Aerosol climatologies from a numerical model and climate observations

Aerosols play an important role in the climate system. They absorb and scatter radiation, and they act as condensation nuclei in cloud-forming processes. Despite their importance, the spatial and temporal variability of atmospheric turbidity--a measure of the amount of aerosol in the atmospheric column--is not well documented. Conventional methods of quantifying turbidity are constrained by the need for clear skies and costly equipment, and this has contributed to the paucity of turbidity data.

Using a physically based, high-resolution spectral radiation model, instantaneous clear-sky beam irradiance is parameterized with simple atmospheric functions and polynomial expansions of the sine of the solar elevation angle. This construction permits analytic integration to obtain an expression for estimating potential daily clear-sky beam irradiation. By forcing the model with monthly-averaged climate data, turbidity is estimated at an array of weather-station locations in South Africa. Findings include new estimates of turbidity trends and variability over South Africa. The methodology makes use of available climate data, is not restricted to clear-sky conditions, and can be applied at any location where the requisite data are available.