The roles of surface heating and cooling in rotor formation

The role of surface heat flux in changing the strength and character of mountain-induced rotor flows has been discussed by several authors (e.g. Kuettner 1959, Doyle & Durran 2002). Here the numerical model Blasius has been used to investigate the effect of lee-slope heating and cooling on rotors generated by a single mountain ridge. This study adds two features to previous investigations. Firstly, the upstream inflow profile is entirely consistent with the surface boundary conditions upwind of and over the hill. It has not been necessary in this study to artificially modify or force the inflow to create the rotors. Secondly, we investigate a wide range of lee-slope cooling and heating, from strongly stable flows to flows which become convectively unstable in the lee of the mountains. We find that the effect of lee-slope surface heating is to decrease the rotor strength. The effect is larger on the second and subsequent downstream rotors than it is on the first rotor. The physical explanation of the effect on the rotors appears to be that increased sub-grid scale turbulence erodes the rotors as the surface heat flux increases.