Effects of the earth's rotation on Boulder downslope windstorm

The 11 January 1972 Boulder Windstorm has been the subject of numerous theoretical and numerical studies. However, most studies are based on an irrotational fluid on a free surface boundary, in which hydraulic jump propagates away with time. Such results are significantly different from observation and the numerical simulation over a rigid surface. In this study, we have performed sensitivity tests to investigate the effect of the Coriolis force on the flow over the Colorado Front Range with the NTU-Purdue nonhydrostatic numerical model.

Without the Coriolis force, the numerical simulations show that the maximum wind at the height of 25 m decreases with time; from 50 m/s after 2 h integration to around 30 m/s or less after 4 h or longer integration. On the other hand, the maximum wind remains around 50-55 m/s after long integration when the Coriolis force is included. The results also indicate that rotation will induce vertical coherence, organizing the flow into vortex columns aligned with the rotation vector. This is another manifestation of the Taylor-Proudman theorem. Meanwhile, stable stratification inhibits vertical flows and tends to decouple horizontal layers, favoring pancake-like vortices with large vertical shear. The relative influence of these two competing effects can be gauged by the Rossby deformation radius, Ld, defined by:

Ld = NH/f = (U/fL)/(U/NH)L = (Ro/Fr)L,

where L is horizontal length scale, U the mean wind, H the height of the mountain, N the Brunt-Väisälä frequency. The Rossby number and Froude number are defined as Ro = U/fL and Fr = U/NH. The value of Ld varies from near zero at strong turbulent-mixing region to more than 100 km in the upper layer away from the mountain. Hence, rotation enhances the downward motion on the highly turbulence area near the mountain peak, which creates a strong mesolow to the downstream region due to subsidence warming. Consequently, it also generates a stronger downslope wind due to a larger pressure gradient. Hence, a hydraulic jump is generated next to the downslope wind. The secondary circulation is further influenced by the Ekman pumping in a rotational flow. Therefore, the rotation makes the flow significantly different from that without rotation.