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Summary: Large discrepancy between observed and simulated precipitation
trends in the ascending and descending branches of the tropical
circulation
Richard P. Allan1
and Brian J. Soden2
Received 25 July 2007; revised 14 August 2007; accepted 22 August 2007; published 26 September 2007.
[1] Observed and model simulated changes in precipitation
are examined using vertical motion at 500 hPa to define
ascending and descending branches of the tropical
circulation. Vertical motion fields from reanalyses were
employed to subsample the observed precipitation data. An
emerging signal of rising precipitation trends in the
ascending regions and decreasing trends in the descending
regimes are detected in the observational datasets. These
trends are substantially larger in magnitude than present-day
model simulations and projections into the 21st century. The
discrepancy cannot be explained by changes in the
reanalysis fields used to subsample the observations but
instead must relate to errors in the satellite data or in the
model parametrizations. This has important implications for
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