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1 JUNE 2000 1951A L L A N Evaluation of Simulated Clear-Sky Longwave Radiation Using
 

Summary: 1 JUNE 2000 1951A L L A N
Evaluation of Simulated Clear-Sky Longwave Radiation Using
Ground-Based Observations
RICHARD P. ALLAN*
Department of Meteorology, University of Reading, Reading, Berkshire, United Kingdom
(Manuscript received 5 January 1999, in final form 3 June 1999)
ABSTRACT
Surface observations from a tropical ocean and a subarctic land-based site are employed to evaluate the clear-
sky surface downwelling longwave irradiance (SDL) simulated using the European Centre for Medium-Range
Weather Forecasts reanalysis (ERA). Comparison of simulated clear-sky and observed all-sky SDL highlights
coincident periods of irradiance variability on various timescales in both datasets. Measurements during cloudy
conditions are subsequently removed using a combination of the measured shortwave and longwave surface
irradiances and recorded rainfall. The most reasonable filtering specifications are determined experimentally;
clear-sky filtered observations of SDL are compared with corresponding simulated values. The root-mean-square
differences between simulated and observed clear-sky SDL are within the observational uncertainty of 10 W
m 2
. Simulated clear-sky SDL is about 8 W m 2
more than the measured tropical values. In the subarctic, the
simulated clear-sky SDL is less than observed values in the winter and greater than observed values in the
summer. The clear-sky SDL differences are explained partially by the differences in ERA moisture profiles and

  

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

 

Collections: Geosciences