The role of atmospheric boundary layer-surface interactions on the development of coastal fronts

摘要

Frictional convergence and thermal difference between land and sea surfaceare the two surface conditions that govern the intensity and evolution of acoastal front. By means of the mesoscale model MM5, we investigate theinfluence of these two processes on wind patterns, temperature andprecipitation amounts, associated with a coastal front, observed on the westcoast of The Netherlands in the night between 12 and 13 August 2004. Themesoscale model MM5 is further compared with available observations and theresults of two operational models (ECMWF and HIRLAM). HIRLAM is not capableto reproduce the coastal front, whereas ECMWF and MM5 both calculateprecipitation for the coastal region. The precipitation pattern, calculatedby MM5, agrees satisfactorily with the accumulated radar image. The failureof HIRLAM is mainly due to a different stream pattern at the surface andconsequently, a different behaviour of the frictional convergence at thecoastline.The sensitivity analysis of frictional convergence is carried out with theMM5 model, by varying land surface roughness length (z₀). For thesensitivity analysis of thermal difference between sea and land surface, wechanged the sea surface temperature (SST). Increasing surface roughnessimplies stronger convergence near the surface and consequently strongerupward motions and intensification of the development of the coastal front.Setting land surface roughness equal to the sea surface roughness means anelimination of frictional convergence and results in a diminishing coastalfront structure of the precipitation pattern. The simulation with a high SSTproduces much precipitation above the sea, but less precipitation in thecoastal area above land. A small increment of the SST results in largerprecipitation amounts above the sea; above land increments are calculated forareas near the coast. A decrease of the SST shifts the precipitation maximainland, although the precipitation amounts diminish. In the situation understudy, frictional convergence is the key process that enhances the coastalfront intensity. A thermal difference between land and sea equal to zerostill yields the development of the coastal front. A lower SST than landsurface temperature generates a reversed coastal front.This study emphasizes the importance of accurate prescription of surfaceconditions as input of the numerical weather prediction model to improvecoastal front predictability.