Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning

Authors: BRUNSELL, NATE - University Of Kansas; MECHEM, D - University Of Kansas; Anderson, Martha

Submitted to: Atmospheric Chemistry and Physics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/6/2011
Publication Date: 4/11/2011
Citation: Brunsell, N., Mechem, D.B., Anderson, M.C. 2011. Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning. Atmospheric Chemistry and Physics. 11:3403-3416.

Interpretive Summary: Solar and atmospheric radiation at the land-surface is partitioned into various components: some is used to evaporate water (latent heat flux), some is used to heat the air above the surface (sensible heat flux), and some goes into heating the soil (ground heat flux). Understanding mechanisms determining the partitioning between sensible and latent heat flux is critical for better modeling and predicting water, energy and carbon cycles – both today and under conditions of climate variability. Vegetation cover amount and soil moisture are considered to be dominant factors in determining how available energy is partitioned. This paper describes an experiment investigating how the spatial scale of heterogeneity in surface vegetation cover and soil moisture affects flux partitioning. A set of Large Eddy Simulations was conducted, artificially modifying scale of variation in vegetation cover and soil moisture. For very small and very large scales of heterogeneity (e.g., more homogeneous landscapes), more of the energy was partitioned into latent heat (evapotranspiration) in comparison with the base case. For moderate scales of heterogeneity (strong blockiness in surface conditions), sensible heat is favored. This is likely because the surface variations tend to set up stronger temperature gradients, enhancing convective heat fluxes.

Technical 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 5 Campaign (CLASIC) field site for June 3, 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 12.8 km 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 radiation. Rather, there is a pronounced change in the partitioning of the surface energy between sensible and latent heat flux. The sensible heat flux is dominant for intermediate 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 important. The simulations showed approximately 50 Wm-2 difference in the spatially averaged latent heat flux. The results reflect a general decrease of the Bowen ratio as the surface conditions transition from heterogeneous to homogeneous. Air temperature is less sensitive to variations in surface heterogeneity than water vapor, which implies that the role of surface heterogeneity may be to maximize convective heat fluxes through modifying and maintaining local temperature gradients. More homogeneous surface conditions (i.e. smaller length scales), on the other hand, tend to maximize latent heat flux. The intermediate scale (1600 m) this does not hold, and is a more complicated interaction of scales. Scalar vertical profiles respond predictably to the partitioning of surface energy. Fourier spectra of the vertical wind speed, air temperature and specific humidity ( ˜ w, ˜ T and ˜q) and associated cospectra ( ˜ w ˜ T, ˜ w˜q and ˜ T ˜q), however, are insensitive to the length scale of surface heterogeneity, but the near surface spectra are sensitive to the mean wind speed.