KINEMATICS AND MICROPHYSICS OF THE TRANSITION ZONE OF THE 10-11 JUNE 1985 SQUALL LINE

A high-resolution composite analysis covering the entire breadth of th e northern portion of a mature leading-line, trailing stratiform squal l-line system reveals that mean subsidence observed in the transition zone consisted of two different types of average downdraft: one at upp er levels that was mechanically forced and one at lower levels that wa s microphysically forced. Both the upper-level and lower-level mean do wndrafts in the transition zone appeared to be the average effect of c onvective-scale vertical drafts associated with convective structures that moved relative to the front edge of the convective line. The stru cture 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 low er-level mean downdraft extended from midlevels to near the surface bu t was maximum near the melting level and was associated with air of lo w equivalent potential temperature. It was likely microphysically driv en by cooling associated with melting and evaporation. The upper-level and lower-level subsidence in the transition zone had little effect o n the radar reflectivity minimum observed at middle to low levels in t he transition zone. The primary microphysical process affecting the de velopment of the reflectivity minimum appears to have been the inabili ty of small ice crystals to form, grow, or persist at midlevels in the transition zone. Consequently, less aggregation could occur in the tr ansition zone just above the melting level than in the secondary band at the same altitude.