Chapter 11: severe weather!! (a)thunderstorms (classification) (b)tornadoes (c)lightning, and (d)hail

Thunderstorm classification a brief review

Thunderstorms often cluster into systems 100 miles or more in size. These are called mesoscale convective systems. MCSs come in many shapes, including squall lines, bow echoes and MCC (complexes).

Fig. 11.2a

Fig. 11.2b

Airmass Thunderstorms

Photo by NSSL

Airmass thunderstorms have a life cycle … with three stages

Mature airmass thunderstorms over the Pacific seen by the Space Shuttle

Airmass Thunderstorms These are scattered small thunderstorms that form in mainly summer. They typically develop in warm, moist air masses away far from any fronts. They occur when the winds aloft are weak (little wind shear) They are also typically short-lived and rarely produce extreme winds and/or hail.

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

Multicell Thunderstorms Life cycle of any one of the cells of a multicell thunderstorm is like any air-mass thunderstorm. The life cycle of the multicell is much different due to the interaction of the cells one with another. The key to the long life of the multicell is the development of the gust front.

Shelf cloud – some examples

Fig

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 appears like a mesoscale cold front. Same effects of pressure check, temperature drop, and wind shift upon passage. Outflow boundary is the remnant of a gust front.

Multicell - cont'd outflow boundary as seen by ground-based radar

Photo by Moller

Multicell storms move slightly to the right of the upper-level wind Multicell movement young cell old cell Photo by Doswell

south north

Why do new cells form mainly on the southern side ?

Supercell Thunderstorms occur most frequently in the southern Great Plains in spring. compared to single cells, supercells are: –rare –longer-lived –larger –organized with separate up- and downdrafts.

LP photo credit: Nguyen

Photo by Bill McCaul low-precipitation supercells

LP supercell

photo credit: Nguyen HP

weak echo region (WER) in a supercell storm 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)

Weak vs strong thunderstorm

supercell - seen by ground-based radar

Cross-section thru supercell - note BWER 54,000 ft tops NW SE Note: lowest 7,000 ft cannot be seen by radar because of the curvature of the earth.

Supercell Thunderstorms Supercells only form when strong upper- level winds blow, and a significant change in wind direction occurs, generally southeasterly at the surface and WSW in the upper troposphere. Wall clouds may be found below the storm’s cloud base -- significant rotation may be seen (tornado spawning area).

Thunderstorm evolution and shear no shear strong shear

Supercell formation in a clockwise-turning hodograph

thunderstorm classification: a summary