Effects of surface heat fluxes on downslope wind storms with elevated inversions

The influence of diurnal variations of surface heat fluxes on downslope windstorms are explored using observations and a model. Observations made on the Falkland Islands (Mobbs et. al, 2005) show a preference for windstorms to occur downstream of a nearly 2-D ridge in early morning. It is shown that most windstorms on the Falkland Islands generally have a short jet length, while those windstorms with a fairly long lee extent are much rarer. Correlations of the surface wind data with soundings taken at synoptic times reveal that the diurnal variation of downslope windstorm behavior is significantly less for those windstorms which occur in the presence of strong inversions aloft in comparison with windstorms that occur without the presence of a strong inversion aloft. A case study is presented showing one particular event that is characterized by a significant extension of the lee jet during the transition from night to day. Large-eddy simulations of a simplified 2-D ridge with a simplified surface heating and cooling regime similar to diurnal heat fluxes, show a propensity for downslope windstorms to occur in the presence of surface cooling. For low inversions, surface heating is shown to mix out the lee jet, resulting in a significantly shortened downstream extension. This is primarily a result of heating on the lee slope, while transport of turbulence generated upstream of the ridge over the ridge is fairly small. For the medium height inversion case, upstream surface heating and turbulent transport over the ridge play a crucial role in preventing communication between the topographic disturbance and the inversion layer. With a switch to surface cooling, restratification of the surface boundary layer reconnects the mountain and inversion so that instabilities may spontaneously grow and form a stagnation zone with significant extent. A lee jet is forced beneath the stagnation zone leading to a typical downslope wind event.