Semidirect radiative forcing of internal mixed black carbon cloud droplet and its regional climatic effect over China



[1] Black carbon (BC) is one of the most important atmospheric aerosols. It can exert a positive radiative forcing by absorbing solar radiation and a negative radiative forcing by acting as cloud condensation nuclei (CCN) at the top of the atmosphere (TOA). Furthermore, cloud droplets with BC can absorb more solar radiation and reduce their single-scattering albedo (SSA), leading to a positive cloud radiative forcing (SSA forcing) at TOA. Here we include such a process into the coupled modeling system (RegCCMS) which consists of a regional climate model (RegCM3) and a tropospheric atmosphere chemistry model (TACM) to investigate BC loading, SSA forcing, and its climatic responses in different seasons over China. The year 2003 was selected as the base year for this study. Results show high BC loadings in Sichuan, Hebei, and Shandong provinces with weak seasonal variations. The national average SSA forcing due to the cloud droplets absorption is 0.04, 0.096, 0.085, and 0.056 W/m2 at TOA, while −0.05, −0.14, −0.12, and −0.077 W/m2 at the surface in January, April, July, and October, respectively. It can reach as much as 0.6 W/m2 at TOA and −0.8 W/m2 at the surface in the regions with high BC concentration and cloud cover. Numerical experiments indicate that internal mixed BC and cloud droplet can absorb more solar radiation, then heat the air, hence reducing the cloud cover and increasing vertical velocity. On the other hand, more solar absorption due to mixture droplets can alter atmospheric and hydrologic cycle, which consequently changes the distribution of cloud cover and eventually affects surface air temperature, precipitation, and solar radiation. Given an above 90% confidence level of statistical significance of linear correlation between SSA forcing and climate changes, surface air temperature changes for 4 months vary from −0.0022 to +0.0103 K and precipitation changes range from −0.0047 in January to 0.029 mm/d in April. These changes show an extremely large seasonal variation; so do those of cloud amount (Max: −0.066%) and solar flux (Max: 0.21 W/m2). Owing to the small SSA forcing, its climatic responses are easily masked by internal model variability in summer season, especially for precipitation and cloud. Even so, small SSA forcing in colder seasons may lead to noticeable local surface drought or cooling. For example, precipitation decreases in Yangtze River Basin reaching 1.2 mm/d in October; surface air temperatures decrease by 0.2 to 0.4 K in east China in April. It is obvious that SSA forcing and its climate responses are weaker compared to BC direct, indirect, and semidirect effects. However, it does have nonnegligible influence on regional climate changes over China.