Evaluation of Tropospheric Moisture Characteristics Between COSMIC-2, ERA5, and MERRA-2 in the Tropics and Subtropics

Abstract:

GNSS radio occultation (RO) receivers onboard the recently-launched COSMIC-2 satellite constellation provide an unprecedented number of high vertical resolution moisture profiles throughout the challenging tropical and subtropical atmosphere. In this study, the distribution of water vapor was investigated using specific humidity profiles from these RO observations and compared to collocated ERA5 and MERRA-2 reanalysis profiles within 40°N-40°S from September-December 2019. Large negative moisture biases were evident within the boundary layer (20-30%) compared to both reanalyses due to super-refraction. The best agreement was observed in the lower troposphere just above the boundary layer as differences were on the order of 5-10%, although some bias hotspots were observed in MERRA-2. In the tropical mid-troposphere, COSMIC-2 generally displayed more moisture than ERA-5 (5-15%) and much less moisture than MERRA-2 (15-30%). Time-series and Hovmoller analyses of moisture differences were conducted during the four-month period. Distinct shifts in the bias were often observed in regions of active deep convection, especially as the location of the ITCZ varied. Overall, COSMIC-2 moisture agreed well with ERA5 and showed much larger differences with MERRA-2. Assimilation of the high resolution COSMIC-2 soundings would likely improve the moisture output of both reanalysis datasets, especially for MERRA-2 within the tropical troposphere.