Elevated Thunderstorms

James T. Moore

Cooperative Institute for Precipitation Systems

Saint Louis University

Dept. of Earth & Atmospheric Sciences

moore@eas.slu.eduCOMET-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-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