Anthropogenic aerosol radiative forcing in Asia derived from regional models with atmospheric and aerosol data assimilation

Abstract

Abstract. An estimate of monthly 3-D aerosol solar heating rates and surface solar fluxes in Asia from 2001 to 2004 is described here. This product stems from an Asian aerosol assimilation project, in which a) the PNNL regional model bounded by the NCEP reanalyses was used to provide meteorology, b) MODIS and AERONET data were integrated for aerosol observations, c) the Iowa aerosol/chemistry model STEM-2K1 used the PNNL meteorology and assimilated aerosol observations, and d) 3-D (X-Y-Z) aerosol simulations from the STEM-2K1 were used in the Scripps Monte-Carlo Aerosol Cloud Radiation (MACR) model to produce total and anthropogenic aerosol direct solar forcing for average cloudy skies. The MACR model and STEM-2K1 both used the PNNL model resolution of 0.45°×0.4° in the horizontal and of 23 layers in the troposphere. The 2001–2004 averaged anthropogenic all-sky aerosol forcing is −1.3 Wm−2 (TOA), +7.3 Wm−2 (atmosphere) and −8.6 Wm−2 (surface) averaged in Asia (60–138° E and Equator–45° N). In the absence of AERONET SSA assimilation, absorbing aerosol concentration (especially BC aerosol) is much smaller, giving −2.3 Wm−2 (TOA), +4.5 Wm−2 (atmosphere) and −6.8 Wm−2 (surface), averaged in Asia. In the vertical, monthly forcing is mainly concentrated below 600 hPa with maximum around 800 hPa. Seasonally, low-level forcing is far larger in dry season than in wet season in South Asia, whereas the wet season forcing exceeds the dry season forcing in East Asia. The anthropogenic forcing in the present study is similar to that in Chung et al. (2005) in overall magnitude but the former offers fine-scale features and simulated vertical profiles. The interannual variability of the computed anthropogenic forcing is significant and extremely large over major emission outflow areas. Given the interannual variability, the present study's estimate is within the implicated range of the 1999 INDOEX result.