Development and Comparison of Empirical Models for All-sky Downward Longwave Radiation Estimation at the Ocean Surface Using Long-term Observations

Abstract. The ocean-surface downward longwave radiation (R_l) 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 were established for ocean-surface R_l 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 R_l at both hourly and daily scales was built. The ocean-surface R_l 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 other models, yielding overall RMSE values of 14.82 and 10.76 W/m² under clear-sky conditions, and 15.95 and 10.27 W/m² 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 R_l also need to be considered besides cloud cover. Overall, the newly developed model has strong potential to be widely used.