Forward modeling of radar observables using a large-eddy simulation model: a new approach to optimization of cloud radar scan strategies

Description

The ARM Climate Research Facility has 15 state-of-the-art scanning Doppler cloud radars continuously operating to characterize the radiatively important cloud structures at the ARM observing facilities. Currently the radars are operating in four different scan modes: vertically pointing (VPT), plan position indicator (PPI), hemispherical sky-range height indicator (HS-RHI), and cross-wind RHI (CW-RHI). Although the radar scan strategies yield valuable information regarding the cloud structures, the scan strategies have not been optimized for particular cloud types and more importantly for comparison with the cloud area/volume fraction simulated by global climate models (GCM), cloud-resolving models (CRM), or large-eddy simulations (LES).

Single-layer stratocumulus clouds were observed for a 24-hour period during the current deployment of the ARM Mobile Facility (AMF1) at Cape Cod, Massachusetts, on November 15, 2012. We have simulated this case using the Weather Research Forecasting (WRF) model in an LES mode with the innermost domain centered at the AMF and having 50-m horizontal resolution and less than 20-m vertical resolution below 1 km. The output from the model run is used to simulate the radar-observed reflectivity in all the scanning modes at the same temporal and spatial resolution as the operational cloud radars. The simulated reflectivity will include the effects of wind shear, attenuation due to gaseous absorption, and beam broadening, among other things. The cloud fraction as deduced from the radar simulation for different scan strategies will be then compared to that of WRF not only to select the best scan strategy for single-layer stratus clouds but also to improve the settings of the existing scan strategies and the scan sequence.