A Case Study of Nonstationary Boundary-Layer Separation and Rotor Formation

Stefano Serafin and Lukas Strauss
Department of Meteorology and Geophysics, University of Vienna

On January 26th 2006, the University of Wyoming King Aircraft (UWKA) documented the occurrence of a wave-induced boundary-layer separation (BLS) event in the lee of the Medicine Bow Range (Wyoming). Remote sensing measurements with the dual-Doppler Wyoming Cloud Radar (WCR) aboard UWKA indicate strong wave activity, downslope winds in excess of 30 m/s within 200 m above the ground and near-surface flow reversal in the lee of the mountain range. Owing to its fine resolution, the radar is also able to capture small-scale coherent vortical structures (subrotors) embedded within the main rotor zone.

A distinctive feature of the observed phenomenon is its unsteadiness, as demonstrated by the BLS line moving upstream for about 8 km in approximately half an hour. Mesoscale simulations with the WRF model at a maximum horizontal grid spacing of 400 m reveal the dynamic forcing leading to this rapid evolution. The upstream motion of the BLS line and of the related rotor appear to depend on the decreasing nonlinearity of the impinging flow, which causes the transition from a flow regime characterized by low-level wave breaking, to another one where trapped lee waves form as a consequence of wave reflection at an elevated neutral level. The observed upstream drift of the rotor is shown to be dynamically consistent with the cessation of wave breaking. The overall evolution of the phenomenon displays striking analogies with documented unsteady Bora events, observed in the Northern Adriatic Sea.

Model simulations are verified against airborne measurements along a number of cross-mountain flight legs, as well as against surface data. Also, a quantification of turbulence intensity in this BLS event, using both high-frequency in situ and radar measurements, is attempted. Given the complex topography and the limited period of time of the observations, measuring turbulence proves to be a challenging task. Preliminary estimates of turbulent kinetic energy and eddy-dissipation rate along the flight trajectory will be presented.

Tuesday August 28, 2012, 2:00 PM – 3:00PM

Refreshments 1:45 PM

NCAR-Foothills Laboratory

3450 Mitchell Lane

Bldg 2 Small Seminar Room (Rm1001)

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Start: 
Wednesday, August 15, 2012
End: 
Tuesday, August 28, 2012