Chapter 8: Organization of isolated deep convection

a brief review

th e re a re 3 b a sic typ es o f th un d ers to rm s , an d th e se ca n occur byth em se lv es o r in 3 b a sic typ es o f m e so sca le o rg a n iza tion

a irm ass ts(s in g le -ce ll, o rd in ary)

m u lt i -ce ll ts su pe rce ll ts

in d iv id u a l th un de rsto rm s

the distinction between the 3 storm types is largely controlled by wind shear

8.1 The role of wind shear

2621

MLkm VV

CAPEBRN

bulk Richardson number:

weak shear strong shear

8.1 The role of wind shear

• no shear• strong shear

quicktime movies:

8.1 The role of wind shear

Weisman: convective storm matrix: buoyancy-shear dependencies. COMET-MetEd module

blue contour: v’=-0.2K near surfacered contour: w (10 m s-1) at 4 kmgreen: qr+qs+qg > 1 g kg-1 at 1 kmarrows: storm-relative flow

no shear strong shear

weakshear

strongshear Wilhelmson-Klemp

(1982) sounding (CAPE=2200 J kg-1)

Brief history of thunderstorm field research

• ’48-’49: Thunderstorm Project (Byers & Braham)• ’55: creation of the NSSL to develop weather radars

and other instruments to better observe thunderstorms (Kessler)

• ’72-’76: NHRE (hail, hail suppression)• ’78: NIMROD (microbursts) (Fujita)• ’79: SESAME• ’82: CCOPE • ’84: JAWS• ’87: PRESTORM (squall lines, MCSs)• ’90: COHMEX• ’95,’97: VORTEX (tornadoes)• ’02: IHOP (convective initiation, low-level jet)• ’07: COPS• ’08-’09: VORTEX-II

The Thunderstorm Project

• Early field project: summer 1946 in Florida, July 1947 in Ohio• Justified in part by need for wx information for the expanding

aviation industry• Ten military aircraft, P61C (“Black Widow”), five each

mission, spaced at 5000’ intervals• Used new radar developments from WW-II (first use of 5 cm

C-band radars)• First meso-net (people recording wx at 5 min intervals during

IOPs)• In-flight data obtained from photographs of instrument panels• focused on determining kinematic and thermal structure and

evolution of thunderstorms

The Thunderstorm Project : thunderstorm stages

• References:– the project report: “The Thunderstorm”– Byers and Braham, 1948: Thunderstorm structure and circulation.

J. Meteorol., 5, 71-86

• Thunderstorm described as composed of a number of relatively independent cells

• Each cell evolves through stages:– “cumulus” stage– mature stage– dissipating stage

The cumulus stage:

• Updrafts throughout, ~ 5 m/s max (15 m/s peak); no downdrafts

• Cell sizes: 2-6 km

• Updraft increases with height but diameter remains about constant ( entrainment).

• LL convergence

• Positively buoyant throughout

• Graupel and rain in-cloud

• 15-30 min in duration

Wind, temperature, and hydrometeors

Surface convergence pattern measured at the time of firstformation of cumulus clouds:

The mature stage:

• Rain first reaches the ground; heaviest rain and strongest turbulence in this stage

• Downdraft forms from above the FL

• Updrafts also remain strong, most intense higher in cell

• Strong surface divergence forms below the heaviest rain, and the cloud outflow forms a gust front at the surface

• Both positive and negative buoyancy is present (v’~ 2 K)

Wind, temperature, and hydrometeors

Surface wind measurements show outflow below the region of radar echo

echo>30 dB

New convergence line ??

The dissipating stage:

• LL divergence

• Downdrafts weaken, turbulence becomes less intense, and precipitation decreases to light rain.

• Lasts about 30 min

Wind, temperature, and hydrometeors

the Thunderstorm Project

• The 3 storm stages have since been interpreted as characteristic of airmass thunderstorms

• Byers and Braham recognize the importance of wind shear: – “strong shear prolongs the mature stage by separating the

precipitating region with downdrafts from the updraft region”

• They also estimate entrainment: – estimated from mass balance: 100% in 2 km– estimated from soundings around storms: 100% in 5 km– discrepancy probably arose from downward motion of mixtures

after entrainment, making the former estimate more reliable

z

m

mcloud

cloud

1

8.2 Airmass Thunderstorms

• Scattered, small, short-lived, 3 stages• Environment has little CAPE, but also little CIN, and little wind shear• They are usually triggered along shallow convergence zones (BL forcing)• Rarely produce extreme winds and/or hail, but may be vigorous with intense

lightning

Photo by NSSL

Mature airmass thunderstorms over the Pacific seen by the Space Shuttle

Hei

ght

(100

s of

ft)

Schematic of the evolution of an airmass storm, as seen by radar

The reason why an airmass thunderstorms is so shortlived is that there is little wind shear, therefore the rainy downdraft quickly undercuts and chokes off the updraft.

Photo by Moller

airmass thunderstorm evolution

Fig. 7.7

8.3 Multicell Thunderstorms• Multicell storms can occur in a cluster, or be organized as one line.

• Individual cells are short-lived like any air-mass thunderstorm, but the multicell cluster is long-lived, due to the ability of old cells to trigger new cells.

• The key to the long life of the multicell is the interaction of the gust front with the ambient LL shear

gust front

shelf cloud above gust front

Uenv

Multicell storms were recognized by Byers and

Braham

Roles of cell lifecycle in multicell storms

• Evidence since the Thunderstorm Project continues to support the relevance of individual cells in thunderstorm systems.

• The sequence on the right shows individual cells and their place in the evolution of a multicellular system.

Ludlam

Multicell Thunderstorms

• Shelf Cloud often indicates rising air over the gust front.

• New cells develop in front of the storm.• Gust front maintained by the cool

downdrafts.• Gust front is typically several miles in

front of the thunderstorm• Gust front appears like a mesoscale cold

front.

• Outflow boundary is the remnant of a gust front.

Multicell storms move slightly to the right of the upper-level wind

multicell movement

youngcell

oldcell

Photo by Doswell

Multicell echo sequence Photo by Moller

(Leary and Houze 1983)

Essential in the development of new cells, and hence the longevity of multicell clusters, is the interaction between the cold pool and low level ambient shear

single-cell vs multicell storms: effect of LL shear

balance between baroclinic & ambient horizontal vorticity leads to deeper ascent – more likley above the LFC (Rotunno, Klemp, Wilhelmson 1987, known as the RKW theory)

no shear

shear

multicell simulatio

n

5 km updraft (color)-1K ’ (contour)

8.4 Supercell Thunderstorms

Fig. 8.15

Fig. 8.16

• Supercell thunderstorms are defined as having a sustained deep-tropospheric updraft ~coincident with a mid-level vorticity maximum– They are typically ‘severe’ (strong horizontal wind gusts, large hail, flash flood, and/or

tornadoes)

• They are rare (<1% in US, <5% in Southern Plains in May), long-lived

• They are easily identifiable on radar– Mesocyclone (sometimes TVS)– elongated anvil (to the east), often with a V notch– a hook-shaped flanking line (@ south side for right movers)– bounded weak-echo region (BWER)– reflectivity often suggests hail presence

• They form under strong shear– see right: composite hodograph– based on 413 soundings– near cyclonic supercells

Supercell Thunderstorms• occur most frequently in the southern Great

Plains in spring.• compared to single cells, supercells are:

– longer-lived– larger– organized with separate up- and downdrafts.

Mesocyclone & hook echo

storm motion to the ENE (70°)radar to the south

3 May 1999 Moore OK F5 tornado:reflectivity animationradial velocity animation

Fig. 8.18

anvil

mesocyclone

photo Josh Wurman

cyclonic supercell storm: visual

aspects

LP

photo credit:Nguyen

Photo by Bill McCaul

low-precipitation supercells

LP supercell

photo credit:Nguyen

HP

storm-relative flow in a supercell

storm motion

Fig. 8.20

composite hodo from ~400 soundings near supercell storms

Fig. 8.23: sfc pressure perturbations (contours – mb), -1K cold pool, rain water @ 1 km (green colors), and updraft @ 1 km (pink)

young supercell

mature supercell

the bounded weak echo region (BWER)

Fig. 8.21 in textbook

RHI

Fig. 8.22

How does the BWER form ?

• As the storm intensifies, the updraft becomes stronger and more erect.• The result are:

– the development of mid-level echo overhang (WER)– a tighter reflectivity gradient (hail is most common just north of the WER)– a shift in cloud top position (right above the WER)

• These are strong indicators of a dangerously severe storm.

Base scan (0.5°) RHI

16.5 km echo tops

NW SE

BWER on radar: range height indicator (RHI) displays

(source: WSR-88D Operations Training Manual)

south to north

west to east

BWER using horizontal & vertical slices(e.g., in soloii)

Fig. 8.19

fallspeed of hailas function of

diameter D

BWER & the hail cascade

Storm classification summary

variables:buoyancy and shear

profiles

Where do we go from here?• Supercell organization: COMET/METED

– Supercell rotation• 8.4.3: mid-level rotation• 8.4.4: low-level rotation

– 8.4.5: storm splitting & supercell propagation– homework #3: Weisman: convective storm matrix: buoyancy-

shear dependencies. COMET-MetEd module

• 9. Mesoscale organization:– Mesoscale Convective Systems: Squall Lines and Bow

Echoes (webcast)– MCSs: BAMEX Science Overview– MCV dynamics (Fritsch 1996)

• 10. Severe weather hazards:– severe weather & storm environment– tornado dynamics– derechoes: straight line winds