The impact of the surface heterogeneity on the energy imbalance problem using LES

Heterogeneous surface conditions modify the flow structure of the convective boundary layer (CBL). We investigated this impact focusing on the net vertical heat transport of mesoscale circulations invoked by surface heterogeneity and on the spatial representativeness of eddy-covariance (EC) point observations. The typical daytime boundary layer was simulated using large eddy simulation (LES), where horizontal heterogeneity was imposed on the ground surface heating as one-dimensional sinusoidal variation. The effect of variation of the wavelength and amplitude on the vertical heat transport was examined.

The net vertical heat transport due to mesoscale circulations is analyzed using a phase averaging method. At a measurement height of 100 m above the ground, the mesoscale transport reaches more than 25 % of the total domain averaged heat flux if the ground surface heatwave amplitude is large (80 % of the mean flux) and the wavelength is larger than 2 km. This vertical heat transport process is mostly neglected in the EC method so that this result becomes one of the significant reasons for the shortage of the surface energy budget, the so-called energy imbalance problem.

Additionally, we evaluated the horizontally averaged surface energy budget excluding the effect of the mesoscale heat transport at the specific height. As the result, our simulated EC point measurements still underestimate this energy budget even excluding the mesoscale mean flow contribution. However, this deficit gets close to zero when the strong mesoscale circulation develops. This underestimate can be strongly attributed to the turbulent organized structure (TOS) developed perpendicular to the primary mesoscale flow structure, which are weakened by the strong horizontal heterogeneity then this underestimates becomes little.