Airborne radar observations of flow and microphysical structure over mountainous terrain

Airborne Doppler radar data collected during several flights in early 2006 provide us with an unprecedented, high-resolution look at the interaction between boundary layer and the free atmosphere as well as providing a look at the microphysical structure of clouds and precipitation adjacent to the terrain. The data examined in this paper were collected using the Wyoming Cloud Radar (WCR) and in situ probes onboard the University of Wyoming King Air (UWKA) research aircraft. The WCR is a 95GHz Doppler radar, which uses a combination of upward- and downward-looking antennas thereby allowing two-dimensional velocities in a vertical plane below the aircraft combined with quasi-vertical velocities above the aircraft.

Several cases flown between January 18th and February 10th, 2006 are considered here. These cases consist of a series of legs over the Snowy Range to the west of Laramie WY. Flight legs extended ~40km from the Saratoga valley west to the Laramie valley to the west of Centennial, WY valley and were aligned along the (vertically-averaged) mean wind direction.

The cases examined here consisted of a completely glaciated, upper-level cloud layer (or layers) coupled with a mixed-phase cloud layer at the top of the boundary layer. Most of these cases appear consistent with supercritical flow evidenced by: the downward slope of the upper cloud layer upwind of the highest terrain and a well-defined hydraulic jump in the boundary layer cloud downwind of the highest terrain. The presence of continuous radar echoes (at least over the highest terrain) between the upper and lower cloud layers allows us to visualize the velocity structure within the highly stratified upper cloud layer and within the turbulent boundary layer as well as the interface between these two layers. The upper cloud layer tended to evolve rapidly during the course of a flight (~3 hours), however it is not clear that the boundary-layer cloud evolved at a similar rate.

Liquid water contents as high as 0.3 gm-3 were encountered on at least one flight, however in other cases liquid water contents were ?0.1 gm-3 and, on occasion, the UWKA was unable to fly low enough to penetrate the mixed-phase cloud layer. The high radar reflectivity in the boundary-layer cloud provides unambiguous evidence of the presence of ice within this layer. The presence of detectible echoes in the lower cloud layer upwind of where the echoes from the upper and boundary-layer clouds merge raises interesting questions the relative importance of a ‘seeder-feeder' mechanism linking the two cloud layers.