1999 Volume 77 Issue 5 Pages 1007-1021
In order to estimate the annual surface shortwave forcing by clouds+aerosols and aerosols, the shortwave flux from pyrheliometer and pyranometer measurements, atmospheric profiles from the radiosonde measurements, and aerosol optical properties retrieved from sky radiometer measurements were integrated with high-accuracy transfer model calculations.
Clear-sky flux was defined from transfer calculations for a pure Rayleigh-scattering atmosphere, with measured temperature and humidity profiles by radiosonde observations. Monthly variation of the clear-sky flux due to the temperature and water vapor variation was 10-30Wm-2. Cloud+aerosol forcing was defined by the difference between the observed flux and the clear-sky flux (positive downward). The annual mean values of the cloud+aerosol surface shortwave forcing was estimated as -81Wm-2, which corresponds to about 24% of the insolation. The aerosol-sky flux is defined with the transfer calculation using the aerosol optical depth retrieved from the sky radiometer measurements. Aerosol forcing was obtained from the differences between the clear-sky flux and the aerosol-sky flux. The mean direct aerosol forcing for 1996, except for March and April, was estimated as -18Wm-2, about 6% of the insolation.
We also performed a sensitivity study of the aerosol-sky flux by varying the weight fraction of soot in aerosols. Among the selected soot fraction, the best estimates were obtained as 10% for January, February and July, 20% for October through December, 5% for May, June and August, and 0% for September. These values are close to the measured seasonal variations of soot fraction in previous studies.
Surface flux calculation with the retrieved aerosol size distributions performed no better than those with the LOWTRAN 7 urban model size distribution, especially in the summer months when the water vapor column amount was large. The necessity of further examination of retrieval methods of aerosol optical properties, using sky radiometer measurements, was suggested.