Structure of horizontal and vertical rotors as observed by Doppler lidar

Over the past two decades Doppler lidar has measured the wind structure of atmospheric rotors in several field projects, including the Mountain Aviation Hazards (MAH) experiment near Colorado Springs, Colorado, the Mesoscale Alpine Programme (MAP) gap-flow project near Innsbruck, Austria, the Cooperative Atmosphere-Surface Exchange Study campaign of October 1999 (CASES-99), and several windstorms or rotor flows of opportunity over the Front Range near Boulder, Colorado. This paper will present a survey of rotor structures that have been observed during these projects. One of the more surprising results of the many field measurement opportunities has been the unexpectedly high frequency of hydraulic jumplike rotor structures, which have been referred to as Type 2 rotors (in contrast to trapped lee waves, which have been called Type 1 rotors). Classic jumplike structure resulting from 30 m/s flow over the Front Range has been observed, including boundary-layer separation, flow reversals of 8 m/s (with a horizontal dimension of ~10 km) downstream of the mountains, and 1-2-km subrotors superimposed on the main rotor (which have been noted in recent numerical studies). Similar structures observed during MAH and MAP will be presented. One of the more interesting occurrences of this kind of rotor was during CASES-99 on a light-wind night. Jumplike structure was observed in ~3-4 m/s drainage flows in shallow gullies in the southeastern Kansas plains.

Other interesting structures noted in the Front Range studies included a train of apparently breaking shear-instability waves near the tropopause. The terrain near Colorado Springs is very three dimensional, leading to a wide variety of both horizontal and vertical rotor structures. Vertical-slice scans revealed horizontal rotors based at the surface and aloft. In one case, an elevated rotor was measured by a sequence of vertical-slice scans, taken at intervals of 5 degrees azimuth, and shown to have a cigar shape with a horizontal extent of ~ 8 km. Rotors with primarily vertical vorticity were found as eddies of flow reversal downwind of major obstacles, such as Cheyenne Mountain. An especially intriguing rotor in a vertical cross section to the west resembled a layer of high-momentum flow rolled up into a nearly spiral-shape, as has sometimes been seen in laboratory flows.