NONLINEAR NUMERICAL-MODEL PREDICTIONS OF FLOW OVER AN ISOLATED HILL OF MODERATE SLOPE

Non-linear model simulations of atmospheric boundary-layer flow over t he hill called Blashaval have been compared with observations and line ar model predictions. Previous studies have shown that linear models c an give good predictions of wind speed at the summit and on the upwind slopes of Blashaval. The non-linear model provided wind speed predict ions of similar accuracy when compared with the mean observed values a t these locations. The published experimental data showed that on the lee-slope the wind speeds at 8m were reduced to approximately 10% of t heir upstream value at the same height. This was associated with an 18 0 degrees change in wind direction compared with the upstream flow, su ggesting that flow separation had occurred. The non-linear model predi ctions of lee-slope wind speed, when used with high-resolution topogra phy data, were significantly better than Linear model predictions. How ever, the non-linear model predicted lee-slope wind speeds that were s till stronger than observed. The non-linear model simulated flow separ ation more readily with a 1 1/2-order turbulence closure than with a f irst-order, mixing-length closure. The configuration of the non-linear model that gave best agreement with observations predicted an 8m lee- slope wind speed that was around 50% of the upstream value.