Comparison of two transmittance algorithms in the community radiative transfer model: Application to AVHRR

Two transmittance algorithms, Optical Depth in Absorber Space (ODAS) and newly developed Optical Depth in Pressure Space (ODPS), are currently available in the Community Radiative Transfer Model (CRTM) version 2. There are noticeable differences of simulated brightness temperature (BT) between the two algorithms for the Advanced Very High Resolution Radiometer's (AVHRR) three infrared channels. For channel 3, the BT difference is dependent on the zenith angle and is mainly caused by the plane-parallel atmospheric assumption used in ODAS. For channels 4 and 5, the differences arise from both zenith angle and gaseous absorption in the two algorithms. In the ODPS algorithm, a reduction in radiance due to chlorofluorocarbons (CFCs) absorption is partially offset by an increase due to a smaller local zenith angle at higher atmospheric levels. A further evaluation of the two transmittance algorithms was performed by comparing AVHRR simulated and observed BTs over ocean. Overall, ODPS is better than ODAS. The BT biases at channels 4 and 5 are dramatically improved in ODPS and very close to zero. At satellite nadir position, an inclusion of CFCs in ODPS reduces bias by 0.25, and 0.15 K for channels 4, and 5, respectively. However, the channel 3 bias shows a strong zenith angle dependence when it is larger than 40 degrees in both the ODAS and ODPS simulations. This bias is due probably to the cloud contamination at larger zenith angles, and additional absorption in the atmosphere associated with other greenhouse gases such as carbon dioxide and water vapor.