Multi-Year Comparisons of High Cloud Frequencies from HIRS, MODIS, and AVHRR

Frequencies of occurrence of high-altitude tropospheric clouds have been extracted from High resolution Infrared Radiation Sounder (HIRS), Advanced Very High Resolution Radiometer (AVHRR), and Moderate-Resolution Imaging Spectroradiometer (MODIS) polar orbiting satellite data. For HIRS and MODIS instruments, the CO2 slicing algorithm is used to infer cloud height and effective emissivity. An optimal estimation algorithm using 11 µm and 12 µm radiance data is used to derive AVHRR cloud height and optical depth. High cloud retrievals from operational HIRS and AVHRR sensors onboard NOAA-16 and NOAA-18 are compared to that of Aqua MODIS for as many as possible of the years 2001-2008. The HIRS and MODIS CO2-slicing algorithms have been made as consistent as is reasonably possible. Clear vs. cloudy discrimination is accomplished with collocated PATMOS-X (AVHRR) cloud detection data for HIRS FOVs and the 1-km MODIS cloud mask (MOD35) for MODIS 5x5-pixel (5-km) regions. In order to address issues affecting sensor to sensor radiance calibration and calculations of clear sky radiances, high-spectral resolution infrared data from the Infrared Atmospheric Sounding Interferometer (IASI) has been used to adjust spectral response functions in the HIRS data. Several updates have been made to the MODIS CO2 slicing algorithm (Collection 6) including the use of high-spectral resolution data from the Atmospheric Infrared Sounder (AIRS) instrument to refine MODIS spectral response functions in the CO2 absorption bands. Results from the three data sets will be compared and analyzed using consistent Level-2 to Level-3 methods. Similarities and differences in high cloud frequencies and distributions will be reported.