Measuring turbulence from airborne in situ and radar data recorded during an event of wave-induced boundary-layer separation

Wave-induced boundary-layer separation (BLS) results from the adverse-pressure gradient forces exerted on the atmospheric boundary layer by internal gravity waves in flow over orography. BLS has received significant attention in recent years, particularly so, because it is a key ingredient in the formation of atmospheric rotors. Traditionally depicted as horizontal eddies in the lee of mountain ranges, rotors develop from the interaction between mountain waves and the atmospheric boundary layer. Our study is based on an observationally documented case of wave-induced BLS, which occurred on 26 January 2006 in the lee of the Medicine Bow Mountains in SE Wyoming. During the event, in situ measurements by the University of Wyoming King Air (UWKA) research aircraft were recorded at a frequency of 25 Hz. Wyoming Cloud Radar (WCR), carried aboard UWKA, measured Doppler vertical wind velocities at multiple levels within the boundary layer at a frequency of up to 20 Hz.

The aim of this work is to compute quantitative measures of turbulence intensity and structure during the observed boundary-layer separation and rotor formation event. Given the complex topography and the limited period of time of the observations, measuring turbulence from airborne in situ and radar data proves to be a challenging task. Some assumptions traditionally taken in turbulence studies (e.g., spatial homogeneity of turbulence or separation of scales) are clearly pushed to their limits, for several reasons. For example, the existence of a spectral gap in the spectral energy density of vertical velocity is hard to demonstrate. This is due to both the limited data available and the physical character of the phenomenon. The occurrence of BLS involves in fact a broad range of relevant scales, from mesoscale to microscale (mean wind - waves - wind pulsations - turbulence). Sophisticated methods to assess the location of the spectral gap, however, exist (e.g., multi-resolution flux decomposition) and are used to recover the correct turbulence averaging interval. From the available high-frequency radar data it is possible to measure turbulence intensity at multiple levels within the boundary layer. Conditions in the relatively unperturbed upstream boundary layer are contrasted with those in the very turbulent rotor region downstream of the mountain.