A robust Observation Operator to assimilate rain microwave radiances into HWRF

To assimilate microwave radiances over clear regions as well as heavily precipitating cells, one needs an observation operator that can accurately simulate the brightness temperatures due to the emission and scattering of the condensation as well as the emission from the background, without straining the computational resources, and while minimizing the impact of poorly modeled variables such as the hydrometeor descriptors. We have developed a candidate operator using canonical correlation analysis. The result is not sensitive to the details of the microphysical variables, it gives quite satisfactory results, and it is very computationally inexpensive. We present this method applied to the Hurricane WRF (HWRF) model and the Hurricane WRF Satellite Simulator for the simulation of the Tropical Rainfall Measuring Mission Microwave Imager (TMI)'s radiances trained on simulations of 2010's Hurricane Earl, and present the results and error statistics of this method. Our approach is in general applicable to any observation operator and can serve as an important tool in the data assimilation toolbox.