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Longwave Radiation from Clear Skies in Niger: A Comparison of Observations with Simple Formulas

Alistair D. CulfInstitute of Hydrology, Wallingford, Oxon, United Kingdom

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John H. C. GashInstitute of Hydrology, Wallingford, Oxon, United Kingdom

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Abstract

Observations of downward longwave radiation from clear skies over Niger, West Africa, are compared with values calculated from atmospheric profiles and with estimates from several equations based on screen-level variables. It is shown that the Eppley pyrgeometer measurements are in error during daylight hours but that this error can be satisfactorily corrected. Measured atmospheric profiles of temperature and vapor density are used to tailor Brutsaert's equation to the dry-season atmospheric conditions in Niger. The adjusted form of Brutsaert's equation describes the data excellently, illustrating the fact that its analytical derivation makes it more easily adaptable than other similar, but empirically derived, equations. There is considerable variation in the output of thew empirical equations, especially during the dry season, but those equations that include a vapor-pressure term perform best.

Abstract

Observations of downward longwave radiation from clear skies over Niger, West Africa, are compared with values calculated from atmospheric profiles and with estimates from several equations based on screen-level variables. It is shown that the Eppley pyrgeometer measurements are in error during daylight hours but that this error can be satisfactorily corrected. Measured atmospheric profiles of temperature and vapor density are used to tailor Brutsaert's equation to the dry-season atmospheric conditions in Niger. The adjusted form of Brutsaert's equation describes the data excellently, illustrating the fact that its analytical derivation makes it more easily adaptable than other similar, but empirically derived, equations. There is considerable variation in the output of thew empirical equations, especially during the dry season, but those equations that include a vapor-pressure term perform best.

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