An Improved FAST Microwave Sea Surface Emissivity Model, FASTEM4

Sea surface microwave emissivity model is crucial for radiance assimilation in supporting weather forecasting. Since the launch of the Advanced Microwave Sounder Unit in 1998, the microwave sensor data have been proven the most contributors to weather forecasting skill. The fast microwave emissivity model (FASTEM) has been used in most of numerical weather service (NWS) centers including NOAA NWS, ECMWF, and Met Office. Although FASTEM is widely used in NWP community, the model improvements at low frequencies below 20 GHz and high frequencies above 85 GHz are necessary. At low frequencies, sea water permittivity is a function of salinity that is not yet taken into account for in the FASTEM. Large bias between satellite observations and radiative transfer simulations at high frequencies has been reported. The operational microwave ocean emissivity model at the present doesn't have the capability to take account for the effect of the surface wind direction, except for a specific model for WINDSAT. The wind directional effect for vertically and horizontally polarized brightness temperatures is about 2 K, significant to radiance simulation and assimilation. In this study, we investigated the effects of permittivity models and generated a new permittivity model by using measurements for fresh and salted water at frequencies between 1.4 GHz and 410 GHz. A modified sea surface roughness model from Durden and Vescky is applied to the surface emissivity calculations. The new improved FAST Microwave Sea Surface EMISSIVITY MODEL, FASTEM4, can be used for the microwave radiance assimilation for weather forecasting and simulations in retrievals for satellite products by taking surface temperature, wind speed and direction, and salinity into account. We will use the new operational and research microwave ocean emissivity model to study the NPOESS ATMS sensor, in particular the effect of wind direction and also for the channels at high frequencies.