The Kinetic Energy Budgets of Two Severe Storm Producing Extratropical Cyclones

Abstract

Eulerian kinetic energy budgets for the synoptic-scale flow over North America were computed for two cases of cyclone development associated with severe prefrontal convection. Horizontal flux convergence constitutes the major energy source in both cases and assumes major importance in maintaining the strength of the upper tropospheric jet maxima. Generation of kinetic energy via cross-counter flow is, surprisingly, a persistent sink in one case and only a weak energy source for the cyclone in the second case. Cross-contour flow toward higher heights is generally found ahead of the upper level troughs, where the jet stream is moving through regions in which the contour gradient weakens downstream. Generation of kinetic energy is largely confined to the lower troposphere, reflecting frictional influence near the earth's surface. Dissipation of kinetic energy, computed as a residual, has local maxima both in the lower troposphere (nearly balancing the generation) and near the jet stream level. Subgrid-scale sources of kinetic energy are apparent in both cases and, at times, are particularly important in regions of widespread deep convection. These sources are associated with longitudinal shear in the polar jet stream and with the presence of deep convection. Convective region budgets show that the maximum rate of kinetic energy gain occurs during periods when the convection increases most rapidly both in intensity and in areal extent.