Semigeostrophic moist frontogenesis in a Lagrangian model

Abstract A geometric technique for solving the Lagrangian conservation form of the semigeostrophic equations is extended to study moist frontogenesis. Model elements are required to conserve θ E and the frontogenesis is forced by a deformation flow. The boundary-layer elements ahead of the surface front are conditionally unstable but initially unsaturated. As they are forced to ascend at the surface front these elements saturate and appear at a new equilibrium position after implicitly releasing latent heat. This leads to the formation of a ‘lens’ of moist air at mid-levels some 200 km behind the surface front. In terms of dry potential vorticity, a positive anomaly has been created in the region where the elements saturate and a corresponding negative anomaly is created at the site of the lens. The inclusion of cooling using a simple model to mimic evaporational effects is found to have a significant effect on the overall evolution of the moist front.

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