Land surface albedo, as an essential biophysical factor, plays an essential role in surface energy balance. Identifying environmental drivers of albedo in the Tibetan Plateau (TP) helps to understand the role of the third pole in responding to environmental change and regulating regional climate. Based on remotely sensed data of albedo, Normalized Difference Vegetation Index (NDVI), snow cover, and soil moisture, this study investigated the effects of land cover (i.e., vegetation, snow) and soil on albedo from the perspectives of spatial, temporal, and spectral (visible, near-infrared, and shortwave) changes of albedo. Generally, changes in shortwave (SW) albedo were primarily driven by changes in snow cover during the growing season (from May to September), predominantly in May and June. The NDVI had larger contributions to visible (VIS) albedo change and was identified as the foremost important driver for VIS albedo in July and August. The correlations between the nearinfrared (NIR) albedo and NDVI were positive in the mid- and late growing season in eastern TP. Soil moisture was negatively correlated with albedo throughout the growing season and was identified as the foremost important driver in August. The NIR albedo was more susceptible than the VIS albedo to changes in soil moisture. The correlations between NDVI and albedo varied across different categories of aridity caused by changing correlations between NIR albedo and NDVI along the aridity gradients, and consequently the VIS and NIR albedo counterbalance can further limit the contributions of vegetation greenness on SW albedo in sub-humid and humid region. Our findings are expected to improve understandings of energy budget simulations over TP region in land surface models.

Influences of the mixing treatments of anthropogenic aerosols on their effective radiative forcing (ERF) and global aridity are evaluated by using the BCC_AGCM2.0_CUACE/Aero, an aerosol-climate online coupled model. Simulations show that the negative ERF due to external mixing (EM, a scheme in which all aerosol particles are treated as independent spheres formed by single substance) aerosols is largely reduced by the partial internal mixing (PIM, a scheme in which some of the aerosol particles are formed by one absorptive and one scattering substance) method. Compared to EM, PIM aerosols have much stronger absorptive ability and generally weaker hygroscopicity, which would lead to changes in radiative forcing, hence to climate. For the global mean values, the ERFs due to anthropogenic aerosols since the pre-industrial are-1.02 and-1.68 W m(-2) for PIM and EM schemes, respectively. The variables related to aridity such as global mean temperature, net radiation flux at the surface, and the potential evaporation capacity are all decreased by 2.18/1.61 K, 5.06/3.90 W m(-2), and 0.21/0.14 mm day(-1) since 1850 for EM and PIM schemes, respectively. According to the changes in aridity index, the anthropogenic aerosols have caused general humidification over central Asia, South America, Africa, and Australia, but great aridification over eastern China and the Tibetan Plateau since the pre-industrial in both mixing schemes. However, the aridification is considerably alleviated in China, but intensified in the Arabian Peninsula and East Africa in the PIM scheme.

Aridity index (AI), defined as the ratio of precipitation to potential evapotranspiration (PET), is a measure of the dryness of terrestrial climate. Global climate models generally project future decreases of AI (drying) associated with global warming scenarios driven by increasing greenhouse gas and declining aerosols. Given their different effects in the climate system, scattering and absorbing aerosols may affect AI differently. Here we explore the terrestrial aridity responses to anthropogenic black carbon (BC) and sulfate (SO4) aerosols with Community Earth System Model simulations. Positive BC radiative forcing decreases precipitation averaged over global land at a rate of 0.9%/degrees C of global mean surface temperature increase (moderate drying), while BC radiative forcing increases PET by 1.0%/degrees C (also drying). BC leads to a global decrease of 1.9%/degrees C in AI (drying). SO4 forcing is negative and causes precipitation a decrease at a rate of 6.7%/degrees C cooling (strong drying). PET also decreases in response to SO4 aerosol cooling by 6.3%/degrees C cooling (contributing to moistening). Thus, SO4 cooling leads to a small decrease in AI (drying) by 0.4%/degrees C cooling. Despite the opposite effects on global mean temperature, BC and SO4 both contribute to the twentieth century drying (AI decrease). Sensitivity test indicates that surface temperature and surface available energy changes dominate BC- and SO4-induced PET changes.