Elevated Thunderstorms
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Transcript of Elevated Thunderstorms
Elevated Thunderstorms
James T. Moore
Cooperative Institute for Precipitation Systems
Saint Louis University
Dept. of Earth & Atmospheric Sciences
[email protected]/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-mid-tropospheric 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 low-level 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 Maxtheta-e CAPE makes sense when the liftingis at or above afrontal zoneand the boundarylayer air is verystable.
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 for00 UTC 28 April 1994
Max e
CAPE for00 UTC 28 April 1994
Monett, MO sounding for 00 UTC 28 April 1994; Max e
Cape = 1793 J kg-1, Surface-based CAPE = 0 J kg-1
Norman, OK sounding for 00 UTC 28 April 1994; Max e Cape = 2479 J kg-1, Surface-based 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