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 factor in their development. At mesoscale lines of convection the heating is again dominant, but it is thought that cooling by the evaporation of precipitation is also a crucial factor in their development and propagation. Although most simple models of synoptic fronts do not attempt to simulate diabatic cooling accurately, data from the FRONTS 87 experiment have identified evidence of a strong mesoscale downdraught below 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 suitable parametrization of diabatic cooling, based on the assumption of small stability to slantwise convection, is used to simulate a band of snow sublimation in developing fronts. the semi‐geostrophic equations are employed in the study of fronts which are formed by deformation or horizontal shear of the geostrophic winds. It is found that the cooling has little influence on the large scales of motion, and does not greatly accelerate the formation of frontal singularity. However, the cross‐frontal flows in the vicinity of the cooling are modified strongly, with the formation of a narrow downdraught of comparable dimensions and intensity to that suggested by the experimental data. the cooling has a weak influence on all the fields at a larger distance from the front; this is discussed in the context of the large‐scale, or geostrophic, response to lines of convection.