A Novel Calibration of Precipitable Water Vapor From HY-2B Scanning Microwave Radiometer by Integrating GNSS and ERA5 Data for Offshore Areas

The Chinese Haiyang-2B (HY-2B) satellite is equipped with a scanning microwave radiometer (SMR) for global marine atmospheric water vapor detection. To calibrate precipitable water vapor (PWV) derived from the SMR accurately, a novel calibration method, referred to as Spatial Correction for Water Vapor (SC-WV), is proposed. This method integrates PWV data obtained from the Global Navigation Satellite System (GNSS) with the fifth generation of the European Centre for Medium-Range Weather Forecasts atmospheric reanalysis products (ERA5). By leveraging the ERA5 data, the spatial variations in water vapor surrounding the GNSS station are calculated, and the GNSS-derived PWV is subsequently corrected to obtain precise measurements at the SMR grid nodes. The calibration of SMR PWV within a 200 km radius around GNSS stations is conducted using GNSS PWV data observed at 47 island stations and 44 coastal stations from the International GNSS Service (IGS) in 2021. Both the proposed SC-WV method and the traditional inverse distance weighting (IDW) method are employed for comparison. The results demonstrate the applicability of the SC-WV method for both GNSS island and coastal stations, enabling the accurate calibration of the SMR PWV at each grid node surrounding the GNSS stations. On the basis of the GNSS PWV measurements obtained from island and coastal stations, the average bias values for the SMR PWV were 0.5 and 0.8 mm, respectively, with corresponding root mean square errors (RMSEs) of 2.6 and 2.8 mm respectively. The consistent accuracy indices observed provide evidence of the robustness and validity of the SC-WV method.