Field measurement of clear-sky solar irradiance in Badain Jaran Desert of Northwestern China

Abstract

The Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) sponsored and conducted an intensive field campaign on dust aerosols in Badain Jaran Desert of Northwestern China from April 20 to June 20, 2010. A set of state-of-the-art broadband radiometers and sun/sky photometers were deployed along with launched radiosonde. In this paper, we compared the simulated solar irradiances by using the SBDART radiative transfer model with those from the ground-based measurements for 69 selected cases of 7 days. It was shown that the averaged aerosol optical depth at 500nm (AOD500) is 0.18±0.09 with AOD500 less than 0.5 for all cases. The single-scattering albedo and asymmetry factor at 675nm are 0.928±0.035, 0.712±0.023, respectively. The AODs retrieved from the CIMEL sun photometer at various wavelengths agree well with those from the PREDE sky radiometer, and the columnar water vapor contents from CIMEL also agree well with radiosonde observations. In the radiative closure experiment, we used a collocated thermopile pyrgeometer with a shadow and ventilator to correct the thermal dome offset of diffuse irradiance measurement. The mean differences between model and measurements are −9.1Wm−2 (−2.6%) for the direct irradiance, +3.1Wm−2 (+2.8%) for diffuse irradiance, and −6.0Wm−2 (−1.3%) for global irradiance, which indicates an excellent radiative closure. Aerosol shortwave direct radiative forcing (ARF) and radiative heating rate are also investigated. The daily mean ARF ranges from −4.8 to +0.4Wm−2 at the top of the atmosphere, −5.2 to −15.6Wm−2 at the surface, and 5.2 to 10.8Wm−2 in the atmosphere. The corresponding radiative heating rates for the whole atmosphere due to dust aerosols are 0.07, 0.11, 0.14, 0.11, 0.10, 0.08, and 0.07K/day for the 7 selected cloudless days. These solar radiative forcing can be considered as the representative impact of background dust aerosol in Northwestern China.