Shortwave radiative closure experiment and direct forcing of dust aerosol over northwestern China

Abstract

Modeled and observed solar diffuse fluxes at the surface usually have unacceptably large discrepancies. It is necessary to address and resolve these discrepancies in order to accurately calculate a reliable aerosol direct radiative forcing (DRF). We present and compare two methods of deriving dust aerosol optical properties from the MFRSR (Multi-Filter Rotating Shadowband Radiometer) observations and the AERONET products. The single-scattering albedo (SSA) values from MFRSR are found to be 10% less than those from the AERONET. This difference is mainly due to the different imaginary part of refractive index retrieved by the MFRSR compared to AERONET. These two sets of dust aerosol optical properties are used in the SBDART model to simulate the shortwave fluxes that are compared with the surface observations to perform the radiative closure experiment. The diffuse simulations using the AERONET-derived aerosol SSA may have significant discrepancies compared with the observed diffuse irradiances. The DRFs at the top of atmosphere (TOA) simulated with the MFRSR-derived aerosol optical properties are positive while the DRFs with the AERONET are negative. The sign of the DRFs at the surface and in the atmosphere using the MFRSR is the same as those using the AERONET while the magnitudes from the MFRSR are much larger. It indicates that dust aerosols with higher absorption as derived from the MFRSR heat the aerosol layer but cool the surface much more than those based on the AERONET, which may have an important impact on the boundary layer processes. Citation: Ge, J. M., J. P. Huang, J. Su, J. R. Bi, and Q. Fu (2011), Shortwave radiative closure experiment and direct forcing of dust aerosol over northwestern China, Geophys. Res. Lett., 38, L24803, doi:10.1029/2011GL049571.