Intercalibration of microwave temperature sounders using radio occultation measurements
Journal article, 2015
This is a study of the usefulness of radio occultation (RO) data for intercomparing different microwave temperature (MWT) sounding instruments. The RO data used are from the Global Navigational Satellite System Receiver for Atmospheric Sounding on the Metop-A and B satellites. The MWT sounders investigated are the Advanced Microwave Sounding Unit-A instruments on the satellites NOAA 15, 16, and 18 and Metop-A. We collocate RO and MWT data and then use these collocations to study various aspects of the MWT instruments. In addition, two different versions of the MWT data are compared: standard operational data (OPR) and the NOAA Integrated Microwave Intercalibration Approach data (IMICA). The time series of monthly mean differences shows that there are robust patterns for each satellite and data version, which mostly drift only slowly over time. The intersatellite spread, measured by the standard deviation of the yearly mean values by all satellites, is between 0.1 and 0.4K, depending on channel, with no significant differences between OPR and IMICA data. The only notable exception is Channel 8 of NOAA 16, which appears to have a time-varying offset of 0.5-1K relative to the other instruments. At this point it is not clear whether this deviation is real or a sampling artifact, so further study is needed. Due to the large number of collocations used, it is possible to also investigate the scene brightness and scan angle dependence of the MWT bias (relative to RO). First results of such an analysis are presented and discussed. Particularly, the investigation of the scan angle dependence is novel, since this bias pattern is difficult to assess without RO data. Further work is needed on these angular dependences, before conclusions are robust enough to include in data recalibration efforts, but our overall conclusion is that RO collocations are a powerful tool for intercomparing MWT sounders.