Numerical Simulations of the Subsynoptic Features Associated with the AVE-SESAME I Case. Part I: The Preconvective Environment

Abstract

A series of mesoscale numerical simulations of the AVE-SESAME I case (10 April 1979) were performed in order to analyze the dynamical processes that result in the production of an environment favorable for the development of severe local convective storms. The investigation focused on the relative contributions of quasi-adiabatic inertial and isallobaric adjustments attributable to the geometry of the tropospheric flow and the fluxes of heat, moisture and momentum from the surface of the earth. The model simulations support many of the conclusions deduced by Kocin et al. in their analyses of the observations taken during the field experiment. The quasi-adiabatic simulations support the existence of a coupled upper-tropospheric and lower-tropospheric jet streak system. However, the dynamical coupling is more complex than the straight line jet streak model utilized by Uccellini and Johnson. The departures are attributable to two sources. First, there is a time-varying curvature in the exit region due to the propagation of a meso-αscale trough through the area while a longer wave trough remains relatively stationary. Second, the exit region experiences significant changes in the mass field due to the presence of differential horizontal thermal advection. These two effects produce significant alterations to the classical exit region patterns of vertical motion and man divergence. In addition, them processes phase with a pattern of significant horizontal variations in the fluxes of heat, moisture and momentum in the planetary boundary layer. The combination of these processes result in the amplification of the low-level pressure tendencies and an increase in the strength of the low-level jet streak. The combination of mass-momentum adjustments associated with the jet streak system and low-level flux gradients results in the creation of significant amounts of buoyant energy and the vertical motion necessary for its release. The simulation experiments suggest that the 6 h increase in buoyant energy over the areas that subsequently experience convection is approximately half the result of the quasi-adiabatic processes and half the result of the surface fluxes of heat and moisture. This study has three major contributions. First, it indicates the possible importance of the phasing of deep tropospheric mass-momentum adjustments with differential surface fluxes of heat and momentum. Second, it extends the understanding of jet-streak exit region dynamics to the case of cyclonically curved flow in the presence of differential horizontal thermal advection. Third, it reveals the rapidity with which circulation patterns associated with a jet streak exit region can change.