Simulation of UV Radiance Using UNL-VRTM

TOA reflected radiances in UV wavelength are simulated using three radiative transfer models: TOMRAD, UNL-VRTM and SCIATRAN. TOMRAD stands for TOMS Radiative Transfer model. It is a vector radiative transfer model and is specifically used for UV region radiance simulation for pure Rayleigh scattering atmosphere with gases absorption. UNL-VRTM is a Unified Linearized Vector Radiative Transfer Model and was specifically designed for the simulation of atmospheric remote sensing observations and for the inversion of aerosol, gas, cloud, and/or surface properties from these observations. It comprises VLIDORT for radiative transfer, a linearized Mie and a linearized T-Matrix code for aerosol single scattering, a Rayleigh scattering module, and line-by-line (LBL) gas absorption calculation with HITRAN data base. SCIATRAN is a comprehensive software package for the modeling of radiative transfer processes in the terrestrial atmosphere and ocean in the spectral range from the ultraviolet to the thermal infrared (0.18–40μm) including multiple scattering processes, polarization, thermal emission and ocean–atmosphere coupling

For simulating the radiances at the OMPS viewing angle, Microwave Limb Sounder (MLS) Ozone profiles are collocated with OMPS radiances and then used as inputs to the forward model. The bias between OMPS observations and simulations is analyzed from each model. It is shown that the UNL-VRTM is the most suitable model for OMPS radiance simulation. Overall, the bias can be attributed to several factors: 1) the scheme of computing the gaseous absorption coefficients gases and Rayleigh scattering in different RTM, 2) surface reflectance, 3) aerosol scattering approximation, 4) the band center wavelength shifts in OMPS, and 5) the errors of input profile to radiative transfer model. The sensitivity studies show that scalar radiative transfer approximation, considering only ozone, and/or the assumption of constant surface reflectance within UV region may cause significant errors to the TOA reflectance. Overall, the differences is less than 5% for OMPS cross-tracking position at all channels. Since SCIATRAN model includes the rotational raman scattering, the difference between measurements and simulations can also be reduced to some extent, particularly for the wavelength larger than 330 nm.