Evaluation of Near-surface Specific Humidity and Air Temperature from Atmospheric Infrared Sounder (AIRS) over Oceans

The state of the near-surface atmosphere, especially air temperature and specific humidity, has profound effects on human health, ecosystem function, and global energy flows. The accuracy of these products is important for weather forecasting, climate modeling, data assimilation, and trend assessment. The Atmospheric Infrared Sounder (AIRS) provides global products of near-surface air temperature and specific humidity estimates. These products have seen continuous improvements in accuracy, resulting in significant reductions in error rates. Despite these improvements, existing studies have not systematically validated AIRS near-surface products in both temporal and spatial perspectives, especially over oceans. This study aims to fill this gap by using the International Comprehensive Ocean–Atmosphere Data Set (ICOADS) as a ground-based reference to evaluate AIRS near-surface air temperature and specific humidity over the ocean from the V7 Level 2 product. Our results show an overall underestimation of near-surface air temperature and specific humidity, with pronounced spatial patterns in the estimation errors. In addition, we observed higher uncertainties near land and found that the products perform better during winter and at night on a global scale, although there are regional exceptions. In terms of time scale, the estimation errors show remarkable stability over a 20-year period, demonstrating the ability of AIRS to capture general temporal characteristics. These findings underline the importance of validating and understanding the retrieval uncertainties of AIRS near-surface products, paving the way for improved climatological applications.