Summary: Examination of Relationships between Clear-Sky Longwave Radiation and Aspects of the
Atmospheric Hydrological Cycle in Climate Models, Reanalyses, and Observations
RICHARD P. ALLAN
Environmental Systems Science Centre, University of Reading, Reading, United Kingdom
(Manuscript received 5 May 2008, in final form 8 December 2008)
ABSTRACT
Relationships between clear-sky longwave radiation and aspects of the atmospheric hydrological cycle are
quantified in models, reanalyses, and observations over the period 1980­2000. The robust sensitivity of clear-
sky surface net longwave radiation (SNLc) to column-integrated water vapor (CWV) of 1­1.5 W m22
mm21
combined with the positive relationship between CWV and surface temperature (Ts) explains substantial
increases in clear-sky longwave radiative cooling of the atmosphere (QLWc) to the surface over the period.
Clear-sky outgoing longwave radiation (OLRc) is highly sensitive to changes in aerosol and greenhouse gas
concentrations in addition to temperature and humidity. Over tropical ocean regions of mean descent, QLWc
increases with Ts at ;3.5­5.5 W m22
K21
for reanalyses, estimates derived from satellite data, and models
without volcanic forcing included. Increased QLWc with warming across the tropical oceans helps to explain
model ensemble mean increases in precipitation of 0.1­0.15 mm day21
K21

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

Collections: Geosciences