Abstract
Global navigation satellite system (GNSS) radio occultation (RO) receivers onboard the recently-launched COSMIC-2 (C2) satellite constellation provide an unprecedented number of high vertical resolution moisture profiles throughout the tropical and subtropical atmosphere. In this study, the distribution and variability of water vapor was investigated using specific humidity retrievals from C2 observations and compared to collocated ERA5 and MERRA-2 reanalysis profiles within 40°N to 40°S from September to December 2019, which is prior to the assimilation of C2 in the reanalyses. Negative C2 moisture biases are evident within the boundary layer, so we focused on levels above the boundary layer in this study. Overall, C2 specific humidity shows excellent agreement with that of ERA5 and has larger differences with that of MERRA-2. In the tropical mid-troposphere, C2 shows positive biases compared to ERA5 (6–12%) and larger negative biases with MERRA-2 (15–30%). Strong correlations are observed between C2 and reanalysis specific humidity in the subtropics (>0.8) whereas correlations are slightly weaker in the deep tropics, especially for MERRA-2. Profile pairs with large moisture differences often occur in areas with sharp moisture gradients, highlighting the importance of measurement resolution. Locations which demonstrated weaker humidity correlations in active convection regions show that ERA5 has a negative specific humidity bias at 3 km in higher moisture environments, whereas MERRA-2 displays a large positive bias at 7 km. However, additional explanations for profile pairs with large moisture differences remain unclear and require further study.
Highlights
Accepted: 23 February 2021Water vapor is the Earth’s most important greenhouse gas as it accounts for nearly twothirds of the natural greenhouse effect [1]
The results of this study focus on evaluating the specific humidity differences between the C2, ERA5, and MERRA-2 datasets throughout the tropics and subtropics
Tropical and subtropical water vapor distribution and variability was explored from September–December 2019 using specific humidity (SH) profiles from the recently launched
Summary
Water vapor is the Earth’s most important greenhouse gas as it accounts for nearly twothirds of the natural greenhouse effect [1] It plays a major role in the global energy cycle as a dominant feedback variable in association with radiative effects and moist dynamics [2], and fast-acting water vapor feedbacks constitute a strong amplification mechanism for anthropogenic climate change, which makes water vapor a key parameter for climate change analysis [3]. Water vapor carries a large amount of latent heat which is released into the atmosphere during condensation and stored again through evaporation [4] This results in the vertical distribution of tropospheric water vapor controlling many aspects of the climate, especially through its influence on shallow and deep convection throughout the tropics [5]. Accurate and consistent tropospheric water vapor measurements are essential for studying water vapor feedbacks on the global energy budget, which is still one of the largest uncertainties in understanding global warming [7]
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