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Variability in clear-sky longwave radiative cooling of the atmosphere

Summary: Variability in clear-sky longwave radiative cooling of
the atmosphere
Richard P. Allan1
Received 16 March 2006; revised 9 June 2006; accepted 2 August 2006; published 18 November 2006.
[1] The longwave radiative cooling of the clear-sky atmosphere (QLWc) is a crucial
component of the global hydrological cycle and is composed of the clear-sky outgoing
longwave radiation to space (OLRc) and the net downward minus upward clear-sky
longwave radiation to the surface (SNLc). Estimates of QLWc from reanalyses and
observations are presented for the period 1979­2004. Compared to other reanalyses data
sets, the European Centre for Medium-range Weather Forecasts 40-year reanalysis
(ERA40) produces the largest QLWc over the tropical oceans (217 W mÀ2
), explained by
the least negative SNLc. On the basis of comparisons with data derived from satellite
measurements, ERA40 provides the most realistic QLWc climatology over the tropical
oceans but exhibits a spurious interannual variability for column integrated water vapor
(CWV) and SNLc. Interannual monthly anomalies of QLWc are broadly consistent between
data sets with large increases during the warm El Nin~o events. Since relative humidity
(RH) errors applying throughout the troposphere result in compensating effects on the
cooling to space and to the surface, they exert only a marginal effect on QLWc. An
observed increase in CWV with surface temperature of 3 kg mÀ2


Source: Allan, Richard P. - Department of Meteorology, University of Reading


Collections: Geosciences