Idealised modelling of landfalling cold fronts

2017-01-16T22:55:49Z (GMT) by Muir, Leslie Charles
Cold fronts in both hemispheres commonly develop over the ocean before making landfall on the upstream coastlines of the continents. As the fronts undergo landfall they interact with sharp gradients in heating at the coastline, diurnal cycles of heating inland of the coastline, and on some continents orography. The effects of land surfaces, sea surfaces, coastlines and orography 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 updrafts. 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 point the cro::;s-front flow vani::;hes. This equilibrium point moves towards the warm air as the surface roughness is increased because the cross-front ageostrophic wind also increases with increasing surface roughness. Surface sensible heating weakens and slows the front::; 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 windspeed in the boundary layer. When the deformation field is allowed to translate, the fronts advance across the ocean towards land. If the fronts reached the coastline between mid-morning and late afternoon, the daytime heating over the land oppose::; the on::;hore advection of cold air, retarding the fronts at the coastline. If the fronts reached the coastline in the evening or early morning, they advance onshore relatively unimpeded. Adding a bell-shaped mountain results in weakening on the upwind slope and a strengthening on the lee slope, with both insulated and free-slip surfaces. The effect of turbulent mixing is to give the fronts a much more constant speed as they cross the mountain. A transition from subcritical flow to supercritical flow is found when increasing the mountain height or decreasing the mountain width. Nocturnal cooling has a similar response of weakening on the upwind slope and strengthening on the lee slope. Surface sensible heating weakens the front on both sides of the mountain as increased turbulent mixing overwhelms the effect of the mountain circulation. Increased upstream strengthening and slowing occurs when a coastline is added on the upstream slope of the mountain in the absence of sensible heating. In the combination of sensible heating, orography and a coastline similar results are found to a combination of numerical experiments with a coastline, sellsible heating and no orography and numerical experiments with no coastline, sensible heating and orography.