Idealized modelling of landfalling cold fronts

The effects of land surfaces, sea surfaces and coastlines on the dynamics of cold fronts are examined in an idealised two-dimensional confluent deformation model of frontogenesis. When the deformation field is stationary, increasing the surface roughness weakens the fronts, while increasing the cross-front ageostrophic wind and frontal updraughts. The frontogenetic effect of deformation balances the frontolytic effect of turbulent mixing, resulting in near-steady-state fronts. In steady state, the surface fronts are located near the point at which the cross-front flow vanishes. This equilibrium point moves towards the warm air as the surface roughness is increased because the cross-front ageostophic wind also increases with increasing surface roughness. Surface sensible heating weakens and slows the fronts during the day, whereas surface sensible cooling strengthens and accelerates them at night. Adding a coastline to the model results in very strong gravity-current-like fronts in the afternoon with a relative flow of cool air towards their leading edge. Above the boundary layer, the synoptic fronts remain unaffected by the coastline. As the turbulent mixing weakens in the late afternoon, the coastal fronts surge inland, advancing faster than the wind speed in the boundary layer. When the deformation field is allowed to translate, the fronts advance across the ocean towards land. If the fronts reach the coastline between mid-morning and late afternoon, the daytime heating over the land opposes the onshore advection of cold air, retarding the fronts at the coastline. If the fronts reach the coastline in the evening or early morning, they advance onshore relatively unimpeded. Hence, the fronts arrive onshore preferentially in the late afternoon or evening.