The Role of Cumulus Convection in the Development of Extratropical Cyclones

Abstract

The goal of this study is to determine whether cumulus convection plays a role in the development of extratropical cyclones, and if it does, to determine the nature of that role. The basic approach is to ascertain whether there is a systematic relationship between the observed extent and degree of convective activity accompanying cyclogenesis and the departure of actual storm evolution from that predicted by large-scale dynamical models. On the basis of intensive analyses of the two storms initially chosen for study, the following hypothesis was formulated, and the balance of the investigation was directed primarily toward ascertaining its validity: In same instances of extratropical cyclogenesis, cumulus convection plays a crucial role in the initiation of development through the release of latent heat in the vicinity of the cyclone center. In such cases, dynamical models that do not adequately simulate convective precipitation, especially as it might occur in an environment that is unsaturated, will fail to properly forecast the onset of development. Further evidence, either to support or refute the hypothesis, was derived from detailed analyses of seven additional storms, cursory examination of 12 others, and both qualitative and quantitative consideration of the physical mechanisms involved. Although not conclusive proof of the hypothesis, the evidence does indeed support it. Significant convection occurred in the center of storms generally only during the early stages of their life history. Latent heat released by convective showers in the vicinity of the Low center appeared to initiate development before such development would have occurred if only the larger scale baroclinic processes were operative. Convective activity not in the immediate vicinity of the Low center did not appear crucial either to the initiation of development or to the trend of continued development following the onset of cyclogenesis.