Impact of surface heterogeneity on energy imbalance: A study using LES

Abstract

Recent observational studies have described the non-closure of the energy balance when the eddy co-variance (EC) method is used for the measurements. We investigated this problem using a numerical simulation of a heterogeneous surface region. A typical daytime boundary layer was simulated, using the large eddy simulation (LES) method in which horizontal heterogeneity was imposed on the ground surface heating as a one-dimensional sinusoidal variation. This horizontal heterogeneity is expected to produce a mesoscale circulation. We decomposed the total vertical heat flux into the EC turbulent flux, the heat flux due to a mesoscale circulation (hereafter, mesoscale flux), and the "residual flux". The sum of the mesoscale flux, and residual flux accounts for the energy imbalance if we estimate the total flux only from the EC method. The numerical results demonstrated that larger amplitude of surface heating caused larger mesoscale flux, but smaller residual flux. As a result, the energy imbalance became minima at some weak amplitude of surface heating. The residual flux was caused by the turbulent organized structure (hereafter, TOS), which is a cluster of thermals moving, with a larger time scale than that of individual plumes. The larger surface heating amplitude weakened the TOS due to the following two mechanisms; (1) the TOS is organized in roll due to the strong horizontal pressure gradient, (2) the higher horizontal wind speed, parallel to the mesoscale circulation, advects the TOS faster then the ergodicity works better. The other cases with gepstrophic winds, resulted in the decrease of the energy imbalance with increasing wind velocity. © 2006, Meteorological Society of Japan.