A Fast Radiative Transfer Model for Simulating High-Resolution Oxygen B-band and its application to DSCOVR EPIC observations

A fast radiative transfer model has been developed for simulating high-resolution

oxygen B-band absorption band. The first order scattering radiance is calculated accurately by using a higher number of layers and streams for all required wavenumber grid points. The multiple-scattering component is extrapolated and/or interpolated from a finite set of calculations in the space of two integrated gaseous absorption optical depths to the wavenumber grids: a

double-k approach. The double-k approach substantially reduces the error due to the

uncorrelated nature of overlapping absorption lines. More importantly, these finite

multiple-scattering radiances at specific k values are computed with a reduced

number of layers and/or streams in the forward radiative transfer model. To simulate an

oxygen B-band spectrum, 33 k values and 99 calculations of radiative transfer are needed to achieve an accuracy of 0.5% for most applications under all-sky conditions and 1.5% for the

most challenging multiple-layer cloud systems (99% of spectrum below 0.5%). This

results in around a hundred-fold time reduction with respect to the standard forward radiative transfer calculation. This fast radiative transfer model for high-resolution oxygen B-band simulation provides a powerful tool for DSCOVR EPIC B-band observation data analysis. It has been applied to DSCOVR EPIC B-band retrievals of the cloud top pressure.