The Impact of Step Orography on Flow in the Eta Model: Two Contrasting Examples

Simulations were performed using the Eta Model with its eta vertical coordinate and stepwise treatment of terrain, and with a substitution of the terrain-following sigma vertical coordinate to investigate the impact of step orography 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 separation on the lee side of the mountains, previously shown to occur in idealized two-dimensional Eta simulations, was also apparent in these real data cases, even for the downslope wind event. The step orography resulted in a substantial underestimate of wind speeds to the lee of the Rockies during the windstorm. Near the surface, both the eta and sigma simulations of the Taiwan blocking event were comparable. For both events, the use of step orography resulted in much weaker mountain waves than occurred when the sigma vertical coordinate was used. Localized vertical velocity perturbations associated directly with the step corners were minor for these cases.