Abstract : 2G.11
The Role of Surface Heating on Lee Waves and Rotors For a Range of Resonant Wave Modes

Victoria Smith, William Thurston, Stephen Mobbs, Andrew Ross, Simon Vosper
Institute for Atmospheric Science

Rotors, formed as a result of flow separation in the adverse pressure field of mountain lee waves can be described as low-level horizontal vortices rotating along an axis parallel to the mountain range. They form beneath the crests of trapped lee waves, and pose a substantial threat to aviation due to their region of recirculation air and severe turbulence.

The role of surface heat flux in changing the strength and character of these rotor flows has been discussed by several authors (e.g. Kuettner 1959, Doyle & Durran 2002). Here the numerical model BLASIUS has been used to conduct a high-resolution investigation into the effect of lee-slope heating on lee waves and rotors generated by flow over a single 2-dimensional mountain ridge. Unlike previous studies, this study uses upstream inflow profiles that are 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 lee waves. We investigate the impact on different lee-wave rotor regimes of a wide range of lee-slope heating, from strongly stable flows to those which become convectively unstable. Increased heating has two principal effects. The first is to decrease both rotor amplitude and strength. Where rotors are initially present beneath several lee waves, the effect of heating is larger on the second and subsequent rotors. The explanation appears to be that increased sub-grid scale turbulence erodes the rotors as surface heat flux increases. The second effect is on the wavelength of the lee wave train. Increased heat flux causes a weakening of the temperature inversion responsible for trapping the lee waves. This increases the wavelength of the lee waves for all regimes. In some cases, two competing resonant wavelengths are observed in the initial no heating simulation. Lee slope heating is then seen to encourage one mode to dominate over the other.