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Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning
Brunsell, Nathaniel A. Mechem, David B. Anderson, M. C.
Brunsell, Nathaniel A.
Mechem, David B.
Anderson, M. C.
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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 50Wm−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.
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Date
2011-04-11
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European Geosciences Union
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Brunsell, N. A., D. B. Mechem and M. C. Anderson: 2011, Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning. Atmospheric Chemistry and Physics, 11, 3403-3416. http://dx.doi.org/10.5194/acp-11-3403-2011