A high-resolution composite analysis covering the entire breadth of the northern portion of a mature leading-line, trailing stratiform squall-line system reveals that mean subsidence observed in the transition zone consisted of two different types of average downdraft: one at upper levels that was mechanically forced and one at lower levels that was microphysically forced. Both the upper-level and lower-level mean downdrafts in the transition zone appeared to be the average effect of convective-scale vertical drafts associated with convective structures that moved relative to the front edge of the convective line. The structure of individual upper-level convective-scale downdrafts suggested that they may have been partially composed of gravity waves excited by the interaction of the penetrative convective updrafts of the mature and dissipating convective cells with the stable ambient flow. The lower-level mean downdraft extended from midlevels to near the surface but was maximum near the melting level and was associated with air of low equivalent potential temperature. It was likely microphysically driven by cooling associated with melting and evaporation.
The upper-level and lower-level subsidence in the transition zone had little effect on the radar reflectivity minimum observed at middle to low levels in the transition zone. The primary microphysical process affecting the development of the reflectivity minimum appears to have been the inability of small ice crystals to form, grow, or persist at midlevels in the transition zone. Consequently, less aggregation could occur in the transition zone just above the melting level than in the secondary band at the same altitude.