A day in the life of a warm front

Abstract

Three numerical model simulations of cyclogenesis are compared to examine the role of boundary layer stratification in enhancing the components forcing the ageostrophic circulation of an idealized warm front. The Sawyer-Eliassen diagnostic equation is applied to examine the contributions of frictional forcing and diabatic heating, as well as confluent and shear geostrophic deformation, to forcing of the secondary circulation of a warm front. The surface heat and moisture flux distribution are varied in each case, in order to evaluate the effect on each component as well as the cyclogenesis. Results confirm previous studies that geostrophic deformation forces strong frontogenesis at the surface, and at mid-levels frontogenesis is weaker and forces primarily by latent heat release. Frontogenetical forcing is modified by small-scale frictional forcing and diabatic heating, which depend upon the surface and boundary layer processes. Although functional forcing comprises less that five percent of total forcing of the warm front, it apparently enhances frontogenesis, partially due to indirect effects on other more dominant processes. The intensity of frictional forcing is strongly dependent on the surface heat flux distribution and track of the cyclone relative to the sea-surface temperature gradient. In the absence of surface fluxes, frictional forcing is negligible. Surface forcing due the diabatic heating is frontolytical, and reduces total forcing at low levels by about twenty percent. The magnitude of the frontolytical forcing by diabatic heating at low levels is only partially dependent on the surface heat and moisture flux distribution. Even in the absence of surface heat and moisture fluxes, the frontolytical forcing persisted, suggesting problems with the cumulus parameterization or errors in the computation of this term. These results remain to be verified against an actual case study of a warm front.