Submitted to: Geophysical Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 30, 2008
Publication Date: December 5, 2008
Citation: Choi, M., Jacobs, J.M., Kustas, W.P. 2008. Assessment of clear and cloudy sky parameterizations from daily downwelling longwave radiation over different land surfaces in Florida, USA. Geophysical Research Letters. 35:L20402. Interpretive Summary: A reliable estimate of net radiation is critical for operational models assessing water use. This requires accurate estimates of incoming and outgoing shortwave and longwave radiation. To provide net radiation over large areas methods using standard weather station data are needed. In this study several existing methods for computing downwelling longwave radiation under clear and cloudy sky were evaluated using eleven experimental sites across a wide range of land cover conditions in Florida. The most accurate approach for estimating downwelling longwave radiation in this region of Florida was found to be the Sellers (1965) equation for clear sky modified for cloudy conditions using the Crawford and Duchon (1999) equation. Even if more complicated parameterizations can be employed using cloud properties and profile temperature and humidity data, this operational method can provide reasonable estimates with relatively high accuracy under typical cloud conditions in Florida. This downwelling longwave formulation when combined with operational methods for determining other radiation components will lead to improved estimates of large area net radiation and modeling regional water use. This in turn will have an immediate impact on managing regional groundwater, surface water, and water supply resources in the Florida region.
Technical Abstract: Clear sky downwelling longwave radiation and cloudy sky downwelling longwave radiation formulas were tested across eleven sites in Florida. The Sellers equation, using air vapor pressure and temperature measurements, provides the best estimates of clear sky downwelling longwave radiation with a root mean square error of less than around 12 Wm-2 across all sites. The Crawford and Duchon’s cloudiness factor with Sellers equation is recommended for calculations under cloudy conditions. This combined approach requires no local calibration and estimates of downwelling longwave radiation with a root mean square error of less than around 13 Wm-2 and squared correlation coefficients that typically exceed 0.9.