MESOSCALE CONVECTIVE SYSTEMS IN WEAKLY FORCED LARGE-SCALE ENVIRONMENTS .1. OBSERVATIONS

During a 24-h period, beginning 1200 UTC 11 May 1982, a series of five mesoscale convective systems (MCSs) developed within a weakly forced large-scale environment. Analyses indicate that the large-scale flow c reated a broad region of potential buoyant energy, but owing to a rest raining inversion and weak large-scale upward motion, convection was i nitiated only where lifting associated with mesoscale features was abl e to eliminate the inversion. The series of MCSs developed sequentiall y and moved eastward across the moist axis. Two of these systems had a large component of motion against the mean tropospheric flow and prop agated in a direction nearly opposite to that of the traveling upper-l evel disturbances. Each system produced an outflow of cold downdraft a ir that spread progressively farther south than that from the precedin g system. This description of the development and evolution of convect ion is very different from traditional ones wherein convection develop s and moves more or less in phase with traveling upper-level disturban ces. Simple analytic models are used to determine the likely mechanism s of upstream propagation. These results suggest that the combined eff ects of both density currents and internal gravity waves produce the u pstream propagation of the region of convection. Density currents domi nate the propagation of convection once it forms, while internal gravi ty waves may help initiate new convection upstream of the region of ex isting convection, thereby producing a jump in the region of convectiv e activity.