Mountain-induced Turbulence: New Insights from Airborne In Situ and Doppler Radar Measurements

In this work, turbulence induced by the presence of mountainous terrain is studied using airborne in situ and cloud radar measurements from the NASA Orographic Clouds Experiment (NASA06), conducted over the Medicine Bow Mountains in SE Wyoming in 2006. During NASA06, two complex mountain flow cases were documented by the University of Wyoming King Air and the Wyoming Cloud Radar flown aboard the research aircraft. The available in situ and remotely-sensed data offer the opportunity to study a variety of turbulent processes over mountainous terrain, ranging from gravity-wave breaking to atmospheric rotors.

Our analysis aims at describing the spatial distribution of turbulence and its intensity. The turbulence indicators we use are the variance of vertical velocity (VAR) and the eddy-dissipation rate (EDR). In order to obtain quantitative estimates of turbulence intensity from the cloud radar, a thorough analysis of uncertainties in the Doppler wind retrieval has proved essential. For example, the pulse-volume averaging effect of the radar beam and the contamination of Doppler velocity by the horizontal wind due to deviations in aircraft attitude have to be taken into account. The results of this analysis indicate that 20% accuracy or better in quantitative estimates can be achieved in regions of moderate to severe turbulence in the lee of the mountains. On the other hand, due to compounding uncertainties, only qualitative estimates of turbulence intensity can be obtained outside of the most turbulent regions.

Two NASA06 events exhibiting high-amplitude mountain waves have been analyzed. “Moderate” turbulence was found in the turbulent breakdown of a large gravity wave over the mountain top, with VAR and EDR reaching 4.8 m² s^-2 and 0.25 m^2/3 s^-1, respectively. Within the rotor circulations, instead, turbulence of the “severe” category was detected, with VAR and EDR ranging between 7.8–16.4 m² s^-2 and 0.50–0.77 m^2/3 s^-1. A unique result of this study is the quasi-instantaneous, two-dimensional display of the spatial distribution of turbulence in the interior of atmospheric rotors, provided by the radar-derived turbulence fields.