The impact of step orography on flow in the Eta Model: Two contrasting examples

Simulations were performed using the Eta Model with its era vertical coordi nate and stepwise treatment of terrain, and with a substitution of the terr ain-following sigma vertical coordinate to investigate the impact of step o rography on flow near high mountains. Two different cases were simulated: ( i) a downslope windstorm along the Front Range of the Rocky Mountains, and (ii) stably stratified flow blocked by high mountains in Taiwan. Flow separ ation on the lee side of the mountains, previously shown to occur in ideali zed two-dimensional Era simulations, was also apparent in these real data c ases, even for the downslope wind event. The step orography resulted in a s ubstantial underestimate of wind speeds to the lee of the Rockies during th e windstorm. Near the surface, both the era and sigma simulations of the Ta iwan blocking event were comparable. For both events, the use of step orogr aphy resulted in much weaker mountain waves than occurred when the sigma ve rtical coordinate was used. Localized vertical velocity perturbations assoc iated directly with the step corners were minor for these cases.