Validation of Infrared Azimuthal Model as Applied to GOES Data Over the ARM SGP

Precise estimates of skin temperature and radiation budget are critical components of weather and climate. Shadowing effects over land cause azimuthal variations in infrared (IR) brightness temperatures that affect large scale daytime estimates of surface temperatures (Ts) and top-of-atmosphere (TOA) long wave (LW) flux. From rotating azimuthal plane scan (RAPS) data of the Clouds and the Earth's radiant Energy Systems (CERES) onboard Tropical Rainfall Measuring Mission (TRMM), Terra and Aqua, Minnis et al., (2004) showed that azimuthal effects arising from shadowing by vegetation or clouds need to be taken account for reducing errors in estimating fluxes at TOA. From a model constructed from 69 days of data from CERES Single Scanner foot print TOA/Surface and clouds (SSF) from TRMM, it was shown that multi-angle IR temperature prediction errors can be reduced by 35% or more over the full range of daylight conditions.

Surface measurements from Infrared thermometer (IRT) and Solar Infrared Radiation Station (SIRS) over Southern Great Plain (SGP) region from Atmospheric Radiation Measurement (ARM) mission provide ground truth to validate the azimuthal model corrections. In this study an attempt is made to demonstrate that the bias in skin temperature induced by the fixed geostationary satellite position can be minimized using azimuthal corrections applied to satellite data. For the present study, a total of 11 clear sky days (7 from summer and 5 from winter) are selected from year 2002 identified by looking at satellite images and using CERES data over SGP. The correlated k-distribution is applied to bring the skin temperatures measured from IRT and SIRS to the TOA over the ARM SGP region. These are then compared with temperatures at TOA from GOES - 8 for the same period. Statistics are computed for the differences between these temperatures before and after azimuthal corrections are applied to GOES - 8 data at the view zenith angle of satellite. Initial results show that for majority of days the azimuthal corrections reduce the differences up to 2 K. Larger corrections are expected for surfaces with terrain variability greater than that over the ARM SGP surface.