Mesoscale heat transport over complex terrain by slope winds - A conceptual model and numerical simulations

Abstract

Vertical heat fluxes induced by mesoscale thermally driven circulations may contribute significantly to the subgrid-scale fluxes in large-scale models (e.g., general circulation models). However, they are not considered in these models yet. To gain insight into the importance and possible parameterisation of the mesoscale flux associated with slope winds, an analytical (conceptual) model is developed to describe the relationship between the mesoscale heat flux and atmospheric and land-surface characteristics. The analytical model allows us to evaluate the mesoscale flux induced by slope winds from only a few profile measurements within a domain. To validate the analytical model the resulting heat flux profiles are compared to profiles of highly resolved wind and temperature fields obtained by simulations with a mesoscale numerical model. With no or moderate synoptic wind the mesoscale heat flux generated by the slope wind circulation may be as large as, or even larger than, the turbulent fluxes at the same height. At altitudes lower than the crest of the hills the mesoscale flux is always positive (upward). Generally it causes cooling within the boundary layer and heating above. Despite the simplifications made to derive the analytical model, it reproduces the profiles of the mesoscale flux quite well. According to the analytical model, the mesoscale heat flux is governed by the temperature deviation at the slope surface, the depth of the slope-wind layer, the large-scale lapse rate, and the wavelength of the topographical features.