Development of a CALIPSO IIR radiance simulator

Numerical radiance simulators can be applied to theoretical error analyses and calibrations of satellite instruments in addition to the simulations of radiances and fluxes under various atmosphere and surface conditions. In this presentation, we report on the development of a computationally efficient simulator to model outgoing clear- and cloudy-sky radiances observed by the Infrared Imaging Radiometer (IIR) on board the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. The present radiance simulator is based on combining the correlated k-distribution method (CKD) with the high-spectral-resolution radiative transfer model (HRTM). The IIR spectral response function is taken into consideration in the computation of atmospheric gaseous transmittance based on the CKD method. The HRTM, which is based on pre-calculated lookup tables (LUTs) of the bidirectional reflection distribution function (BRDF), transmissivity and effective emissivity for water and ice clouds, is employed to simulate cloudy-sky radiances at the IIR channels. Numerical results demonstrate the accuracy and efficiency of the IIR fast radiance simulator in comparison with a combination of the rigorous line-by-line radiative transfer model (LBLRTM) and discrete ordinate radiative transfer (DISORT) models. Case studies for different cloud scenes identified based on observations by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the CALIPSO platform are presented to show the comparison between simulations and IIR observations. In the process of simulation, the cloud information can be obtained from CALIOP; the profiles of atmospheric gaseous transmittance are produced, using the CKD method with the Modern Era Retrospective Analysis for Research and Applications (MERRA) products.