Morphing Storms, Texas-Style

摘要

Springtime is severe thunderstorm time across the eastern two-thirds of the United States. For thunderstorms to become severe, two criteria are usually met: (1) the release of instability in the air mass must is delayed, allowing it to build to near-explosive levels, and (2) winds must be sheared, i.e., increase in speed (and, at times, change in direction) with altitude.The wind shear is doubly important. First, it ensures that the updraft and downdraft remain separated (i.e., downdrafts falling vertically down through the updraft choke it off). Shear also promotes the development of one or more vertical vortices. When a counterclockwise vortex coincides with the updraft, a mesocyclone develops, conferring rotation to the entire thunderstorm. The resulting supercell often produces large hail and strong to violent tornadoes. If a pair of counter-rotating vortices develops, the pair can accelerate chilled downdraft air to the surface as a series of violent downbursts.The forward surge of damaging wind distorts the line of storms into a bow shape.