Improvement of microwave land emissivity calculation in the CRTM

Satellite observed brightness temperature (Tb) in various spectral channels is assimilated through the JCSDA Community Radiative Transfer Model (CRTM) on the NCEP Gridpoint Statistical Interpolation (GSI). It has long been known that the amount of microwave observations assimilated over land in the current operational GSI is far less than over ocean. One of main reasons is inaccurate surface emissivity calculation in the CRTM because of high spatial variability of surface emissivity and its spectra, which is largely unknown over different surfaces. Thus, inaccurate surface emissivity results in large errors in CRTM simulated satellite brightness temperatures over land and rejection of satellite data in GSI, especially for surface sensitive channels.

This study focuses on the microwave land emissivity model developed by Weng et al. (2001), which is applied for all microwave sensors in the CRTM. This model calculates emissivity from a two-stream radiative transfer solution, and then modifies Fresnel equations for reflection and transmission at layer interfaces. Obviously, surface characteristics, which are a function of soil types and vegetation types, play an important role in quantify the land emissivity. However, these parameters are absent in the microwave emissivity model in the CRTM.

We add soil and vegetation characteristics as well as canopy optical information in the emissivity model. The NOAA 18 Advanced Microwave Sounding Unit-A (AMSU-A), which has a multi-channel microwave temperature/humidity sounder, is selected to investigate brightness temperature simulation with the improved emissivity calculation. The sensitive experiments show a reduction of bias and root-mean-square error in simulated brightness temperature, as well as an increase in the number observations assimilated and a decrease in penalty compared to the results when using a previous land surface emissivity scheme.