A fast radiance simulator for the solar bands of NPP/VIIRS instrument

A new algorithm is developed to efficiently calculate TOA solar reflectance for visible through near infrared spectral region in both clear and cloudy atmospheres. Existence of both multi-layered clouds and a non-Lambertian surface is allowed. A fast TOA radiance simulator is subsequently designed for narrow-band satellite-based instruments, such as NPP/VIIRS, by considering the bulk scattering properties of different kind of cloud particles. The performance of this simulator is validated with respect to both computational accuracy and efficiency. For single-layer cloud with a non-Lambertian surface, the simulation biases between this simulator and the 128-stream DISORT are limited to 1% if the satellite viewing zenith angle is smaller than 70°. The simulator runs 500 hundred times faster than 128-stream DISORT if only a single-layer cloud exists in an atmospheric column and the surface is Lambertian. However, if both multi-layered clouds and a non-Lambertian surface are taken into account, the relative error increases to 4~5% and the computing time ratio of the fast simulator to DISORT decrease to 90.