Editorial Type:
Article
Article Type:
Research Article

Evaluation of Simulated Clear-Sky Longwave Radiation Using Ground-Based Observations

Richard P. Allan Department of Meteorology, University of Reading, Reading, Berkshire, United Kingdom

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Print Publication:
01 Jun 2000
DOI:
https://doi.org/10.1175/1520-0442(2000)013<1951:EOSCSL>2.0.CO;2
Page(s):
1951–1964
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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 near-surface temperature in comparison with radiosonde ascents. A primary limitation of the radiometric measurements is the lack of information regarding cloud amount; if model-simulated clear-sky fluxes and cloud radiative forcing are to be fully evaluated, it is highly desirable that such information should accompany surface-based radiation data.

* Current affiliation: Hadley Centre for Climate Prediction and Research, Meteorological Office, Bracknell, Berkshire, United Kingdom.

Corresponding author address: Dr. R. P. Allan, Hadley Centre, The Met. Office, London Road, Bracknell, Berkshire RG12 2SY, United Kingdom.

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 near-surface temperature in comparison with radiosonde ascents. A primary limitation of the radiometric measurements is the lack of information regarding cloud amount; if model-simulated clear-sky fluxes and cloud radiative forcing are to be fully evaluated, it is highly desirable that such information should accompany surface-based radiation data.

* Current affiliation: Hadley Centre for Climate Prediction and Research, Meteorological Office, Bracknell, Berkshire, United Kingdom.

Corresponding author address: Dr. R. P. Allan, Hadley Centre, The Met. Office, London Road, Bracknell, Berkshire RG12 2SY, United Kingdom.

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