/0 6' Stor'/re"ative Environ'enta" #e"icit, Superce"" Co'posite Para'eter,

;un6ers Stor' Motion

n addition to good 0;( $m shear, it is favoura"le for development of rotating updrafts

to have a se?uence of wind shear vectors over small la!ers turning cloc$wise with

height. his results in a curved hodograph, the line that connects the arrow heads of the

wind vectors of a vertical profile when presented in a horizontal plane. *urved

hodographs are also possi"le with at!pical vertical wind profiles, so it is much easier to

see this in hodograph plots than from the wind "ar"s adJacent to a sounding.

n a Cagrangian sense of motion, a storm is affected "! its surrounding winds. Cow

level storm;relative winds are ingested into the updraft. 3ach small la!er of verticalshear "ears horizontal vorticit!, which is ingested and tilted into the vertical, increasing

the total rotation of an updraft. he surface on a hodograph diagram swapped out

"etween the hodograph line connecting the 0;' $m winds and the storm motion vector

is e?uivalent to the rotation that is gained. t will follow that a storm motion following

the hodograph line will not gain much rotation, "ut a deviant motion to the right of the

hodograph ma!. +or a more complete discussion refer to the %et3d module. n


24/29

practice, a straight, sufficientl! long hodograph e.g. >0 $ts 0;( $m shear4 ma! produce

"oth left;moving and right;moving supercell storms as the downdraft ma! force cells

to o"tain a deviant motion: split cells4, while a cloc$wise;curved hodograph favours

right;moving supercell storms. he updraft is forced "! non;h!drostatic vertical

pressure gradient forces to occur to the warm side of the hodograph.

8upercells often are a"le to develop when 0;' $m 8R3B is greater than &0 m2s2, while

also the chance for tornadoes increases with larger 8R3B. he 8upercell *omposite

#arameter consists of 8R3B,


25/29

1*. Stor'/re"ative Moisture 5"o9 and Mid


26/29

wea$er midupper flow4. he vectors point in the direction the "ul$ of the anvil would

"low to.

n some occasions, 6+8 shows strongl! diverging upper 8R wind vectors. his is a good

signal the model has produced a large convective s!stem with mesoscale updrafts

confirm with convective precipitation4.

+inall!, this map can "e used as !et another alternative wa! to determine the presence

of deep insta"ilit!, "ecause it is onl! plotted where Cifted nde- is smaller than 2.

13. 1/7 6' Shear, ICAPE and ICI>

his map is convenient to Judge where shear crosses areas of insta"ilit!. he &;D $m

"ul$ shear vector is another version of deep la!er shear, "ut e-cludes the 0;& $m la!er.

his can "e of use "esides the 0;(0;& $m shear map, especiall! in cases when the

hodograph is straight and 0;& $m shear strong and thus part of the 0;( $m shear. t then

ma$es sense to loo$ what amount of shear is availa"le a"ove & $m. he D $m level is

found to "e of more value than ( $m "!


27/29

long; versus short;lived supercells.

*A#3 stands for ntegrated *A#3, and has the units @m2, not @$g. t was first defined

"! %apes &EE'4 as the sum of *A#3Ndpg for all parcels in a column which have

*A#3I0. his ma$es it independent of the choice of parcel. nstead, a deeper la!er of

parcels that have *A#3 gives higher values for the same *A#3 than if onl! a shallow

la!er has *A#3. +or e-ample, a &00 h#a thic$ la!er giving 00 @$g *A#3 will result in

the same *A#3 value as a shallower 0 h#a la!er of &000 @$g a"out 00 $@m 24. he

parameter is plotted e-perimentall!, as its advantage over other versions of *A#3 in

operational meteorolog! has never "een tested.

t ma$es sense that if as a storm develops, all air from low levels will "e ta$en into the

storm, and the total energ! released "! all parcels in a column of one s?uare meter

diameter is *A#3. n practice, the map will loo$ ver! similar to %C*A#3, e-cept when

parcel la!er thic$ness differs over the area, or when elevated parcels over a sta"le

"oundar! la!er have *A#3, where %C*A#3 ma! "e a"sent. 8o, this parameter has

characteristics of "oth %C*A#3 and %)*A#3.8imilarl!, *9 is the integrated negative "uo!anc! counterpart, a sum of all *9 of all

parcels in a column that have positive *A#3. his is topped in this implementation at

(00 h#a and parcels to 700 h#a due for computing resource reasons4. n the map, the

smallest vectors indicate ver! small *9 sums, while larger and thic$er vectors impl!

higher *9.


28/29

17. =ncapped Laer :epth, !/% 6' :eep Convergence, 1/ 6' Shear +ectors

Related to *A#3 in the previous map, )ncapped Ca!er epth is an integrated

parameter that shows how deep of a la!er in a column through the lower troposphere

contain parcels with *A#3 greater than 0 @$g and *9 less than 0 @$g. haven%t seen

this parameter mentioned or used before, so please refer to this document and contact me if you

intend to include it in a study.

n other words, it shows the integrated depth of all uncapped parcels. Cow values

"lue4 indicate that onl! a shallow la!er is capa"le of releasing *A#3 to form a

thunderstorm, whereas high values red4 indicate *A#3 can "e released from an! level

in the lowest ';> $ilometers. he pro"a"ilit! of thunderstorms indeed appears toincrease with )ncapped Ca!er epth. Also the persistence of storms after nightfall,

when *A#3 "ecomes more elevated, can "e forecast "etter with this parameter than an!

other have seen so far. he position and coverage of storms is generall! ver! well

indicated "! this parameter, it wor$s in m! e-perience "etter than either *A#3 or 6+8

convective precipitation rate.

eep convergence is a useful addition to the map lineup. t shows regions of mesoscale


29/29

ascent, which are sometimes more and sometimes less esta"lished than convergence of

the &0 meter wind. t occasionall! shows e-panding rings of deep convergence when

6+8 "lows itself up into a "ig convective s!stem. Regions of convergence, s!noptic scale

lift and )ncapped Ca!er epth are good indicators for storm development.

&;> $m shear vectors are supplied ?ualitativel!, onl! when larger than 2. ms per

vertical $ilometer, the e?uivalent of & ms shear over 0;( $m, often used as

appro-imate minimum threshold for the more severe multicell and supercell storms.

Oscar van der Velde, 2007

http:www.lightningwizard.com

http:lightningwizard.estofe-.org
http://www.lightningwizard.com/http://lightningwizard.estofex.org/http://www.lightningwizard.com/http://lightningwizard.estofex.org/

Documents

Convective Weather Maps