The Prevailing Altostratus Clouds near the Solomon Islands Influence the Quality Control of COSMIC-2 Bending Angle Data

The quality control of global radio occultation data, a necessary step in data assimilation, is usually carried out either globally or in latitudinal bands. This study shows that latitude-based quality control for COSMIC-2 bending angle observations during the summer of 2023 results in the removal of a large number of “outliers” near 8-km altitude, where measurements are generally considered more reliable than those in the middle and lower troposphere. Further analysis reveals that these outliers are predominantly found near the Solomon Islands, where altostratus clouds with cloud tops exceeding 6 km frequently occur in summer. Another region with a high concentration of outliers is in the lower troposphere within the Intertropical Convergence Zone. Because of their longer wavelengths compared to infrared and microwave observations, global radio occultation data are less affected by clouds, making them particularly valuable in cloudy conditions. By implementing a local quality control in each 2.5° 2.5° longitude and latitude region, we mitigate excessive removal of outliers in cloudy areas between 6 and 8 km and between 2 and 4 km altitude. The deviation between bending angle data and the 2.5° 2.5° local average varies more uniformly with longitude than it does with the latitudinal average. Since the quality of COSMIC-2 bending angle data does not depend solely on longitude, local quality control is more appropriate.