Download Elevated Thunderstorms

Elevated Thunderstorms
James T. Moore
Cooperative Institute for Precipitation
Systems
Saint Louis University
Dept. of Earth & Atmospheric Sciences
moore@eas.slu.edu
COMET-RFC/HPC Hydrometeorology Course
6-13 December 2000
Definition of an “Elevated Thunderstorm”
(Colman 1990, MWR)
• An elevated thunderstorm occurs above a frontal
inversion
• It is isolated from surface diabatic effects which
are often considered fundamental to thunderstorm
development
• Colman’s criteria included:
– observation must lie on the cold side of an analyzed
front, showing a clear contrast in temperature, dew
point, and wind
– the station’s temperature, dew point, and wind must be
qualitatively similar to immediately surrounding values
– the surface air on the warm side of the analyzed front
must have a higher e than the air on the cold side of
the boundary
Elevated Thunderstorms (cont.)
(Colman 1990, MWR)
• These cold sector MCSs generally fit the Maddox
frontal or meso-high type flash flood scenarios
• Elevated thunderstorms can occur during any time
of year; they usually as associated with heavy
rain/snow or hail; nearly all winter-season
thunderstorms over the United States east of the
Rockies (excluding Florida) are of the elevated
type.
• Climatology of elevated thunderstorms reveals a
bimodal variation; primary maximum in April,
secondary maximum in September
Elevated Thunderstorms (cont.)
(Colman, 1990, MWR)
• Form in regions of moderate-strong, low-midtropospheric warm air advection and extremely
stable PBL; typically a shallow front associated
with a strong frontal inversion
• Tend to form in the left exit region of the lowlevel jet
• Form in hydrostatically stable environment with
strong baroclinicity and vertical wind shear
(veering)
• Elevated thunderstorms tend to appear near the
gradient in stability with a potential upstream
source of buoyant air
Elevated Thunderstorms (cont.)
(Colman, 1990, MWR)
• Colman believed that elevated thunderstorms
formed in convectively stable air associated with
NO positive surface-based CAPE.
• According to Colman – these storms appear to be
the result of frontogenetical forcing in the
presence of weak symmetric instability and not the
result of upright convection.
• However, more recent studies suggest that
elevated max e CAPE >>0 can be found in the
area of many of these elevated thunderstorms.
Elevated Thunderstorms Frequency
Colman, 1990
(MWR)
Elevated Thunderstorms Climatology
Colman, 1990
(MWR)
Elevated Thunderstorms Climatology
Colman, 1990
(MWR)
Maximum Theta-e CAPE
Using the Max
theta-e CAPE
makes sense
when the lifting
is at or above a
frontal zone
and the boundary
layer air is very
stable.
Market, 1997
Elevated Convective Instability
Trier and Parsons, 1993; MWR, vol. 121, 1078-1098
24 h Rainfall Ending 7 June 1993 Based Upon Cooperative Reports
Surface
Analysis for
12 UTC 6
June 1993
Precipitable
Water in mm
for 12 UTC 6
June 1993
Comparison of PBL CAPE and Max
Theta-e CAPE for Elevated Convection
850 mb isotachs and wind vectors (l) and e for 12 UTC 6 June 1993
(r)
850 mb e
advection for
12 UTC
6 June 1993
Plot of e vs.
Pressure for
Topeka, KS and
Monett, MO for
12 UTC 6 June
1993
850 mb MTVs (l) and 500 mb heights/vorticity (r) - 12 UTC 6 June 1993
850 mb
Moisture
Convergence for
12 UTC
6 June 1993
200 mb isotachs and
wind vectors for 12
UTC 6 June 1993;
solid line depicts
cross section
200 mb
Divergence
for 12 UTC
6 June 1993
Ageostrophic
Direct
Thermal
Circulation
along path of
Cross section
for 12 UTC
6 June 1993
MB-enhanced IR Satellite Imagery for 1101 UTC 6 June 1993
MB-enhanced IR Satellite Imagery for 1301 UTC 6 June 1993
MB-enhanced IR Satellite Imagery for 1701 UTC 6 June 1993
MB-enhanced IR Satellite Imagery for 2101 UTC 6 June 1993
24 h Rainfall Ending 28 April 1994 Based Upon Cooperative Reports
Surface Analysis for 00 UTC 28 April 1994
Surface e
Analysis for
00 UTC 28
April 1994
Isentropic Cross Section with Normal Wind Components for 00UTC
28 April 1994 from INL, MN to SIL, LA
Vertical
Difference of e
500 mb to 850 mb
at 00 UTC
28 April 1994:
Note Negative
(dashed)
indicates
convective
instability
e vs. Pressure
at Monett, MO
for 00 UTC
28 April 1994
Cross Section of e INL, MN to SIL, LA – 00 UTC 28 April 1994
Mean parcel
CAPE for
00 UTC
28 April 1994
Max e
CAPE for
00 UTC
28 April
1994
Monett, MO
sounding for 00
UTC 28 April
1994; Max e
Cape = 1793 J
kg-1, Surfacebased CAPE = 0
J kg-1
Norman, OK
sounding for
00 UTC 28
April 1994;
Max e Cape
= 2479 J kg1, Surfacebased CAPE
= 0 J kg-1
Skew-T Plot of Monett, MO Sounding (left) and Vertical Motion
on the 306 K Surface for 00 UTC 28 April 1994 (right)
Wind vectors and isobars on the 306 K surface (left) and
moisture transport vectors on the 306 K surface (right) for
00 UTC 28 April 1994
Moisture
Stability Flux on
the 306 K
surface at 00
UTC 28 April
1994. Note:
Positive Values
indicate regions
of destabilization
Ageostrophic
Direct
Thermal
Circulation
00 UTC
28 April 1994;
taken from
NW South
Dakota to SW
Arkansas
Regions Where Elevated Thunderstorms Form
• North of shallow, but strong baroclinic zones;
north of a quasi-stationary or warm front
• Within zones of strong vertical wind shear;
especially from surface – 500 mb, large veering
with height of winds (typically from E-NE to
S-SW)
• Within the northern gradient of low-level e
• Downstream from a maximum in moisture
transport; in the region of moisture convergence
• Often in the entrance region of an Upper-Level Jet
(ULJ) streak
Regions Where Elevated Thunderstorms Form
Within a region of moderate 850 mb e advection
Within a region of relatively high max-e CAPE
Within a region of elevated convective instability
Within a region of high surface-500 mb mean
relative humidity
• Typically within an area of moisture stability flux
> 0 implying dynamic destabilization and
moistening of the lower layers
• Within a region characterized by moderate
isentropic upslope
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