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Severe thunderstorm evolution is synthesized, using published and unpublished studies of radar, instrumented aircraft, visual and surface observations. These observations reveal the existence of a downdraft (originating at 7–10 km AGL) on the relative upwind side of the updraft. Air decelerates at the upwind stagnation point, is forced downward and mixes with air below which then reaches the surface through evaporative cooling and precipitation drag. The initially rotating updraft is then transformed into a new mesocyclone with a divided structure, in which the circulation center lies along the zone separating the rear blank downdraft from the updraft. This process appears to result, in part, from tilting of horizontal vorticity into the vertical. It is proposed that the zone of strong vertical velocity gradient across which the mesocyclone comes to be positioned is also characterized by a strong temperature gradient and is the genesis region of strong tornadoes. Although no direct observations are available yet, we further propose that the strong temperature contrast plays a potential modulating role in tornadogenesis by solenoidal generation of vorticity, in analogy with the extratropical cyclone, to which the transformed mesocyclone bears a striking resemblance.
Lemon et al. (Sat,) studied this question.