Evolution of Downdrafts and Rotation in an Illinois Thunderstorm

Abstract

This paper discusses multiple-Doppler radar observations of a non-severe Illinois thunderstorm occurring on 29 May 1978. The vertical wind shear was fairly rectilinear on this day with the storm motion being related to the wind at a height of 2.5 km. The cell examined had a radar top of 10 km, and high reflectivities [60 dB(Z)]. Emphasis is placed on the evolution of the updraft, downdrafts, and rotation in this cell. Typical magnitudes of the updraft, downdrafts and vorticity associated with the cell were 12 and 8 m s?1, and 5 ? 10?3 s?1, respectively. Four downdrafts were identified in the cell: a downdraft upshear of the updraft, downdrafts on the left and right flanks, and a downshear downdraft. The main downdrafts were the upshear downdraft during the growth period of the cell, and the left flank downdraft during the mature and dissipation periods of the cell. The location of upshear downdraft at mid to upper levels resembles Lemon and Doswell's conceptual model (1979) and three-dimensional cloud model results of severe storms. A vorticity couplet in mid-levels intensified as the downdrafts developed along the flanks of the cell, primarily through the tilting term in the vorticity equation. This vorticity couplet advected downwind of the updraft. The structure of this nonsevere cell is compared with that of severe storms.