Pressure and Temperature Perturbations within a Squall-Line Thunderstorm Derived from SESAME Dual-Doppler Data

Abstract

Some dynamic and thermodynamic properties of a convective cell within a squall line that occurred on 6 June 1979 were studied based on dual-Doppler observations. The domain under investigation had a horizontal dimension of 27 km × 27 km with 12 levels in the vertical. The grid spacing used was 1 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment. Fields of deviation perturbation pressure, density and virtual temperature were recovered from a three-dimensional wind field using the thermodynamic retrieval method. These retrieved fields were then subjected to internal consistency checks to determine the level of confidence. Our findings demonstrate that thermodynamic retrieval is feasible when random errors inherent in the radial wind components are minimized by proper smoothing. Errors in the computation of vertical velocity can be substantially reduced when a variational approach is used with the anelastic continuity equation applied to the vertically integrated horizontal mass divergence as an integral constraint. Results show that the gust front (GF) is primarily responsible for vigorous convection in the storm. Distinct features of strong wind shear, pressure change and temperature contrast are evident across the GF. The derived pressure and temperature perturbations are closely related to the updraft–downdraft structure. In particular, high pressure forms on the upshear side of an updraft with low pressure on the downshear side. The orientation of maximum pressure gradient across an updraft is in the direction of the environmental shear vector. Strong perturbation temperature gradients occur in the vicinity of an updraft with warning on its upwind side and cooling on its downwind side. The appearance of a downdraft in the immediate vicinity of an updraft is of importance in affecting the magnitude and distribution of pressure and temperature perturbations within the storm.