Reprocessing the HIRS data to infer global cloud cover properties and trends

The frequency of occurrence of tropospheric clouds has been extracted from NOAA/HIRS polar orbiting satellite data from 1979 onwards using CO2 slicing to infer cloud amount and height. Algorithm adjustments for instrument noise, sensor to sensor differences, viewing angle, spectral response shifts, calculated versus measured radiance biases, changing CO2 and O3 amounts, and orbit drift have been studied. The HIRS sensor has been flown on fifteen satellites from TIROS-N through NOAA-19 and METOP-A forming a 30-year record. Wylie et al. (2005) presented a CO2 slicing cloud record from 1979 to 2001 from the HIRS/2 sensors on TIROS-N through NOAA-14. Subsequent investigation of cloud products from the next generation HIRS/3 sensors, beginning with NOAA-15, revealed differences with those from the HIRS/2 sensors due to calibration errors. High spectral resolution infrared data from AIRS has been used to adjust spectral response functions in the recent HIRS data; Satellite Nadir Overpasses (SNO) will be used to intercalibrate the HIRS sensors before AIRS. For the period from 1995 through 2009, the cloud properties from HIRS/2 are now in family with those from HIRS/3 sensors. Trends in cloud cover and high cloud frequency are found to be small but significant in these data.