Document Type

Article

Publication Date

2012

DOI

10.1029/2011JD016623

Publication Title

Journal of Geophysical Research: Atmospheres

Volume

117

Issue

D5

Pages

D05304 (1-25)

Abstract

Evaluating the representation of processes controlling tropical and subtropical tropospheric relative humidity (RH) in atmospheric general circulation models (GCMs) is crucial to assess the credibility of predicted climate changes. GCMs have long exhibited a moist bias in the tropical and subtropical mid and upper troposphere, which could be due to the mis-representation of cloud processes or of the large-scale circulation, or to excessive diffusion during water vapor transport. The goal of this study is to use observations of the water vapor isotopic ratio to understand the cause of this bias. We compare the three-dimensional distribution of the water vapor isotopic ratio measured from space and ground to that simulated by several versions of the isotopic GCM LMDZ. We show that the combined evaluation of RH and of the water vapor isotopic composition makes it possible to discriminate the most likely cause of RH biases. Models characterized either by an excessive vertical diffusion, an excessive convective detrainment or an underestimated in situ cloud condensation will all produce a moist bias in the free troposphere. However, only an excessive vertical diffusion can lead to a reversed seasonality of the free tropospheric isotopic composition in the subtropics compared to observations. Comparing seven isotopic GCMs suggests that the moist bias found in many GCMs in the mid and upper troposphere most frequently results from an excessive diffusion during vertical water vapor transport. This study demonstrates the added value of water vapor isotopic measurements for interpreting shortcomings in the simulation of RH by climate models.

Rights

© 2012 American Geophysical Union.

"AGU allows the final published article (version of record) to be placed in an institutional repository or personal website if the AGU copyright statement is clearly visible, and the post is made 6 months after the official publication by AGU. AGU also supports governmental and institutional repositories and access is available to AGU content through CHORUS, usually 12 months after publication."

Original Publication Citation

Risi, C., Noone, D., Worden, J., Frankenberg, C., Stiller, G., Kiefer, M., Funke, B., Walker, K., Bernath, P., Schneider, M., Bony, S., Lee, J., Brown, D., & Sturm, C. (2012). Process-evaluation of tropospheric humidity simulated by general circulation models using water vapor isotopic observations: 2. Using isotopic diagnostics to understand the mid and upper tropospheric moist bias in the tropics and subtropics. Journal of Geophysical Research: Atmospheres, 117(5), 1-25, Article D05304. https://doi.org/10.1029/2011JD016623

ORCID

0000-0002-1255-396X (Bernath)

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