Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning 1Dept. of Geography, University of Kansas, Lawrence, KS, USA
11 Apr 2011
2Hydrology and Remote Sensing Laboratory, USDA. Beltsville, MD, USA
Received: 22 June 2010 – Published in Atmos. Chem. Phys. Discuss.: 23 July 2010 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
Campaign (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 12.8 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
Revised: 22 March 2011 – Accepted: 06 April 2011 – Published: 11 April 2011
Citation: Brunsell, N. A., Mechem, D. B., and Anderson, M. C.: Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning, Atmos. Chem. Phys., 11, 3403-3416, doi:10.5194/acp-11-3403-2011, 2011.