Surface Temperature Observations from AVHRR in FIFE

Abstract

Observations of the surface radiometric temperature by the AVHRR sensor on board the NOAA-9 satellite during the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment conducted in central Kansas during 1987 are presented. The satellite observations were corrected for atmospheric effects using a path radiance model (MODTRAN3) and radiosonde measurements. Problems with this approach include the nonsimultaneity of the soundings with the overpass and errors involved in profile measurements. For the former, soundings before and after the overpass were interpolated to the time of the overpass. For the latter, some of the errors arise from the ±0.5°C uncertainty in the dry- and wet-bulb temperatures, which can produce up to a ±14% relative uncertainty in the water vapor. To overcome this uncertainty, the water vapor profiles were adjusted until the channel 4 and 5 temperature differences over a large reservoir were reduced to zero. This adjusted profile was then used over the entire site. The results are compared to ground broadband temperature readings at 10 sites and to aircraft results from the thermal channel of the NS001 sensor on the C-130 aircraft. The AVHRR values were found to be 5° to 6°C warmer than the average of the ground measurements. This difference is attributed to the fact that the ground measurements were made preferentially on well-vegetated surfaces while the AVHRR integrates over the entire site, which includes many warm surfaces.