Dust aerosol optical properties retrieval and radiative forcing over northwestern China during the 2008 China-US joint field experiment

The Atmosphere Radiation Measurements Program's Ancillary Facility (AAF/SMART-COMMIT) was deployed to Zhangye (39.082 degrees N, 100.276 degrees E), which is located in a semidesert area of northwest China, during the period of late April to mid June in 2008. We selected 11 cases to retrieve dust aerosol optical depth (AOD), Angstrom exponent, size distribution, single-scattering albedo (SSA) and asymmetry parameter (ASY) from multifilter rotating shadowband radiometer (MFRSR) measurements. These cases are dominated by large particles with Angstrom exponent values ranging from 0.34 to 0.93. The values of AOD at 0.67 mu m range from 0.07 to 0.25. The mean SSA value increases with wavelength from 0.76 +/- 0.02 at 0.415 mu m to 0.86 +/- 0.01 at 0.870 mu m, while the mean ASY value decreases from 0.74 +/- 0.04 to 0.70 +/- 0.02. Before estimating dust aerosol direct radiative forcing, a radiative closure experiment was performed to verify that the retrieved aerosol optical properties and other input parameters to the radiative transfer model appropriately represent atmospheric conditions. The daytime-averaged differences between model simulations and ground observations are -8.5, -2.9, and -2.1 W m(-2) for the total, diffuse, and direct normal fluxes, respectively. The mean difference in the instantaneous reflected solar fluxes at the top of atmosphere (TOA) between the model and CERES observations is 8.0 W m(-2). The solar aerosol direct radiative forcing (ARF), averaged over a 24 h period, at the surface is -22.4 +/- 8.9 W m(-2), while the TOA ARF is small and has an average value of only 0.52 +/- 1.69 W m(-2). The daily averaged surface aerosol radiative forcing efficiency at 0.5 mu m is -95.1 +/- 10.3 W m(-2) tau(-1). Our results illustrate that the primary role of dust aerosol is to alter the distribution of solar radiation within the climate system rather than to reflect solar energy to space. We assess the satellite aerosol optical depth products from Mutiangle Imaging Spectroradiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) observations by comparing them with our ground-based retrievals. Reasonable agreements with the ground-based observations are found for the MISR product and MODIS Deep Blue product.