Thunderstorm environment

1
Thunderstorm environment
2
Sample supercell environment hodographs
3
Streamwise vorticity and helicity(Davies Jones,
Lilly)

• Streamwise vorticity component of 3D vorticity
  along the storm-relative horizontal flow
  
• ws (v-c). w/v-c
• Helicity vertical integral of (v-c). w, usually
  over the lowest 3 km
  
• Longevity of supercells may be due to large
  helicity of environment (Lilly)

4
m/s2
F2-F5 tornado environments Johns (1993)
J/kg
5
Thompson Edwards, 2000 An Overview of
Environmental Conditions and Forecast
Implications of the 3 May 1999 Tornado Outbreak.
Wea. Forecasting, 15, 682699
The 3 May 1999 outbreak included two prolific
supercells that produced several violent
tornadoes, with ambient parameters of stability
and shear comparable to those of past tornado
outbreaks in the Great Plains. However, not all
aspects leading to the evening of 3 May
unambiguously favored a major tornado outbreak.
The problems included subtle processes
contributing to convective initiation, the roles
of preexisting boundaries, and storm-relative
flow. This work reveals several specific aspects
where conceptual models are deficient.
http//www.spc.noaa.gov/publications/thompson/3may
99/waf.htm
6
(No Transcript)
7
12 Z 3 May 99
8
00 Z 4 May 99
9
Lines 300 mb divergence (convergence dashed)
Colors conditional symmetric instability (Pe
CAPE CIN 20 Z 3 May
10
(No Transcript)
11
Meso-analysis 20 Z 3 May
12
12 UTC 3 May 1999
00 UTC 4 May 1999
Blue qw Potential instability??
13
00 UTC 4 May 1999
00 UTC 4 May 1999
14
Purcell (Norman)
15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
Storm cell A formed along this roll, in between
the 2 drylines
20
x
21
Storm A right mover
photographer
22
dryline
23
(No Transcript)
24
Errors in prediction 24 h ETA for 00Z 4 May,
compared to observed winds (barbs) _at_ 500 mb
Trof location seems right but wind speeds undere
stimated (the X marks are local speed maxima)
25
Errors in prediction 24 h ETA for 00Z 4 May,
and obs (barbs) _at_ 300 mb There seems to be a s
harper trof over the TX Panhandle
wind speeds are underestimated
26
24 hr forecast
27
(Purcell is 20 km south of OUN. The 00 Z OUN
balloon failed at 9 km)
Observed - more shear in all layers - double the
helicity
28
m/s2
F2-F5 tornado environments Johns (1993)
Environs of tornadoes in this study
J/kg
29
Conclusions (Thompson and Edwards 2000)

• Severe storms subtly depend on ambient shear and
  CAPE, both of which are difficult to predict
  
• Shear models may miss mesoscale aspects of jet
  and mesoscale low-level flow
  
• CAPE is very sensitive to surface conditions and
  uplift along shallow convergence lines
  
• The outbreak, or lack thereof, depends on the
  presence and strength of shallow boundaries,
  which are generally not predicted
  

30
Triggering deep convection

• (Wilson Scheiber 86, Koch and Ray 1997,
  Kingsmill 1995) CI along radar fine-lines.
  
• land surface variations
• helical rolls in the sheared, convective BL
  (HCR)
  
• density-current-like flows
• outflow boundary of a convectively-generated cold
  pool.
  
• sea breezes
• bores/solitary waves
• topography (anabatic convergence)

31
Why convection first develops over mountains

Anabatic flow Less diurnal heating required
G,C CCL E,A convection temperature
Denver sounding, 12 Z before severe storm
32
Synoptic aspects of severe weather breakouts
Severe storm area (sometimes MCCs)
33
Synoptic conditions for severe wx developing over
Laramie Range or Front Range
34
Miller type-I sounding (loaded gun of the souther
n Great Plains
strong CAP- lots of CAPE)
35
Upper-trope flow
Low-level, capped moist air
Elevated mixed layer
Miller type-I sounding trajectories
36
Miller type-II sounding A more moist tropical
sounding
little CIN little-moderate CAPE
37
Miller type-III sounding cold-air aloft low tro
popause little CIN little CAPE strongly forc
ed synoptically thunderstorms along strong conve
rgence line, may be severe
38
Miller type-IV sounding desert storm deep, dry,
well-mixed ABL high-base TS little CAPE may
produce strong microbursts