EARLY GROWTH OF MESOSCALE CONVECTIVE COMPLEXES - A MESO-BETA-SCALE CYCLE OF CONVECTIVE PRECIPITATION

The evolution of precipitation fields associated with several mesoscale convective complexes (MCCs) has been inferred from radar reflectivity data. In almost all cases examined, including those described in the literature, the system's convective ensemble undergoes a marked modulation in intensity, on a meso-beta time scale, during the early growth stage of its meso-alpha-scale life cycle. This beta-scale convective cycle is most evident in time series of volumetric precipitation rate due to convective echo, and is characterized by a temporal maximum, followed by a minimum approximately 0.5-1 h later, superimposed on an otherwise several-hour, monotonically increasing trend. While the latter is due to a concurrent, several-hour increase in the areal extent of convective echo, the beta-scale perturbation is dominated by a modulation in intensity of the existing convective entities that the system constitutes at that stage of growth. The beta-scale cycle also marks the onset of a sustained. increased growth rate of the MCC's stratiform precipitation production. While these conclusions are based on only a few cases, their dynamical implications on the poorly understood processes accompanying MCC growth are significant. Is this meso-beta-scale convective cycle a generalized, fundamental feature accompanying the development of many MCCs? Could the cycle be a "fingerprint" of a dynamic, upscale transition of the incipient convective system from its convective-beta-scale stage into a nascent meso-alpha-scale system? This possibility is discussed by relating the results to numerical simulations and other studies of mesoscale convection in the literature.