ANALYSIS OF A SMALL, VIGOROUS MESOSCALE CONVECTIVE SYSTEM IN A LOW-SHEAR ENVIRONMENT - PART II - EVOLUTION OF THE STRATIFORM PRECIPITATION AND MESOSCALE FLOWS

The evolution of the mesoscale Row and precipitation distribution are investigated for a small mesoscale convective system (MCS) that evolve d in a nearly barotropic environment exhibiting moderate instability a nd weak wind shear. Observations primarily from a single Doppler radar detail the growth of the MCS from the merger of several clusters and lines of vigorous convective cells into a mature state consisting of a weaker convective line trailed by an expanding stratiform precipitati on region. Analysis of radar reflectivity reveals that this stratiform region formed in situ in the presence of weak mesoscale updraft as de caying convective cores coalesced, rather than through rearward advect ion of ice particles directly from the convective region. In the absen ce of sufficient low-level shear, the MCS collapsed rapidly as it assu med the structure of the archetypal convective line and trailing strat iform precipitation region. Velocity-azimuth displays reveal mesoscale updrafts of about 70 cm s(-1) during the active convective stage. In the mature stratiform region, the lower-tropospheric mesoscale downdra ft (similar to 40 cm s(-1))exceeded the mesoscale updraft (similar to 10 cm s(-1))above it, and the level separating the two was relatively high at 6.5 km, about 2 km above the 0 degrees C level. As the MCS clo ud-top anvil area colder than -52 degrees C peaked near 60000 km(2), t he cloud top descended at rates of 20-40 cm s(-1) despite weak but sus tained mesoscale updraft within the upper part of the cloud. A rear in flow jet was observed before convective activity peaked, remained stro ng while the deep convection diminished, and became the main Bow featu re as the MCS decayed. This jet subsided from approximately 7 km at th e rear end to near the surface at the leading edge of the convection. A weaker ascending front-to-rear current was found above this rear inf low jet. No midlevel mesoscale cyclonic vortex was apparent in the ech o structure of the maturing MCS. Indirect estimates of mesoscale vorti city, based on Lagrangian conservation of radar reflectivity, indicate that cyclonic rotation was present in the mesoscale downdraft region, and anticyclonic rotation occurred aloft. The magnitude of this vorti city is about half the Coriolis parameter. A positive potential vortic ity anomaly is found at midlevels within the MCS, and this anomaly int ensifies in depth and in strength as the system matures. This growth i s consistent with the diabatic heating profile estimated from a 1D clo ud model.