A "zenith angle" effect results in the erroneous inference of IR brightness
temperatures at pixels that are at relatively high viewing angles relative to
nadir and are inferred to be considerably colder than they are in reality.
Failure to correct for this effect results in discontinuities at the boundaries
of adjacent geostationary satellites as demonstrated in
Figure 1. A relatively crude correction for zenith angle effects on GPI precipitation
estimates was determined by comparing retrieved IR temperature information
from the GOES satellites which have a relatively large region of overlap and
thus allowed the comparison of retrieved IR brightness temperatures from the
same earth locations but at varying viewing angles (Joyce and Arkin
1997). More recent work by Joyce et al. (1999) shows
that the viewing angle effect is dependent on the temperature of the retrieved
pixel temperature, and that a seasonal adjustment is necessary in the extratropical
regions (Figure 2). It is the incorporation of the results of these studies that
enables the production of a nearly seamless map of IR data.
IR limb correction subroutines are available at the CPC
ftp server. The IR correction needs original IR Tb, satellite zenith angle, Julian day, and pixel latitude
for inputs and returns a limb corrected Tb.
Joyce, R. J. And P. A. Arkin, 1997: Improved estimates of
tropical and subtropical precipitation using the GOES Precipitation
Index. J. Atmos. Ocean Tech., vol. 14, 997-1011.
Joyce, R. J., J. E. Janowiak and G. F. Huffman, 2001: Latitudinal
and seasonal dependent zenith angle corrections for geostationary satellite
IR brightness temperatures. (Journal of Applied Meteorology, in press).