Direct Aerosol Forcing Calculated at the ARM Southern Great Plains Site

The continuous measurements of direct and diffuse solar radiation, water vapor column amount, and aerosol optical depth provided at the ARM SGP site permit us to calculate directly the actual magnitude of the direct aerosol forcing. Our methodology employs the clear sky detection algorithm of Long and Ackerman (2000) to identify cloudless periods. We then fit the downward solar flux at the surface during these periods with an empirical function, which provides us with a continuous mathematical representation of the surface flux under aerosol conditions. The flux under completely clear skies (no aerosol) can be arrived at using two different techniques. In the first, we search our fits for the maximum function value, which corresponds to the minimum aerosol optical depth, in each month. This value represents our best estimate observation of the surface flux in the absence of any aerosol. The direct aerosol forcing is calculated then as the difference between the value of the fitted function at any time and the value of the maximum function at that same time. The validity of this approach is verified by carrying out detailed radiative transfer calculations using measured values of water vapor amount and optical depth for selected events. In the second method, we use measured inputs of atmospheric temperature and water vapor profiles in our radiative transfer model and simply calculate the clear-sky flux. This approach relies, however, on the ability of the model to represent accurately solar radiative transfer. Our previous studies indicate that this approach is accurate at the 10 W/m2 level. The direct aerosol forcing values from either method are then aggregated into daily and monthly values for the 2001 year. By weighting these values by the fraction of cloudless sky and daylight, we can arrive at the monthly mean and annual mean direct aerosol forcing for this site. Using our radiative transfer model results, we can apportion approximately this surface aerosol forcing between top-of-atmosphere forcing and atmospheric absorption. The results of this analysis will be presented and compared to the current estimates of aerosol forcing from models and satellites for the central United States.