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David R. Brooks, Forrest M. Mims III, and Richard Roettger

Abstract

An inexpensive two-channel near-IR sun photometer for measuring total atmospheric column water vapor (precipitable water) has been developed for use by the Global Learning and Observations to Benefit the Environment (GLOBE) environmental science and education program and other nonspecialists. This instrument detects sunlight in the 940-nm water vapor absorption band with a filtered photodiode and at 825 nm with a near-IR light-emitting diode (LED). The ratio of outputs of these two detectors is related to total column water vapor in the atmosphere. Reference instruments can be calibrated against column atmospheric water vapor data derived from delays in radio signals received at global positioning satellite (GPS) receiver sites and other independent sources. For additional instruments that are optically and physically identical to reference instruments, a single-parameter calibration can be determined by making simultaneous measurements with a reference instrument and forcing the derived precipitable water values to agree. Although the concept of near-IR detection of precipitable water is not new, this paper describes a first attempt at developing a protocol for calibrating large numbers of inexpensive instruments suitable for use by teachers, students, and other nonspecialists.

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David R. Brooks, Christopher F. England, Carry E. Hunt, and Patrick Minnis

Abstract

An intercomparison between radiative parameters determined from visible and infrared channels of the METEOSAT-1 and GOES-2 geosynchronous satellites has been carried out using data obtained over the central Atlantic Ocean for 5 November 1978. Hourly visible-infrared measurement pairs at a nominal resolution of 5 km (METEOSAT) or 8 km (GOES) have been stored in 1° × 1° longitude-latitude regions. For the infrared intercomparisons, the GOES 11.5 μm radiance has been compared to METEOSAT infrared counts. The scatter in partly cloudy regions is interpreted as being caused by meteorological differences arising from differences in measurement time between the two data sets. For the visible intercomparison, the GOES measurements for clear and cloudy scenes have first been converted with the aid of scene-dependent angular reflectance and albedo models to estimates of the filtered shortwave radiance that GOES would have measured had it been in the METEOSAT position. This value has then been compared to METEOSAT counts for the shortwave channel. The results indicate that earlier METEOSAT calibrations made from airplane overflights of a limited variety of surfaces are applicable to much larger areas of cloud and ocean.

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