A Parameterization Scheme for Correcting All‐Sky Surface Longwave Downward Radiation Over Rugged Terrain.

Accurate surface longwave downward radiation (SLDR) is crucial for understanding mountain climate dynamics. While existing algorithms notably improve the accuracy of clear‐sky SLDR, a terrain correction algorithm that can correct remotely sensed and model‐simulated all‐sky SLDR on a large scale remains largely unexplored. Here, we propose a parameterization scheme for estimating all‐sky SLDR in rugged terrain. We primarily improve the estimation of nearby terrain thermal contribution by considering topographic asymmetry and incorporate the effects of ice cloud thermal scattering under low water vapor conditions. We validate the reliability of our model using the Discrete Anisotropic Radiative Transfer (DART) model, demonstrating a good agreement with a bias value of −12.8 W/m2 and a RMSE value of 28.2 W/m2. Further evaluation against the Essential Thermal Infrared Remote Sensing (ELITE) SLDR product at three TIPEX‐III in situ sites, located near the bottom of deep valleys with predominantly flat surfaces, indicates significant improvement in our model, reducing the mean bias by 7.4 W/m2 and the mean RMSE by 4.1 W/m2. Post‐terrain correction, the ELITE SLDR difference map exhibits a spatial pattern of "small in the northwest and large in the southeast" in the study area, with the maximum differences reaching 67 W/m2 in the daytime and 54 W/m2 at nighttime. Comparison with existing methods reveals similar improvements due to the consideration of terrain effects. Overall, our SLDR correction model shows enormous potential for correcting remotely sensed and model‐simulated SLDR products on a large scale. Plain Language Summary: Accurate measurement of surface longwave downward radiation (SLDR) is essential for understanding the climate in mountainous areas. However, current methods struggle to correct all sky SLDR data on a large scale when using remote sensing technology. We develop a new method to better estimate nearby terrain thermal contribution considering the effects of uneven terrain. Additionally, we take into account the impact of ice cloud scattering when there is low water vapor. We utilize the ELITE product as a reference to test our model at three locations from the TIPEX‐III project. Our model showed significant improvements, reducing the average bias by 7.4 W/m2 and the RMSE by 4.1 W/m2 When comparing ELITE SLDR data with our corrected SLDR maps, we observed a pattern of lower values in the northwest and higher values in the southeast of the study area. The differences reached up to 67 W/m2 during the day and 54 W/m2 at night. Our method, which includes the effects of terrain, showed similar improvements to other methods designed for rugged terrain. Overall, our SLDR correction model has strong potential for enhancing the accuracy of SLDR measurements over mountainous regions. Key Points: We propose a method for correcting all‐sky surface longwave downward radiation (SLDR) in rugged terrain, with mainly improving the estimate of nearby terrain thermal contributionThe reliability of our model is demonstrated through comprehensive comparisons with Discrete Anisotropic Radiative Transfer (DART) simulations and in situ observationsOur method exhibits significant capability in correcting remotely sensed and model‐simulated SLDRs across extensive geographic scales [ABSTRACT FROM AUTHOR]