Variability of clouds over Southeast Tibetan Plateau: The roles of aerosols

The southeastern part of the Tibetan Plateau (TP) is the area with the most intensive human activities on the TP, and this place collects most of the precipitation of the TP during summer. In recent year, the characteristics of cloud and precipitation over this region have drawn more attention due to the construction of the Sichuan-Tibet Railway. Even though a lot of progress has been achieved by previous research, the effect of local aerosols on clouds and precipitation over the Tibetan Plateau is still unclear. With the combination of the Himawari-8 geostationary satellite data from 2015 to 2021 and MERRA-2 and ERA-5 reanalysis data, this study investigated the cloud effect of aerosols over the southeastern part of the Tibetan Plateau during summertime. The results show that the daily-averaged concentrations of local aerosols (including dust, carbonaceous aerosols and sulphate aerosols) over the southeastern part of the TP has remained stable in recent years, and the distribution of aerosols performs significant regional characteristic influenced by anthropogenic activities as well as transportation through strong westerlies during monsoon season. Based on domain averaged daily AOD, polluted and clean days are picked out for comparison. During polluted days, all three types of aerosols are seen more abundant near the surface associated with the anomalous downward motions, especially in the southeastern-central part of the selected region, leading to higher AOD over the whole region. For the carbonaceous and sulphate aerosols, we found that the increases in aerosol loading are mainly contributed by local emissions rather than strengthened transportation from the surrounding area such as the Sichuan basin. For the dust, of which mass concentration dominates the local aerosol loading, the anomalous northeastern wind distribution suggests that the higher accumulation is possibly due to the stronger transportation from the desert region of northern China. The cloud property trends are found associated with higher aerosol loading. With unfavorable water vapor transport conditions, an increase in aerosol loading leads to a decrease in the occurrence frequency of clouds, the cloud optical thickness (COD), and the effective radius of cloud droplet particles (re), findings consistent with the Twomey effect. However, the hygroscopicity and CCN forming potential of aerosols can be significantly enhanced by mixing with anthropogenic sulfate aerosols, leading to an increase in re and occurrence frequency of clouds with water vapor convergence. In addition, the orographic uplifting over the regions of Yarlung Zangbo river valley, Nyenchen Tanglha mountain range in the southern area and Tanggula mountain range in the northern area also contributes to the condensation and coagulation processes of activated aerosols, thus promoting the generation and development of clouds over the mountainous region. The impact of aerosols on the characteristics of convective clouds is more significant than that of stratus clouds, and its effect is subject to water vapor conditions. The occurrence frequency of convective clouds will increase when there is sufficient water vapor, while when the water vapor is insufficient, the effective radius of cloud droplet particles will decrease due to the Twomey effect, further leading to cloud optical thickness increase. The results of our research are of great significance for predicting and evaluating the characteristics of cloud and precipitation along the Sichuan-Tibet Railway, and also warrant further investigations using both modeling and stationary observational studies.