A Study on Sulfate Optical Properties and Direct Radiative Forcing Using LASG-IAP General Circulation Model

The direct radiative forcing (DRF) of sulfate aerosols depends highly on theatmospheric sulfate loading and the meteorology, both of which undergostrong regional and seasonal variations. Because the optical properties ofsulfate aerosols are also sensitive to atmospheric relative humidity, inthis study we first examine the scheme for optical properties that considershydroscopic growth. Next, we investigate the seasonal and regionaldistributions of sulfate DRF using the sulfate loading simulated from NCARCAM-Chem together with the meteorology modeled from a spectral atmosphericgeneral circulation model (AGCM) developed by LASG-IAP. The globalannual-mean sulfate loading of 3.44 mg m^-2 is calculated to yield theDRF of -1.03 and -0.57 W m^-2 for clear-sky and all-sky conditions,respectively. However, much larger values occur on regional bases. Forexample, the maximum all-sky sulfate DRF over Europe, East Asia, and NorthAmerica can be up to -4.0 W m^-2. The strongest all-sky sulfate DRFoccurs in the Northern Hemispheric July, with a hemispheric average of-1.26 W m^-2. The study results also indicate that the regional DRFare strongly affected by cloud and relative humidity, which varyconsiderably among the regions during different seasons. This certainlyraises the issue that the biases in model-simulated regional meteorology canintroduce biases into the sulfate DRF. Hence, the model processes associatedwith atmospheric humidity and cloud physics should be modified in greatdepth to improve the simulations of the LASG-IAP AGCM and to reduce theuncertainty of sulfate direct effects on global and regional climate inthese simulations.