Spatial and temporal features of mountain wave related turbulence

Turbulence kinetic energy (TKE) in the atmospheric boundary layer (ABL) is generated by buoyant thermals and mechanical eddies while it is suppressed by static stability and dissipated into heat by the effects of molecular viscosity. By observing the various terms in the TKE budget we can determine whether TKE is increasing or decreasing with time in the ABL and learn about the characteristics of turbulence for different atmospheric conditions. This knowledge is needed for the evaluation and improvement of turbulence parameterizations in atmospheric numerical models which find their application in numerical weather prediction, agricultural meteorology and hydrology, air quality modelling, wind energy, etc.

The Terrain-induced Rotor Experiment has provided an unprecedented data set that allows the investigation of temporal and spatial turbulence characteristics in a variety of atmospheric conditions in complex terrain. Three towers were installed by NCAR at three different locations in Owens Valley. Each tower was 35 m tall and instrumented with multiple CSAT3 ultrasonic anemometers collecting data with a sampling rate of 60 Hz. A case study is presented of a high wind event in Owens Valley that occurred on 02 March 2006 during the T-REX Intensive Observation Period (IOP) 1. Terms in the TKE budget are calculated including the dissipation rate which is an important but poorly understood term in turbulence parameterizations. The dissipation rate reaches maximum values of 0.5 m² s^-3 and approximately balances the mechanical TKE production term during the passage of a strong downslope flow converging with an along-valley flow during this IOP. It is shown also that the buoyancy term is more than an order of magnitude smaller than both the mechanical term and the dissipation rate. Earlier in the same IOP, during a clear dry daytime convective situation and a stable night-time situation, values of dissipation rate are much smaller than those during the high wind event. The results are compared and supplemented with data collected from other instruments in the field study including a scanning aerosol lidar. A comprehensive picture of the spatial and temporal turbulence characteristics in the surface layer arises which will lead to better understanding of turbulence in complex terrain.