Chapter 8: Organization of isolated deep convection
a brief review
there are 3 basic types of thunderstorms, and these can occur by
themselves or in 3 basic types of mesoscale organization
individual thunderstorms
airmass ts
(single-cell, ordinary)
multi-cell ts
supercell ts
the distinction between the 3 storm types is largely
controlled by wind shear
8.1 The role of wind shear
Fig. 8.1
Fig. 8.2
weak shear
bulk Richardson number:
BRN 

CAPE
1
V6 km  VML
2

2
strong shear
8.1 The role of wind shear
quicktime movies:
• no shear
• strong shear
8.1 The role of wind shear
no shear
Weisman: convective storm matrix:
buoyancy-shear dependencies.
COMET-MetEd module
blue contour: qv’=-0.2K near surface
red contour: w (10 m s-1) at 4 km
green: qr+qs+qg > 1 g kg-1 at 1 km
arrows: storm-relative flow
weak
shear
strong
shear
Wilhelmson-Klemp (1982)
sounding
(CAPE=2200 J kg-1)
Fig. 8.3
strong shear
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)
• ’04: BAMEX
• 07: COPS
• ’09-’10: 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 Cband 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 first
formation 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 (qv’~ 2 K)
Wind, temperature,
and hydrometeors
Surface wind measurements show outflow below the region of radar echo
New convergence
line ??
echo
>30 dB
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:

1
m
mcloud
z
cloud
– 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
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
height
(100s 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
Uenv
shelf cloud
above gust
front
Multicell storms were recognized by Byers
and Braham
(the Thunderstorm Project, 1948-49)
Byers and Braham recognized the
importance of cold pool building by
decaying cells in the triggering of
new cells.
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.
Role of cell lifecycle in multicell storms
•
The sequence on the right shows individual cells and
their place in the evolution of a multicellular system.
Fig. 8.10
Ludlam
Hobbs and Rangno 1985
(small multicell Cb over Cascades)
young
cell
old
cell
Photo by Moller
Photo by Doswell
Multicell echo sequence
(Leary and Houze 1983)
single-cell vs multicell storms: effect of LL shear
no shear
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)
multicell
simulations
5 km updraft (color)
-1K q’ (contour)
wh, ambient 0-1 km
wh, solenoidal
multicell simulations:
cluster migration towards region with higher CAPE
8.4 Supercell Thunderstorms
•
Supercell thunderstorms are defined as having a sustained deeptropospheric 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
Fig. 8.15
Fig. 8.16
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
3 May 1999 Moore OK F5 tornado:
reflectivity animation
radial velocity animation
storm motion to the ENE (70°)
radar to the south
Fig. 8.18
mesocyclone
cyclonic supercell
storm: visual aspects
anvil
photo Josh Wurman
LP
photo credit:
Nguyen
Photo by Bill McCaul
low-precipitation
supercells
LP supercell
HP
photo credit:
Nguyen
storm-relative flow in a supercell
young supercell
Fig. 8.15
interpret this inflow low
using Bernouilli eqn
0.5rv2+p’=constant
Fig. 8.23: sfc pressure perturbations
(contours – mb), -1K cold pool, rain water @ 1
km (green colors), and updraft @ 1 km (pink)
composite hodo from ~400 soundings near supercell storms
Fig. 8.20
mature supercell
the bounded weak echo region (BWER)
RHI
Fig. 8.22
Fig. 8.21 in textbook
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.
BWER on radar: range height indicator (RHI) displays
Base scan (0.5°)
RHI
16.5 km echo tops
NW
SE
(source: WSR-88D Operations Training Manual)
BWER using horizontal & vertical slices
(e.g., in soloii)
south to north
west to east
Fig. 8.19
BWER & the hail cascade
fallspeed of hail
as function of
diameter D
Where do we go from here?
• covered in 2011: Section 8.4 Supercell dynamics: COMET/METED
– Supercell rotation
• 8.4.3: origin of mid-level rotation
• 8.4.4: solenoidal vorticity and the mesocyclone
– 8.4.5: storm splitting & supercell propagation
– homework #3: Weisman: convective storm matrix: buoyancy-shear
dependencies. COMET-MetEd module
• not covered in 2011: 9. Mesoscale organization:
– Mesoscale Convective Systems: Squall Lines and Bow Echoes
(webcast)
– MCSs: BAMEX Science Overview
– MCV dynamics (Fritsch 1996)
• not covered in 2011: 10. Severe weather hazards:
– severe weather & storm environment
– tornado dynamics
– derechoes: straight line winds
Storm classification
summary
variables:
buoyancy and shear profiles