THE SPLIT WINDOW RETRIEVAL ALGORITHM FOR SEA SURFACE TEMPERATURE DERIVED FROM SATELLITE MEASUREMENTS

Abstract

Because remote measurements of the earth's surface from satellite altitude are affected by the earth's atmosphere, measurements of surface parameters must be adjusted. One of the major factors which affects radiances in the atmospheric window regions is absorption due to water vapor. Various techniques for correcting measured earth radiances have been employed. One of the more promising techniques is sometimes called the “split window” technique, even though the basic technique has broader applications than just a split window. A channel is being added to the Advanced Very High Resolution Radiometer (AVHRR) specifically to allow use of this technique. Starting with NOAA D, satellites in the TIROS-N series will provide two measurements in the 10 to 12 ym region, one channel at 10.3 to 11.3 μm and one at 11.5 to 12.5 μm. Because the atmospheric transmittance is greater at 10.3 to 11.3 μm than at 11.5 to 12.5 μm, the measurement at 10.3 to 11.3 μm contains more signal from the earth's surface and less signal from the earth's atmosphere. In its simplest form, the retrieval method uses the difference between these two measurements to estimate a second difference between the measurement at 10.3 to 11.3 μm and a measurement at 10.3 to 11.3 μm that would be obtained if the instrument were located at the earth's surface. This second difference is the correction for atmospheric attenuation at 10.3 to 11.3 μm. If the area is not too moist and the atmospheric transmittance is relatively close to unity, the correction takes the form of a constant times the difference in the two measurements. Basic prinicples of the technique, its advantages, limitations, and modifications for moist areas are discussed. The discussion also includes results obtained from TIROS-N measurements at two separate windows at 4 and 11 μm to estimate the accuracy technique .