Climate Change 2001:
Working Group I: The Scientific Basis
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Figure 7.1: Comparison between an observational estimate from satellite radiances and two model simulations of the complex structure of midtropospheric water vapour distribution for the date May 5. At any instant, water vapour is unevenly distributed in the atmosphere with very dry areas adjacent to very moist areas. Any modification in the statistics of those areas participates in the atmospheric feedback. The observed small-scale structure of the strong and variable gradients (a) is not well resolved in a simulation with a climate model of the spatial resolution currently used for climate projections (b), but simulated with much better fidelity in models with significantly higher resolution (c). (a): Distribution of mean relative humidity in layer 250 to 600 mb on May 5, 1998, as retrieved from observations on SSM/T-2 satellite (Spencer and Braswell, 1997). Missing data are indicated by black areas and the retrieval is most reliable in the latitude band 30°S to 30°N. (b): Relative humidity at about 400 mb for May 5 of an arbitrary year from a simulation with the GFDL R30L14 atmospheric general circulation model used in the AMIP I simulation (Gates et al., 1999; Lau and Nath, 1999). In small polar areas (about 5% of the globe) some relative humidities are negative (set to zero) due to numerical spectral effects. (c): Relative humidity at 400 mb from the ECHAM4 T106 simulation for May 5 of an arbitrary year (Roeckner et al., 1996; Wild et al., 1998).

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