Strong downslope airflow in and near mountainous terrain is capable of damaging surface structures and creating very dangerous flight conditions. In addition to numerical model simulations, studies of downslope wind storms have primarily relied on in situ measurements by research aircraft and remote sensing observations by ground-based lidar systems. Observations by conventional weather surveillance radars have been more difficult to incorporate in these kinds of studies because the radars require hydrometeor targets in the flow and they may suffer from serious ground clutter problems in complex terrain.

The present study examines an unusual east-wind downslope event at Mt. Washington, New Hampshire, using data from a scanning 35-GHz Doppler cloud radar and a scanning 9-GHz Doppler precipitation radar located near the western base of the mountain. The high sensitivity of these short-wavelength radars, the close ranges involved (< 4 km), and the presence of plentiful hydrometeors in the flow combined to allowed the downslope layer to be detected, delineated, and measured with excellent detail. Although the winds were only of modest strength (12 m/s), the case is informative because the radar observations are some of the finest available of downslope flow in terms of spatial (37 m), temporal (60 s), and velocity resolution (0.1 m/s). The downslope flow extended along and above the mountain's western slope in a laminar-like layer that was overrun by cloud-filled westerly flow. The downslope layer shrank from 1.0 to 0.1 km depth in 8 hours. As the laminar flow approached the base of the mountain and the radar site, it broke into waves, hydraulic jumps, rotors, and chaotic turbulence features that were clearly revealed by the 35-GHz cloud radar's RHI scans. Vertical profiles of the horizontal wind vector above the radar site were provided at 15-minute intervals using velocity-azimuth-display (VAD) analysis of PPI scans by the 9-GHz radar. In-situ measurements from the Mount Washington Observatory on the mountain's summit, from radiosondes launched at the radar site, and from regional synoptic meteorological stations provide important additional information for understanding factors that produced and extinguished the down-flowing winds. The radar and supplemental data will be presented to describe the characteristics of the downslope flow and its environment.