A physical parameterization for cloudy-sky downward longwave radiation: Validation for tropical and subtropical regions in Brazil

Downward longwave radiation is a crucial component of the surface radiation balance and is essential for a better understanding of the greenhouse effect. It is strongly influenced by atmospheric temperature, water vapor content, and cloudiness. This study proposes a new model to estimate downward longwave radiation under cloudy sky conditions, suitable for the application of remote sensing data. The Santa Barbara DISORT Atmospheric Radiative Transfer code was employed to develop parameterizations for cloud and atmospheric processes. The simulations indicated that the emissivity of a stratiform cloud when the cloud is thicker than 300 m (epsilon = 0.985) approaches that of a blackbody. It is shown that atmospheric emissivity and transmittance can be parameterized as functions of precipitable water content below cloud base. The model was validated using observational data from six different sites in Brazil. The parameterizations of primary model variables (atmospheric layer emissivity, atmospheric transmittance, and precipitable water below the cloud base) generally showed good agreement with radiosonde data. The parameterization of precipitable water in the sub-cloud atmosphere yielded good results, with a maximum RMSE of 0.44 g cm- 2. The developed model presents an average overestimation of only 14 W m- 2, corresponding to a percentage error of 3.5 %, and an overall average RMSE of 18.3 W m- 2 (4.6 %). It shows satisfactory performance and is suitable for application in tropical and subtropical regions, as the underlying physics of this method is consistent worldwide.