Cloud properties from Atmospheric Infrared Sounder and evaluationwith Cloud-Aerosol Lidar and Infrared Pathfinder SatelliteObservations

摘要

The Atmospheric Infrared Sounder (AIRS) onboard the NASA Aqua satellite wasprimarily designed to provide atmospheric temperature and water vapor profiles, and thehigh spectral resolution of this instrument also allows the retrieval of cloud properties(especially cirrus). We present a retrieval of cloud pressure and effective emissivity, basedon a weighted x~2 method using channels around the 15-micron CO2 absorption band. Thecloud property retrieval is applied to all data, and tests based on the retrieved parametersdetermine cloud amount in a second step. Cloud-Aerosol Lidar and Infrared PathfinderSatellite Observations (CALIPSO), launched as part of the A-Train in 2006, made itpossible to evaluate the AIRS retrieved cloud height by comparing it to the height ofmaximum backscatter and of the "apparent middle" of the highest cloud layer determinedby 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 geometricallythinner (less than 1 km) than high clouds (in general between 1.5 and 3 km), (2) highclouds are also much more heterogeneous, and (3) CALIPSO only samples a smallfraction of the AIRS footprint, the comparison is very encouraging. Zonal averages ofhigh-, midlevel-, and low-cloud amount are compared to those of other data sets forJanuary and July. Compared to our cloud retrieval, distributions of cloud pressureprovided by AIRS L2 data reveal a strong bias toward lower pressures for low clouds.