Frontogenesis in the Presence of Surface Heating

Abstract

Observations indicate that frontal zones that form in the presence of strong surface sensible heating have a structure that is markedly different from those that form adiabatically. These differences include: a highly asymmetric low-level jet, the presence of an isothermal pool of warm air in the vicinity of the jet, an enhancement of the baroclinicity on the cold side of the jet, and an extension of the region of low Richardson number into the cold air. In order to investigate the dynamics responsible for these differences, a Lagrangian model was developed that allowed for the incorporation of a surface sensible heat flux parameterization into the semigeostrophic theory of deformation induced frontogenesis. The model was tested by comparison against the analytic solution that exists in the special case of adiabatic frontogenesis in a constant potential vorticity fluid. Having established the validity of the model, the effects that surface sensible heating has on frontogenesis in a nonuniform potential vorticity fluid were investigated. It is shown that the frontal zones generated by means of the model have the same structural characteristics as those of the observed fronts which formed in the presence of strong surface sensible heating. It is also shown that surface heating can lead to both an increase in the maximum baroclinicity and a reduction in the minimum stratification associated with a frontal zone. Both of these modifications can be expected to result in an increase in the rate at which instabilities along the front can develop. This then provides for an indirect mechanism by which surface heating contributes to the development of cyclonic disturbances by modifying the background flow rather than by direct interaction with the growing cyclone.