Latitudinal and Seasonal Dependent Zenith Angle Corrections for Geostationary Satellite IR Brightness Temperatures

Since late October 1998, the National Center for Environmental Prediction (NCEP) Climate Prediction Center (CPC) has archived half hourly, geostationary satellite, full disk, full resolution, "window" (10.7-11.5 microns) InFrared (IR) brightness temperature data area files from the GMS-5, GOES-10, GOES-8, METEOSAT-7, and the METEOSAT-5 satellites using the Man-computer Interactive Data Assimilation System (McIDAS) environment provided by National Environmental Satellite Data Interactive Service (NESDIS). Beginning February 1999, from McIDAS area files, CPC began computing and archiving among other products; global, 60o S-60o N, half-hourly, merged geostationary satellite mean 0.5o IR temperature. During processing of mean 0.5o IR temperatures, mislocation of cold cloud due to parallax is removed using a linear function of cloud top height with temperature. It was apparent through a visual loop of this data that GOES-10 and METEOSAT-5 IR, at large limb angles, were generally 5-30 K colder than temperatures obtained over the same locations from GMS-5 (nearer to nadir) only one half hour earlier. By collocating IR from one GOES satellite for which zenith angles are restricted to less than 26.5o and ranging the domain of zenith angles from the other GOES from 44o - 78o, limb-darkening errors are determined to increase with zenith angle and toward limb temperatures of 230 K (indicating cloud effects) with maximum values greater than 20 K. The procedure was repeated for the two METEOSAT satellites. A temperature dependent zenith angle correction derived from an average of these studies is universally applied to IR from each satellite and does a very good job at evenly removing bias in overlap regions for all satellites, temperatures, and zenith angles over the tropics from satellite overlap comparisons, however, over-correction increases from about 25oS and 25oN, toward the poles. To determine latitudinal dependence of the satellite zenith angle IR correction table (developed from tropical IR observations), the ratio of accumulated differences in uncorrected observed mean 0.5 degree brightness temperatures (obtained from overlapping satellites) over the differences in their IR corrections, were binned for every 0.5 degrees latitude from 60N to 60S for Feb 27-28, 1999. The ratio was near 1 in the tropics and smoothly drops off to about 0.5-0.6 at 60N and 60S, however, the ratio decreases quicker in the northern hemisphere where colder winter surface temperatures decrease the warm surface-cold cloud contrast, and thus reduce zenith angle dependency. The latitudinal dependency was repeated for April 11 - 18, May 24 - June 2, and July 21-30, 1999, and the ratio increases in the northern hemisphere and decreases in the southern hemisphere compared to February profile indicating a seasonal dependence. After applying this latitudinal and seasonal adjustment to the zenith angle correction, the resulting IR bias is smaller with smooth bias patterns resembling inter-satellite calibration differences more than satellite geometry dependency. Root mean square error of the limb to nadir view IR is reduced by approximately 50% for most satellite pairs at the larger angles. A comparison of 5 K histograms of limb corrected mean 0.5 degree IR obtained from zenith angles of 74-76 degrees nearly match the histograms from the corresponding nadir satellite histograms.