Land use feedback on clear sky downward longwave radiation: A land use adapted model

The existing models for downwelling longwave (LW) radiation employ different formulations for clear sky entissivity calculation. Even though both the temperature and water vapor affect emissivity, most models use either a lump humidity parameter or only one of these effects because of existing correlation between the two parameters. These parameters are further affected by heterogeneous land use patterns and temporal changes in atmospheric circulation patterns. In this paper, a model considering the nonlinear temperature and water vapor pressure effects superpositioned in one equation to account for the net impact on clear sky emissivity is investigated. Furthermore, the developed model is enhanced to become robust and adaptable for different land uses. Ground radiometer and meteorological data applicable in the subtropical climate of Saint Johns River Water Management District (SJRWMD), FL, during the spring season are utilized for model development. The new model is tested against pyrgeometer data gathered above a crop in Bondville, IL, a rangeland in Ft. Peck, NIT, and an urban setting in Deland, FL. When the new parameterization is validated using 24 h average downwelling LW flux, it yields a Nash-Sutcliffe Efficiency (NSE) greater than 0.5, normalized Mean Bias Errors (MBE), and Root Mean Square Errors (RMSE) of less than 5% and 7%, respectively, with the latter being the smallest deviations with respect to existing formulations. Copyright (C) 2007 Royal Meteorological Society.