Evaluation of RTTOV-SCATT Performance With a Vector Discrete Ordinate Method for an Atmosphere Including Nonspherical Raindrops

With the recent availability of realistic azimuthally randomly oriented (ARO) raindrops, it is essential to evaluate the difference of radiative transfer simulations with scattering between the ARO raindrops (i.e., Chebyshev rain drop and liquid spheroid) and the commonly used spherical raindrop. Here, the scattering module of radiative transfer for TOVS (RTTOV-SCATT) was evaluated by a reference atmospheric radiative transfer simulator (ARTS) for the frequencies between 10.65 and 36.5 GHz. The two vector scattering solvers used by ARTS were discrete ordinate iterative (DOIT) and RT4. Both scattering solver difference (SSD) and scattering property difference (SPD) are assessed. It is shown that the brightness temperature (BT) difference between RTTOV-SCATT and DOIT can exceed 2.5 K, statistically higher than the value obtained from previous studies. There exists a size parameter threshold that influences the trend of SSD, and this threshold increases slightly with frequency. RT4 tends to produce lower BTs than DOIT, and the biases range from −0.1 to −0.8 K for different frequencies and surface types. In addition, the permittivity model difference dominates the SPD (biases and STDs range from 0.1 to 0.6 K and 0.4–1.3 K, respectively). The nonspherical shape and orientation yield higher horizontal polarization BT and lower vertical polarization BT. The additional polarization difference introduced by ARO raindrops is 0–3.2 K as a function of total column integrated rain water content. Thus, it is believed that this study offers the error sources and will help the improvements of RTTOV-SCATT.