Characteristics of Satellite Sampling Errors in Total Precipitable Water from SSMIS, HIRS, and COSMIC Observations

This study quantifies the characteristics of different satellite sampling errors in the time series of total precipitable water (TPW) derived from Constellation System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation, Special Sensor Microwave Imager Sounder (SSMIS), and High-resolution Infrared Radiation Sounder (HIRS) during the overlapping time period of January 2007 to December 2013. Gap-free data from ERA5 reanalysis of the European Centre for Medium Range Weather Forecasts are used as reference values. All TPW data are first compared with microwave radiometer measurements from Atmospheric Radiation Measurement Program. In general, they are consistent, with all their regression coefficients being greater than 0.77. Discrepancies in global TPW time series can be mainly attributed to the inherent sampling errors of these three different satellite remote sensing systems. COSMIC has small sampling errors in higher latitudes. But it has scarce samples in tropical regions, which leads to a large sampling error of 3.00 mm in the estimation of global TPW. Sampling in SSMIS is more uniform with mean errors less than 0.5 mm. But the sampling is only over the ocean. Sampling errors in HIRS are larger in tropics and north subtropical areas due to clear sky biased sampling. Moreover, it is significantly correlated with the variability of TPW, whereas the sampling error in COSMIC is less influenced by TPW. Sampling errors will be reduced and more consistent global TPW time series will be derived by simply combining the multisensor samplings together.