Improved Algorithm to Derive All-Sky Longwave Downward Radiation From Space: Application to Fengyun-4A Measurements

Longwave downward radiation (LWDR) is an important parameter that modulates the Earth’s radiation and energy balance and is also a key variable that affects global warming. Currently, although many reanalysis LWDR products and satellite-based algorithms are available, their coarse spatiotemporal resolutions, as well as the difficulties in organizing the corresponding driving parameters, seriously limit their applications. As China’s new generation geostationary satellite, Fengyun-4A (FY-4A) provides higher spatial and temporal resolutions (4 km@nadir, 15 min at full disk mode) at longwave infrared channels, which can routinely monitor the changes of the Earth’s radiation in near real time, and therefore, provide great potentials in generating various high-accuracy radiation products. Unfortunately, the existing official LWDR products of FY-4A can only provide estimates under clear skies, and their accuracy still has much room for improvement. For the above-mentioned points, an improved general all-sky parameterization algorithm is proposed based on readily available input variables, such as land surface temperature (LST), column water vapor (CWV), and cloud-top temperature (CTT). Then the new algorithm is applied to FY-4A, aiming to derive a believable all-sky LWDR. The validation results show that the new algorithm does show a noticeable improvement over the original one by reducing the relatively large errors in LWDR under extremely cold and dry conditions (flux range < 150 W/m ² ), as well as the large bias in the polar and high altitude regions. Moreover, the new method can generate a more reliable LWDR than that of the FY-4A official product in terms of both spatiotemporal continuity and accuracy, with RMSE less than 22 W/m ² and bias less than 0.5 W/m ² under all-sky conditions. The easy-to-use and believable performance of the new algorithm provides an opportunity to accurately derive all-sky LWDR from FY-4A and similar satellite missions with high resolutions.