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Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning

by N. A. Brunsell, D. B. Mechem, M. C. Anderson
Atmospheric Chemistry and Physics ()

Abstract

The role of land-atmosphere interactions under heterogeneous surface conditions is investigated in order to identify mechanisms responsible for altering surface heat and moisture fluxes. Twelve coupled land surface – large eddy simulation scenarios with four different length scales of surface variability under three different horizontal wind speeds are used in the analysis. The base case uses Landsat ETM imagery over the Cloud Land Surface Interaction Cam- paign (CLASIC) field site for 3 June 2007. Using wavelets, the surface fields are band-pass filtered in order to maintain the spatial mean and variances to length scales of 200 m, 1600 m, and 12.8km as lower boundary conditions to the model (approximately 0.25, 1.2 and 9.5 times boundary layer height). The simulations exhibit little variation in net ra- diation. Rather, there is a pronounced change in the par- titioning of the surface energy between sensible and latent heat flux. The sensible heat flux is dominant for intermedi- ate surface length scales. For smaller and larger scales of surface heterogeneity, which can be viewed as being more homogeneous, the latent heat flux becomes increasingly im- portant. The simulations showed approximately 50Wm−2 difference in the spatially averaged latent heat flux. The re- sults reflect a general decrease of the Bowen ratio as the surface conditions transition fromheterogeneous to homoge- neous. Air temperature is less sensitive to variations in sur- face heterogeneity than water vapor, which implies that the role of surface heterogeneity may be to maximize convective heat fluxes through modifying and maintaining local temper- ature gradients. More homogeneous surface conditions (i.e. smaller length scales), on the other hand, tend to maximize latent heat flux. The intermediate scale (1600m) this does not hold, and is a more complicated interaction of scales.

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