Severe convective storms, theory Pieter Groenemeijer FMI Helsinki, 2 May 2005.

severe convective storms, theory

Pieter Groenemeijer

FMI

Helsinki, 2 May 2005

“one-slide introduction” of myself

I am Pieter Groenemeijer

• M.Sc. in Physics and Astronomy atUtrecht University

• Oklahoma University (spring semester 2002)

• ESWD (European Severe Weather Database)

• “Sounding-derived parameters associated with large hail and tornadoes in the Netherlands“

• Co-initiator of ESTOFEX (with Johannes Dahl and Christoph Gatzen), Oct, 2002.

my contribution this morning

1. Ingredients-based forecasting- instability- lift

2. Storm structure- wind shear: multicells and supercells- other factors: linear convective systems

_________________________________________ (short break)

• Convection parameters• Severe weather hazards

- a study in Holland

5. A case

Questions, discussion

what will we discuss?

severe convective storms:

storms that produce hazardous weather like:

• lightning• heavy rain (leading to flash floods)• strong winds (straight-line winds)• large hail• tornadoes

ingredients-based forecasting (Doswell, 2004)

• What is“ingredients-based forecasting”?

an “ingredient” is something necessary for some event to occur

I will cover the theory by exploring those

ingredients

ingredients for convective storms

1. latent instability

2. lift (rising motion)

instability

• lapse rate definition: dT/dz > 1.0 C/km in dry air

or: dT/dz > moist adiabatic lapse rate in saturated air

these are the definitions of

absolute and conditional instability

instability

• layer definition:when lifting a layer, saturation occurs and

dT/dz becomes > moist adiabatic lapse rate

Or equivalently: theta-e (and theta-w) decrease with height

potential instability

instability

a convective bubble is more like a parcel than a layer...

• parcel definition:parcel becomes warmer than environment after lift

latent instability (Normand, 1937)

several “convective parameters” are based on the concept of latent instability:

• CAPE (in all its forms)• LI (Lifted Index)• Showalter Index

instability

parcel theory

parcel theory

EL

level source

d'

g zT

TCIN

v

v

EL

LFC

d'

g zT

TCAPE

v

v

limitations of parcel theory

Realize that parcel theory is a simplification of reality:

• what in reality is a parcel? is it undiluted?

• and its environment? is it not influenced by convection?

objection:

We neglect pressure perturbation forces!(come back to that later)

lift

latent instability ≠ storms

• a “cap”, CIN may be present, or• entrainment may inhibit the development of

convective storms

lift • can weaken the “cap”, or • is associated with convergence at the surface:

- helps to sustain initiating convective bubbles

lift and convective inhibition

entrainment

we have identified...

two ingredients for convective storms...

• latent instability• (sufficient) lift

okay... but when should we become worried about extreme events?

are other ingredients required?

storm structures / convective modes

• some severe events are associated with particular storm structures (or convective modes)

multicell line multicell clusters isolated supercell

EXAMPLES from my home country

storm structures / convective modes

• some severe events are associated with particular storm structures (or convective modes), others are not, e.g.:

- strong tornadoes are known to occur mostly with supercell storms

- extreme rainfall and lightning can occur with any storm structure, but generally...

anticipating storm structure is very important to predict the quantity and quality of the severe weather that may occur

factors influencing storm structures

1. vertical wind shear

2. other factors

vertical wind shear

• vertical wind shear has a strong influence on convective organisation

it affects • storm propagation• vertical speeds in up- and downdrafts• storm longevity

storm in weak vertical shear

weak shear:single-cell storms

1. updraft grows2. precipitation forms3. cold pool forms

and spreads out4. updraft ceases5. storm ceases

reality

a gust front made visible by blowing dust and sand

new cells form at the edge of the cold pool....

storm in moderate vertical shear

moderate shear: multicell storms

1. updraft grows

2. precipitation forms

3. cold pool forms and spreads out >>>>>

4. updraft ceases

5. storm ceases

time

1. new updraft grows


3. cold pool forms and spreads out >>>>>

4. updraft ceases

5. storm ceases

1. new updraft grows


3. cold pool grows and spreads out

4. updraft ceases

5. storm ceases

new cells form at the edge of the cold pool....

RKW-theory

from Rotunno, Klemp and Wilhelmson, 1988

when horizontal vorticity produced by the cold pool

and that of the environments are roughly equal

the strongest lift will occur

RKW-theory

from Xue et al., 1997

no vertical wind shear

RKW-theory

from Xue et al., 1997

low-level vertical wind shear

RKW-theory

RKW-theory is not undisputed...

it seems to work better in the laboratory than in reality


multicell cluster

the cells may not be distinguished by a radar scanning at a low elevation....


multicell line:

squall line

watch the cells forming at the front of the system that move backward w.r.t. the system

storm in strong vertical shear

strong shear:supercell storms

supercell

definition:a supercell is a storm with a persistent, deeprotating updraft (that is, a mesocyclone)

a few characteristics:

• very strong updrafts• often: very strong downdrafts...resulting in a high potential for severe weather

• don’t move with the mean wind

hodographs

storm-relative helicity

vertical shear

implies horizontal

vorticity

storm-relative helicity

zSRH avg d ωcv storm-relative helicity

(e.g. Davies, 1985;Droegemeier et al., 1993)

hodographs

right-moving supercell

left-moving supercell

hodographs

LP supercell near Waynoka, OKApril 17th 2002 Tornado Team Utrecht

Mesocyclone near Selby SD June 8th 2002 Tornado Team Utrecht

supercells on (Doppler) radar


three ingredients for the most severe convective storms...

• latent instability• (sufficient) lift• vertical wind shear




note that I didn’t say that CAPE should by higher than some threshold. Storms have caused F4 tornadoes with only a few 100’s of J/kg available!

other factors than wind shear that influence storm structure...

It is hard to predict if and how quickly storms will cluster into a linear MCS.-MCS’s often form when cold pools formed by multiple storms merge

Factors favoring clustering into an MCS:-strong lift

(e.g. caused by an intense shortwave trough, frontal wave)

-convective initiation along a boundary-weak cap (low CIN)

bow echoes

Convective systems may develop into bow echoes.

Amsterdam

Rotterdam

The Hague

Image made at KNMI

bow echoes

Image made at KNMI

Amsterdam

Rotterdam

The Hague

1639 UTC

bow echoes

Image made at KNMI

17 July 2004 - Image by Patrick Weegink

ingredients-based forecasting (Doswell, 2004)

an “ingredient” is something necessary for some event to occur

• helps with information overload• helps prevent overlooking important factors• prevents “tunnel-vision”




certain parameters may help to quantify those

convective parameters

but, beware....


Total totals index (TOTL) = T850 + Td850 - 2 * T500 [°C]

K index = T850 + Td850 - T500 - (T-Td)700 [°C]

Sweat index = 12*Td850+20*(TOTL-49)+2*U850+5*U500+125*(0.2+sin(f)) where f=(wind direction500-wind direction850), U=wind speed[kts], TOTL=0 if TOTL<49

but, beware, some parameters....


...combine different physical atmospheric properties (moisture, temperature, wind shear) into one parameter in some “magical way”


K index = T850 + Td850 - T500 - (T-Td)700 [°C]






K index = T850 + Td850 - T500 - (T-Td)700 [°C]



...come with a list of thresholds, that may not be valid in your forecast region (if at all...)




K index = T850 + Td850 - T500 - (T-Td)700 [°C]




...require no physical understanding of the weather situation




K index = T850 + Td850 - T500 - (T-Td)700 [°C]




...require no physical understanding of the weather situation

...don’t increase one’s understanding either.

you can not find out what went wrong and do better next time!

* = will discuss this later on

my convective parameters

parameter for prediction of remarks

CAPE

(if not available:

LIFTED INDEX)

instability beware of different parcels that are lifted

CAPE RELEASED BELOW 3 KM*

low-level instability buoyant parcels close to the surface can cause rapid vortex stretching tornadoes

INSTABILITY parameters



forcing term of differential vorticity advection

upward motion,

convective initiation

upward motion in numerical models may be contaminated by the convection itself....forcing term of

temperature advection

upward motion,

convective initiation

or, alternatively Q-vector divergence or PV-analysis

LIFT parameters




0-6 km BULK SHEAR

convective organisation remark: convective organisation is strongly influenced by the amount of lift as well

0-1 km BULK SHEAR*

tornadoes

0-3 KM STORM-RELATIVE HELICITY

potential for supercell convection

0-1 KM STORM-RELATIVE HELICITY*

potential for tornadoes

WIND SHEAR parameters




MOISTURE AT MID-LEVELS*

strong downdrafts if low

MOISTURE AT LOW LEVELS*

strong downdrafts if low deep, dry boundary layers cause evaporative cooling and a high potential for strong wind gusts

LCL HEIGHT* tornadoes tornadoes unlikely with LCL > around 1500 m

WIND SPEED AT 850 hPa*

wind gusts vertical transport of horizontal wind speeds (very) relevant for wind speed in downdrafts

other parameters


study done at Institute for Marine and Atmospheric Research Utrecht

Sounding-derived parameters associated with large hail and tornadoes in the Netherlands

My M.Sc thesis research...

Basic idea

1. Find soundings taken in the proximity of severe weather events (here: tornadoes)

2. Find if they have special characteristics (w.r.t. other soundings)

method: look at parameters that represent something physical and that have been studied before

Proximity soundings

What is a proximity sounding…?

Used definition:• within 4 hours of the sounding

(before or after)

• within 100 km from a point thatis advected by the 0-3 km meanwind from the sounding locationat the sounding time

• radiosonde observations

Dec 1975 – Aug 2003

(thanks to KNMI, DWD, KMI)

• severe weather reports from Dutch voluntary observers (VWK)

Data sets

Sinds 1974

Vereniging voor Weerkunde en Klimatologie (VWK)

http:/www.vwkweb.nl

Data

soundings associated with: number

hail (2.0 - 2.9 cm)

hail (>= 3.0 cm)

tornadoes F0

tornadoes F1

tornadoes F2

waterspouts

thunder (1990-2000 only)

46

47

24

37

6

26

2045

all soundings 67816

Most-unstable CAPE (MUCAPE)Number of events

maximum

median

75th perc.

25th perc.

MUCAPE high with hail; less with tornadoes…

US studies: MUCAPE highly variable with tornadoes. Strong tornadoes may occur with low CAPE when shear is high

Most-unstable CAPE released below 3 km A.G.L.

MUCAPE<3km high with F0, not with F1+

US studies: Davies (2004) has found a relation between tornado occurrence and high CAPE below 3km (in his study mixed-layer CAPE)...

(most-unstable) LFC height (m)

LFC relatively low with tornadoes (esp. F0)…

US studies: Davies (2004) has found a relation between low LFC and tornado occurrence

LCL height (50 hPa mixed layer parcel)

US studies: Low LCL favors significant tornadoes, e.g. Craven et al. (2002)

LCL not sign. diff. between tornadic and thunder

LARGE HAIL F0 F1+

Average soundings

note the distribution of parcel buoyancy with height

0-6 km A.G.L. bulk shear (m/s) (1)

US studies: strong tornadoes and (very) large hail often occur with supercells. These are associated with >20 m/s 0-6 km shear (e.g. Doswell&Evans, 2003)

0-6 km A.G.L. bulk shear (m/s) (2)

likelihood of hail increases with 0-6 km shear, but the majority of hail events occur with moderate shear

0-1 km A.G.L. bulk shear (m/s)

0-1 km shear high with F1, esp. F2 tornadoes...and with wind gusts

US studies: strong 0-1 km shear favours sign. tornadoes (e.g. Craven et al., 2002).

0-1 km A.G.L. storm-relative helicity (m2/s2)

0-1 km shear high with F1, esp. F2 tornadoes..

US studies: high values favor supercell tornadoes (e.g. Rasmussen, 2003).

• MUCAPE and 0-6 km bulk shear are useful predictors for large hail, especially when combined

• most large (> 2cm) hail in the Netherlands is associated with multicells rather than supercells

Conclusions of the study

• F1 and esp. F2 tornadoes occur with higher-than-average 0-1 km shear and SRH.

• F0 tornadoes (and waterspouts) occur with lower-than-average 0-1 km shear values

• (MU)CAPE is not extreme with tornadoes and thereby has limited value for tornado forecasting..

Conclusions of the study

Submitted to Atmospheric Research

• MUCAPE released below 3 km / low LFC heights seem to be important for the formation of weaker (and likely non-supercellular) tornadoes….

(but of course we rather want to forecast the stronger tornadoes)

• LCL heights are probably not as much a limiting factor for tornado development in the NL than in much of the U.S.A.

i.e. LCL heights are practically always low enough here for tornadoes

Some conclusions

using convective parameters23th June, 2004analysis prepared in cooperation with Christoph Gatzen (ESTOFEX)

photo: Christian Schöps

source: ESTOFEX

23 June, 2004: 500 hPa height, wind speed

23 June, 2004: 850 hPa height, theta-e

23 June, 2004: MUCAPE, deep layer wind shear

23 June, 2004: MUCAPE, low level wind shear

23 June, 2004: LCL height

23 June, 2004: LFC height

Sounding from the action area. It indicates...

• rather weak CAPE

• most of it below 3km

• winds veer strongly with height (indicating helicity)

• strong low level wind shear

In this case, the forecast didn’t work out. The favourable veering of wind wind height in the lowest km, was not at all predicted by most numerical models that forecasted SWly winds instead of SEly winds.

Conclusion and highlights• the ingredients-based methodology can help to structurize the forecasting process

• for severe convection the essential ingredients are:• latent instability (CAPE)• lift• vertical wind shear (20 m/s…40 kts is supercell threshold)

• Convective parameters with a single obvious physical meaning are probably the most useful.

Most important for forecasting….HAIL CAPE and convective modeTORNADOES 0-1 km shear, SREH and convective modeWIND GUSTS 850 hPa wind, dry low or mid-levels and

convective mode

ReferencesCraven, J. P., H. E. Brooks, and J. A. Hart, 2002: Baseline climatology of sounding derived parameters associated with deep, moist convection. Preprints, 21st Conference on Severe Local Storms, San Antonio, Texas, American Meteorological Society, 643–646.

Davies, J. M., 2002: On low-level thermodynamic parameters associated with tornadic and nontornadic supercells. Preprints, 21st

Conf. on severe local storms, Kananaskis Park, Alberta, Canada, Amer. Meteor. Soc., 558–592.

Davies, J. M., 2004: Estimations of CIN and LFC Associated with Tornadic and Nontornadic Supercells. Wea. Forecasting, 19, 714–726.

Fujita, T. T., 1971: Proposed Characterization of Tornadoes and Hurricanes by Area and Intensity, SMRP Research Paper No. 91, University of Chicago.

Doswell, C. A. III, and J. S. Evans, 2003: Proximity sounding analysis for derechos and supercells: An assessment of similarities and differences. Atmos. Res., 67-68, 117–133.

Rasmussen, E. N., 2003: Refined supercell and tornado forecast parameters. Wea. Forecasting, 18, 530–535.

Severe convective storms, theory Pieter Groenemeijer FMI Helsinki, 2 May 2005.

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