A Midlatitude Squall Line with a Trailing Region of Stratiform Rain: Radar and Satellite Observations

A squall line exhibiting an extensive trailing region of stratiform precipitation passed over the observational network of the National Severe Storms Laboratory on 22 May 1976. Satellite imagery and conventional radar observations document its evolution from a broken line of thunderstorms to a system of mesoscale proportions, and single-Doppler radar observations describe aspects of its mature structure. Satellite measurements of cloud-top temperature showed the system to be a mesoscale convective complex (MCC). The life cycle of the system exhibited the stages of development seen in tropical cloud clusters. At maturity, two prominent mesoscale flow regimes were identified at midlevels: one marked by inflow into the system's front and continuing toward its rear, and another associated with inflow entering the extreme rear of the system. The rear inflow was associated with a cyclonic midlevel vortex in the stratiform precipitation region. It produced a concavity, or “notch”, in the back edge of the precipitation echo. Shortly after the appearance of the notch, a downwind segment of the leading convective line accelerated forward. The notch persisted through the dissipating stage, at which time secondary notches also formed. The last remnant of the stratiform precipitation area took the form of a chain of three comma-shaped vortices, whose origin could be traced in time back to the primary and secondary notches. The inflow at the front of the system spanned both the leading convective and trailing stratiform regions. Convective-scale velocity maxima were superimposed on this front-to-rear flow in the convective region, while a broad maximum of the rearward current occurred in the stratiform region, just above the melting layer. This rearward system-relative flow apparently promoted the broad structure of the precipitation area. Slowly falling ice particles originating at convective cell tops were evidently advected rearward and dispersed over a 50–100 km wide region, whereupon their melting produced a prominent radar bright band.