Severe frontal rainband, Pt. 3, Derived thermodynamic structure

Pressure, buoyancy, and virtual potential temperature perturbations are calculated from wind fields derived from Doppler radar data taken in a surface cold front. The dynamics of the front are similar to a density current. This hypothesis is also suggested by accompanying numerical simulations of cold air outflows. The updraft at the leading edge of the cold air mass is maintained in conjunction with an upward directed pressure force. The average maximum updraft is >7 m sec super(-) super(1) without any appreciable potential instability present in the undisturbed warm-sector sounding. The buoyancy and virtual potential temperature data reveal a front with a substantial fraction of the cooling taking place within the first 2 km of a frontal zone. Thus, the aspect ratio (width-depth) of the front, even after the filtering associated with the interpolation and retrieval processes, is slightly less than 1. The frontogenesis for the shear in the alongfront wind and the thermal gradient are discussed. The gradient of these quantities in the lower levels is maintained by confluence and eventually destroyed by tilting of the gradients into the horizontal. The thermal fields are locally influenced by diabatic processes in the frontal updraft and behind the front. The cooling taking place in the cold air is apparently related to evaporation and melting of hydrometeors. The virtual potential temperature reduction with this cooling is >0.5 K. Considerable alongfront variations in the pressure, wind, and precipitation field occur as a result of the presence of a 13-km wave. These variations in the wind field are due to the influence of the waves on the rate of frontogenesis experienced by a parcel as it moves through the frontal zone. The primary factor for the changes in frontogenesis in the direction parallel to the surface front is the variation in the confluence term.