Cloud properties from Atmospheric Infrared Sounder and evaluation with Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations

Abstract

The Atmospheric Infrared Sounder (AIRS) onboard the NASA Aqua satellite was primarily designed to provide atmospheric temperature and water vapor profiles, and the high spectral resolution of this instrument also allows the retrieval of cloud properties (especially cirrus). We present a retrieval of cloud pressure and effective emissivity, based on a weighted χ2 method using channels around the 15‐micron CO2 absorption band. The cloud property retrieval is applied to all data, and tests based on the retrieved parameters determine cloud amount in a second step. Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), launched as part of the A‐Train in 2006, made it possible to evaluate the AIRS retrieved cloud height by comparing it to the height of maximum backscatter and of the “apparent middle” of the highest cloud layer determined by CALIPSO. The retrieved cloud height of about 66% (80%) of AIRS high (low) clouds lies within 1.5 km of the apparent middle of the CALIPSO cloud layers. Comparing cloud pressures shows an agreement in cloud height of 72% (59%) for high (low) clouds within 75 hPa. Keeping in mind that (1) low clouds are geometrically thinner (less than 1 km) than high clouds (in general between 1.5 and 3 km), (2) high clouds are also much more heterogeneous, and (3) CALIPSO only samples a small fraction of the AIRS footprint, the comparison is very encouraging. Zonal averages of high‐, midlevel‐, and low‐cloud amount are compared to those of other data sets for January and July. Compared to our cloud retrieval, distributions of cloud pressure provided by AIRS L2 data reveal a strong bias toward lower pressures for low clouds.