Comparison of water vapour products retrieved from Metop-A by Radio Occultation, Infrared and Microwave systems

Abstract. Water vapour (WV) is an essential climate variable (ECV) and different Earth Observing Systems (EOS) have been used to monitor and characterise its distribution, transportation and interplay in different phenomena. Metop-A satellite carried since 2006 four of such systems: (i) Infrared Atmospheric Sounding Interferometer (IASI), (ii) Advanced Microwave Sounding Unit (AMSU-A), (iii) Microwave Humidity Sounder (MHS), and (iv) Global navigation satellite system Receiver for Atmospheric Sounding (GRAS) performing radio occultation (RO) measurements. These systems operate at different frequencies, with different acquisition geometries, and also use different retrieval schemes to obtain vertically resolved WV profiles in the troposphere. Therefore, they provide independent measurements of WV, each with its limitations and strengths. Their characterisation via comparison with other datasets is important to estimate their systematic differences and uncertainties, which must be known when it comes to their use as climate data records (CDR) in climate monitoring and change studies. In this study, the water vapour product part of the Rutherford Appleton Laboratory (RAL) Infrared and Microwave System (IMS) scheme used in the European Space Agency (ESA) Water Vapour Climate Change Initiative (WV_cci) is compared to GRAS-RO WV data, part of Radio Occultation Meteorology Satellite Application Facility (ROM SAF) CDR, v1.0. The comparison uses ERA-Interim analysis and the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) as references and presents the systematic differences in different latitude bands, gauges the influence of cloud contamination on the statistics, as well as the differences between profiles over land and water, and during day and nighttime. Results in the lower troposphere (LT) show a significant difference between the data sets, where RAL IMS is wetter than ERA-I and GRUAN, especially in mid and high latitudes and over water (up to 14 % ppmv) regardless of clouds, whereas GRAS-RO is drier than the references (5.3 % ppmv at maximum w.r.t. GRUAN and during night). In the upper troposphere (UT), both data sets are drier than ERA-I (16.8 % ppmv, RAL IMS; 7.2 % ppmv, RO) and wetter than GRUAN (up to about 20 % ppmv). In terms of variability, smoothing GRAS-RO, ERA-I and GRUAN with RAL IMS WV averaging kernels (AK) reduces the magnitude of their median absolute deviation (MAD) and increases their similarities (LT: RO, 8.78 % ppmv; RAL IMS, 12.75 % ppmv. UT: up to 36.33 % ppmv).