The role of latent heating in warm frontogenesis



The role of latent heating from individual cloud processes associated with one part of a springtime extratropical cyclone, the warm front, is assessed using high-resolution modelling. Condensation and cloud droplet nucleation are the largest sources of latent heat along the frontal surface and together produce rates of horizontal frontogenesis that are of the same order of magnitude as the deformation and tilting terms at mid levels; however, near the surface latent heating does not cause strong frontogenesis. In the vertical, stabilization caused by condensation and cloud nucleation is nearly everywhere higher than stabilization caused by dry dynamics, and is the primary process through which high static stability is found along the frontal surface. The frontogenesis and static stability tendency equations are combined to form an expression for the frontal slope tendency. While dynamic processes lead to increases in frontal slope, latent heating often counteracts this tendency. This indicates that the direct effect of latent heating on the thermal structure of the front is to decrease the slope and in that sense weaken the warm front.