Role of the dispersive fluxes in the Surface Energy Balance over heterogeneous surfaces, a LES study

The surface energy balance links together the energy fluxes at the surface. It has been shown that energy budget does not close on time-scales shorter than several hours (Wilson et al., 2002). This problem has been extensively studied: past investigations looked at storage processes (Lindroth et al., 2010), advection (Kochendorfer and Paw, 2011), spatial variability (Mauder et al., 2010), issues linked to the footprint of the measurement (Schmid, 1997), or meteorological conditions (Franssen et al., 2010). Only in rare occasions, the budget has been closed within satisfactory margins.

The dispersive fluxes are a spatial analogy to the standard Reynolds decomposition using time averages. They provide spatial correlation of the time-averaged fluxes and allow to quantify exchange of energy that are not governed by turbulent motions. This framework has been extensively used to study canopy flows (Finnigan, 2000).

This contribution evaluates the role of dispersive fluxes on the surface energy balance using large-eddy simulations of the atmospheric boundary layer in neutral conditions over heterogeneous surface conditions. The surfaces studied are composed of patches of different sizes and different temperatures. Dispersive fluxes are compared to the turbulent fluxes and show a non-negligible contribution to the transport of heat. Hence, results suggest that the dispersive fluxes provide a representation of surface heterogeneity that otherwise would not appear in the advection process.