THE ROLE OF SNOW SUBLIMATION IN FRONTOGENESIS

Recent work on the influence of moist processes at synoptic fronts has shown how condensation heating can be a strong and even a dominant fa ctor in their development. At mesoscale lines of convection the heatin g is again dominant, but it is thought that cooling by the evaporation of precipitation is also a crucial factor in their development and pr opagation. Although most simple models of synoptic fronts do not attem pt to simulate diabatic cooling accurately, data from the FRONTS 87 ex periment have identified evidence of a strong mesoscale downdraught be low a synoptic frontal surface. There has been speculation that such a downdraught is forced by the sublimation of falling snow. Since snow is more efficient at cooling the atmosphere, for a given precipitation rate, than evaporation of rain, rainfall parametrizations are likely to underestimate the influence of such cooling. In this paper, a suita ble parametrization of diabatic cooling, based on the assumption of sm all stability to slantwise convection, is used to simulate a band of s now sublimation in developing fronts. The semi-geostrophic equations a re employed in the study of fronts which are formed by deformation or horizontal shear of the geostrophic winds. It is found that the coolin g has little influence on the large scales of motion, and does not gre atly accelerate the formation of frontal singularity. However, the cro ss-frontal flows in the vicinity of the cooling are modified strongly, with the formation of a narrow downdraught of comparable dimensions a nd intensity to that suggested by the experimental data. The cooling h as a weak influence on all the fields at a larger distance from the fr ont; this is discussed in the context of the large-scale, or geostroph ic, response to lines of convection.