Doppler Radar Study of the Trailing Anvil Region Associated with a Squall Line

Abstract

The kinematic structure and reflectivity distribution within a region of widespread precipitation associated with a summertime midlatitude (Illinois) squall line, as revealed by an analysis of Doppler radar data, are presented and discussed. The squall line moved in a southeasterly direction while active convection forming on its leading edge moved in a northeasterly direction. Decaying thunderstorms and their anvils merged to form the extensive region of stratiform precipitation which trailed the squall line. An extension of the VAD (Velocity Azimuth Display) method, or the EVAD (Extended VAD) method, has been developed for the analysis of single-Doppler radar data. In contrast to the VAD method, which requires knowledge of the particle fall velocities (or assumptions regarding it) to calculate the divergence of the horizontal wind, the EVAD method yields the vertical distributions of both the particle fall speed and the divergence. Also presented are results from a multiple-Doppler (MDOP) analysis of data from three radars. A function-fitting technique has been used for the MDOP analysis which filters out motions of high wavenumbers and yields the mesoscale motions. The MDOP method yields the horizontal distributions of the winds, their divergence and the vertical air velocity, while the EVAD method yields their horizontal averages. The EVAD method gave weak (<10 cm s?1) ascending air motion below about 1-km height, descent (peak magnitude about 25 cm s?1) between approximately 1 and 5-km height and ascent (peak approximately 35 cm s?1) above 5-km height. The vertical air velocity calculated by the MDOP method also shows ascent below about 1-km height. Above that height, the MDOP method gave descending motion in the eastern part of the trailing anvil (adjacent to and behind the squall line convection), coinciding with a region of low and dissipating reflectivities. Further to the rear, in the western part of the anvil, the vertical air motion calculated by the MDOP method is qualitatively similar to that found by the EVAD method. It is speculated that the descending motion in the eastern part of the trailing anvil may be part of the subsidence associated with a new line of convection which formed ahead of the old line as it dissipated. The divergence field calculated by the MDOP method shows a banded structure especially pronounced at the middle and higher levels. The component of the horizontal wind relative to the squall line and transverse to it, calculated by the MDOP method, suggests the following: above approximately 3-km height, air entered the trailing anvil at its front and ascended towards its rear; at heights between approximately 3 and 6.5 km, environmental air entered the rear of the anvil and descended towards its front in a height interval from near the ground to 3 km; below the latter flow regime, the air moved from the front to the back of the anvil in a layer whose thickness increased towards the rear of the anvil.