Direct and indirect effects of anthropogenic aerosols on regional precipitation over east Asia

Abstract

[1] A regional coupled climate-chemistry-aerosol model is developed. It is used to assess the direct and indirect effects of anthropogenic sulfate and carbonaceous aerosols on regional climate over east Asia with a focus on precipitation. The simulated direct and first indirect effects for the most part reduce the solar radiation and hence decrease the surface temperature, while the second indirect effect generates both negative solar forcing and a substantial positive long-wave forcing. It decreases the precipitation, but because of the cancelling effect, surface temperature does not change very much. With the interactively model-calculated current aerosol loading and the combined direct/semidirect/ first indirect effect, the simulated precipitation is reduced by about 10% in the fall and winter and by about 5% in the spring and summer. The second indirect effect has the largest impact, by itself decreasing the fall and winter precipitation from about 3% to 20%, depending on the autoconversion scheme assumed. The semidirect effect on precipitation is relatively small. An empirical orthogonal function analysis of climatological precipitation over east Asia since the last century shows a decreasing trend of the leading modes over most of China in the fall and winter, which is generally geographically consistent with the distribution of the model-simulated precipitation reduction from anthropogenic aerosols.

Sulfur cycle and sulfate radiative forcing simulated from a coupled global climate-chemistry model

“…Regional sulfate forcing from GCCM reaches In East Asia for example, weaker summer monsoon and stronger winter monsoon emerge under the forcing of anthropogenic sulfate, leading to reduced moisture flux from the ocean to the East Asian continent in both seasons. The changes are in many aspects similar to those simulated with the regional coupled climate-chemistry model by Huang et al (2007) who found regional radiative forcing and surface cooling in East Asia (note: area definition might be different from ours) to be −4.08 W m −2 and −0.35 K, respectively. They also found a reduction of cloud fraction by 0.8% and precipitation by more than 10%; whereas the values from GCCM (2.4%, and 10.3%, respectively) are of similar orders.…”

“…Shindell et al (2001) also used online model to illustrate that the OH concentration could be reduced by about 10%, which would be significant to many chemical processes. Besides global models, regional models are also tending towards online coupling (e.g., Giorgi et al 2002;Grell et al 2005;Forkel and Knotche 2006;Huang et al 2007), as the radiative effects can be particularly strong on the local scales.…”

Aerosol optical properties and radiative forcing in the high Himalaya based on measurements at the Nepal Climate Observatory-Pyramid site (5079 m a.s.l.)

“…Measurements of aerosol chemical composition and aerosol optical depth in the Nepal Himalaya have clearly shown the build up of aerosols in the pre-monsoon season during the winter and early spring, with relatively high values of light absorbing particulate matter including dust and black carbon (Carrico et al, 2003). Very recently, new insight into the mechanisms of aerosol transport from dust and pollutions sources in central and southeastern Asia to the Tibetan and Himalayan regions was also provided by Huang et al, 2007 andRamanathan et al, 2007. These studies indicate that aerosol in-situ and columnar concentrations over the south slope of Himalayas are strongly affected by the development of the boundary layer, responsible for transporting pollution aerosols upward, contributing to changes in solar irradiance Dumka et al, 2008;Ramachandran, 2008). Assessing the impact of aerosol particle on the local energy balance in the high altitude regions of Himalayas is therefore of importance for the whole area of HKH regions.…”

Preferred response of the East Asian summer monsoon to local and non-local anthropogenic sulphur dioxide emissions

“…Subsequently, however, this cold and dry anomaly is advected into Asia, where it induces atmospheric and surface feedbacks over Asia and the Western North Pacific (WNP), which also weaken the EASM. In spite of very different perturbations to the local aerosol burden in response to Asian and European sulphur dioxide 1 3 , decadal changes in sensible heat flux over the Tibetan Plateau (Ding et al 2009;Duan et al 2013), anthropogenic aerosol (Xu 2001;Menon et al 2002;Huang et al 2007;Liu et al 2009Liu et al , 2011Chen et al 2012;Zhang et al 2012;Guo et al 2013;Hwang et al 2013;Jiang et al 2013;Ye et al 2013;Wang et al 2013;Zhou et al 2013;Polson et al 2014;Song et al 2014), and natural decadal variability (Lei et al 2011).…”