Wednesday, 6 October 2004: 10:45 AM
Vertical shear is known to play a large role in determining the structure and evolution of severe local storms. Numerical simulations and observational studies both suggest a strong relationship between vertical shear and storm morphology. However, the evolution of severe storms is difficult to predict when the vertical shear is evolving over the lifetime of the storm. Storms beginning in an environment supportive of multicells may later experience vertical shear adequate for supercells. In this case, little is known from previous idealized numerical studies about the expected subsequent storm evolution as these studies are generally performed assuming the environmental wind profile is unchanging. Conversely, simulations using observed data and direct observations themselves contain too many other influences to isolate the role of the changing wind shear.
In this study, idealized experiments are conducted in which the environmental vertical shear is forced to change at a prescribed rate, in a horizontally-homogeneous manner, over the lifetime of a storm. Simulations using curved, straight and tail (curved at low levels and straight at upper-levels) hodographs whose shear increases in time will be presented as well as simulations in which the shear evolves from a straight hodograph to a tail hodograph. Finally, simulations in which the shear change is applied later in the storm lifetime will be presented. In some cases, the simulated storm transitions smoothly between storm types while, in other cases, the final storm is a different type than in either control simulation. The timing of shear change is found to be especially important for a curved hodograph, with no development of a right flank supercell when the shear change is delayed one hour.
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