SPATIAL VARIABILITY OF TURBULENT FLUXES AND ROUGHNESS LENGTHS IN HAPEX-MOBILHY

Surface-based and aircraft measured fluxes over the heterogeneous surf ace in HAPEX-MOBILHY are analyzed for the ten flight days when cloud c over above the boundary layer was minimal. The fair-weather climatolog y of the spatial variation of surface fluxes is estimated to provide a n assessment of the generality of previous case studies appearing in t he literature. For the 10-day averages, greater heating over the fores t generates a forest breeze which leads to rising motion and a modest increase of boundary-layer cloud cover at the forest edge. The exchang e coefficients and effective roughness lengths are computed for local averages (15 km scale) and for regional averages (100 km scale) intend ed to represent a range of grid sizes in numerical models of the atmos phere. The effective roughness length for momentum over the mixed agri cultural region for both scales is on the order of 1 m, apparently due to bluff roughness effects associated with scattered trees, edges of small woods and other obstacles. This roughness length value is an ord er of magnitude larger than values used in numerical models for the sa me region, which are based on the dominant vegetation type. The spatia lly varying effective roughness length for heat is computed for use in those models which use surface radiation temperature to estimate surf ace heat flux. The effective roughness lengths for heat are found to b e smaller than those typically used in numerical models of the atmosph ere.