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 flow and precipitation distribution are investigated for a small mesoscale convective system (MCS) that evolved in a nearly barotropic environment exhibiting moderate instability and 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 precipitation region. Analysis of radar reflectivity reveals that this stratiform region formed in situ in the presence of weak mesoscale updraft as decaying convective cores coalesced, rather than through rearward advection of ice particles directly from the convective region. In the absence of sufficient low-level shear, the MCS collapsed rapidly as it assumed the structure of the archetypal convective line and trailing stratiform precipitation region. Velocity–azimuth displays reveal mesoscale updrafts of about 70 cm s21 during the active convective stage. In the mature stratiform region, the lower-tropospheric mesoscale downdraft (;40 cm s21) exceeded the mesoscale updraft (;10 cm s21) above it, and the level separating the two was relatively high at 6.5 km, about 2 km above the 08C level. As the MCS cloud-top anvil area colder than 2528C peaked near 60 000 km2, the cloud top descended at rates of 20–40 cm s21 despite weak but sustained mesoscale updraft within the upper part of the cloud. A rear inflow jet was observed before convective activity peaked, remained strong while the deep convection diminished, and became the main flow feature as the MCS decayed. This jet subsided from approximately 7 km at the 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 inflow jet. No midlevel mesoscale cyclonic vortex was apparent in the echo structure of the maturing MCS. Indirect estimates of mesoscale vorticity, 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 vorticity is about half the Coriolis parameter. A positive potential vorticity anomaly is found at midlevels within the MCS, and this anomaly intensifies in depth and in strength as the system matures. This growth is consistent with the diabatic heating profile estimated from a 1D cloud model.