Development and comparison of empirical models for all-sky downward longwave radiation estimation at the ocean surface using long-term observations

The ocean-surface downward longwave radiation (Rl) is one of the most fundamental components of the radiative energy balance, and it has a remarkable influence on air–sea interactions. Because of various shortcomings and limits, a lot of empirical models have been established for ocean-surface Rl estimation for practical applications. In this paper, based on comprehensive measurements collected from 65 moored buoys distributed across global seas from 1988 to 2019, a new model for estimating the all-sky ocean-surface Rl at both hourly and daily scales was built. The ocean-surface Rl was formulated as a nonlinear function of the screen-level air temperature, relative humidity, cloud fraction, total column cloud liquid, and ice water. A comprehensive evaluation of this new model relative to eight existing models was conducted under clear-sky and all-sky conditions at daytime/nighttime hourly and daily scales. The validation results showed that the accuracy of the newly constructed model is superior to that of other models, yielding overall root mean square error (RMSE) values of 13.44 and 8.34 W m−2 under clear-sky conditions and 15.64 and 10.27 W m−2 under all-sky conditions at hourly and daily scales, respectively. Our analysis indicates that the effects of the total column cloud liquid and ice water on the ocean-surface Rl also need to be considered in addition to cloud cover. Overall, the newly developed model has strong potential to be widely used.