Water vapor is one of the most important greenhouse gases. As it drifts through the atmosphere, the vapor absorbs heat. The warm, moist air rises, leaving the planet’s surface and emitting that heat back out into the upper atmosphere. To determine how well we understand the behavior of water vapor at high altitudes, Jiang et al. compared three different reanalyses—long-term records of changing weather patterns over time—to satellite observations.
The Microwave Limb Sounder (MLS) is an instrument aboard NASA’s Aura satellite that takes measurements of the atmosphere’s composition, humidity, and temperature at altitudes of roughly 8 kilometers and above. The researchers used such measurements to examine the accuracy of water vapor calculations from two versions of NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA and MERRA2) reanalysis and the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalyses—all of which provide climate modelers with estimations of how much water is in the atmosphere. The authors specifically investigated the amounts of water in the upper troposphere and lower stratosphere and how much water was transferred between the two layers. This was the first study of NASA MERRA2 reanalysis since its release in October 2015.
They found that the reanalyses were fairly inconsistent with the MLS observations. The reanalyses overestimated the average amount of water in the upper troposphere, overshooting the MLS observations by about 150%. The team also found that the MLS measured water vapor traveling between the two layers slower than the reanalyses predicted. Vertical transport within the lower stratosphere in the tropics was 168% faster than the MLS measures according to the ECMWF, whereas MERRA numbers were only 10% faster than the MLS ones. The rate of horizontal water movement was also skewed: MERRA had water moving 106% faster in the Northern Hemisphere and up to 45% slower in the Southern Hemisphere, and ECMWF was 16% faster in both hemispheres.
These findings highlight our incomplete understanding of water vapor’s distribution and variability in the upper troposphere and lower stratosphere. The researchers call for further study on water vapor behavior at these high altitudes, as the large variance between the reanalyses could hinder the production of accurate climate models. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2015JD023752, 2015)
—Cody Sullivan, Freelance Writer
Citation: Sullivan, C. (2016), The forgotten water vapor at high altitudes, Eos, 97, doi:10.1029/2016EO045075. Published on 5 February 2016.