Airborne radar observations of breaking waves/rotors in the lee of the Medicine Bow Mountains in SE Wyoming, USA

The NASA06 field campaign occurred during Jan/Feb 2006 in SE Wyoming, with a primary objective to study the microphysics associated with orographic precipitation formation over the Medicine Bow Mountains. The Medicine Bow Mountains are a dome-shaped range in SE Wyoming that rise 1000 m above the surrounding terrain, with a slight elongation of the highest terrain running along a SSW to NNE axis. As part of the measurement campaign, the University of Wyoming King Air (UWKA), highly instrumented for collecting both in situ measurements and remote sensing (with the airborne Wyoming Cloud Radar, WCR), flew repeated passes oriented along the wind direction at altitudes as low as 700 m above the highest terrain. Precipitation as well as lofting of snow from the surface provides scatterers for the WCR measurements. The multiple-antenna configuration allows for dual-Doppler synthesis and corresponding retrieval of the two dimensional flow field in the vertical plane beneath the aircraft.

On two occasions during the campaign (26 Jan and 5 Feb) the UWKA and WCR captured measurements of rotor/breaking wave events in the valley 12 to 15 km downstream of the highest peaks. For both cases, in situ data collected from the UWKA at roughly 3000 m above the valley floor reveal well-developed waves with maximum amplitudes of 20 m s-1 on 26 Jan and 10 m s-1 on 5 Feb. On both days the primary wave had a half wavelength of approximately 6 km. While the 26 Jan case appears to be much stronger, measurements from the UWKA only capture the beginning of the event. The 5 Feb case, somewhat weaker, was captured during 4 passes over one hour.

Evident on both days are strong down-slope winds in excess of 30 m s-1 within 200 m of the surface. Also on both days there exists a separation of flow that occurs on the windward side of the valley, with some evidence suggesting a reversal of flow below the crest near the surface. Numerous smaller eddies, or sub-rotors, are present near the surface and downwind of the crest. In this study we will examine the spatial and temporal scales of both the large rotor/wave-breaking events as well as the smaller scale sub-rotor features. For both days we will present analyses of the turbulence observed at magnitude and scales large enough to be resolved by the radar measurements, roughly 50 m. The 5 Feb case allows for further examination of the temporal evolution over the one hour observation period. For the 26 Jan case we will describe the initial development of the breaking wave event. Finally, we will describe very preliminary modeling efforts for these two cases. A companion paper (submitted as a poster presentation) will describe the basic meteorology and provide background for this analysis.