Aerosol sulfate loading and shortwave direct radiative forcing over the North Atlantic Ocean. Nemesure, S., Benkovitz, C. M., and Schwartz, S. E. 7th Atmospheric Radiation Measurement (ARM) Science Team Meeting, San Antonio, TX, Mar. 3-7, 1997.

Shortwave radiative forcing of climate by anthropogenic sulfate aerosols is estimated to be equal in magnitude but opposite in sign to that of greenhouse warming, with a global annual average value of approximately -1 W m-2 uncertain to at least a factor of two. Estimates of the contributions to this forcing by the direct effect are -0.4 W m-2. It is therefore necessary to accurately and efficiently represent this forcing in climate models, specifically including spatial and temporal variability. Here we explore a method to expedite the process for determining this forcing. The method utilizes an approach where the forcing is computed precisely at several discrete radii and then integrated over an arbitrary aerosol size distribution. Additionally, the forcing is calculated at several values of relative humidity, solar zenith angle, and aerosol optical thickness. The parameters can be interpolated to provide the forcing at specific intermediate values. Alternatively, an empirical relationship between the forcing and the above mentioned variables can be utilized to further reduce computation time. At present, the calculations are restricted to ammonium sulfate particles over an ocean surface. The advantage of the ocean surface is the constant and low albedo compared to the highly variable albedo of land surfaces. Ultimately, the sensitivity of forcing to surface albedo and composition will be included.

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