Xiaoning Xie

On the Robustness of the Weakening Effect of Anthropogenic Aerosols on the East Asian Summer Monsoon with Multimodel Results

Using outputs from 10 CMIP5 models with fixed sea surface temperature, we investigate the fast response of the East Asian summer monsoon (EASM) and summer precipitation in East China to anthropogenic aerosols. To address this topic, we employ two commonly used EASM indices that can represent zonal and meridional land-sea thermal contrast, respectively. The results reveal that the notion of a weakened EASM in response to increased anthropogenic aerosols is a robust one, as well as decreased precipitation in East China. The ensemble mean of decreased precipitation in the aerosol run was about 6.6% in comparison to the CTL run and could be enlarged to 8.3% by excluding the experiments with the aerosol direct effect only. Convective precipitation was found to be the primary contributor (>80%) to the reduction of total precipitation. The combination of direct and indirect effects of aerosols can decrease solar radiation reaching the Earth’s surface and eventually modulate large-scale EASM circulation and suppress summer precipitation in East China. The uncertainties and discrepancies among the models highlight the complexity of interaction in aerosol-precipitation processes when investigating present and future changes of the EASM.

Aerosol-cloud-precipitation interactions in WRF model: Sensitivity to autoconversion parameterization

Cloud-to-rain autoconversion process is an important player in aerosol loading, cloud morphology, and precipitation variations because it can modulate cloud microphysical characteristics depending on the participation of aerosols, and affects the spatio-temporal distribution and total amount of precipitation. By applying the Kessler, the Khairoutdinov-Kogan (KK), and the Dispersion autoconversion parameterization schemes in a set of sensitivity experiments, the indirect effects of aerosols on clouds and precipitation are investigated for a deep convective cloud system in Beijing under various aerosol concentration backgrounds from 50 to 10000 cm−3. Numerical experiments show that aerosol-induced precipitation change is strongly dependent on autoconversion parameterization schemes. For the Kessler scheme, the average cumulative precipitation is enhanced slightly with increasing aerosols, whereas surface precipitation is reduced significantly with increasing aerosols for the KK scheme. Moreover, precipitation varies non-monotonically for the Dispersion scheme, increasing with aerosols at lower concentrations and decreasing at higher concentrations. These different trends of aerosol-induced precipitation change are mainly ascribed to differences in rain water content under these three autoconversion parameterization schemes. Therefore, this study suggests that accurate parameterization of cloud microphysical processes, particularly the cloud-to-rain autoconversion process, is needed for improving the scientific understanding of aerosol-cloud-precipitation interactions.

Analytical Studies of the Cloud Droplet Spectral Dispersion Influence on the First Indirect Aerosol Effect

Atmospheric aerosols (acting as cloud condensation nuclei) can enhance the cloud droplet number concentration and reduce the cloud droplet size, and in turn affect the cloud optical depth, as well as the cloud albedo, and thereby exert a radiative influence on climate (the first indirect aerosol effect). In this paper, based on various relationships between cloud droplet spectral dispersion (ɛ) and cloud droplet number concentration (Nc), we analytically derive the corresponding expressions of the cloud radiative forcing induced by changes in the cloud droplet number concentration. Further quantitative evaluation indicates that the cloud radiative forcing induced by aerosols for the different ɛ−Nc relationships varies from −29.1% to 25.2%, compared to the case without considering spectral dispersion (ɛ = 0). Our results suggest that an accurate description of ɛ − Nc relationships helps to reduce the uncertainty of the first indirect aerosol effect and advances our scientific understanding of aerosol-cloud-radiation interactions.

Impact of East Asian summer monsoon circulation on the regional aerosol distribution in observations and models

The East Asian summer monsoon (EASM) can change the spatio-temporal distribution of aerosols by influencing the aerosol horizontal and vertical transports and the wet deposition of aerosols over East Asia. In this paper, we examined the aerosol optical depth (AOD) during summer together with the intensity of the EASM based on moderate-resolution imaging spectroradiometer products on board the Terra satellite and the modeling results from the NCAR Community Atmospheric Model 5.1 in the mid-latitude monsoonal East Asia (20–45° N, 105–130° E). Our results from both observations and simulations show positive correlations of AOD with the monsoon intensity over the Northeast Asia sub-region (32.5–45° N, 105–130° E), and negative correlations with that over the southeast Asia sub-region (20–32.5° N, 105–130° E). The observed and simulated AODs were much larger over the northern sub-region and much smaller over the southern sub-region in the strongest monsoon years compared with those in the weakest monsoon years. The model results suggest that the mechanism responsible for the north-south difference in the aerosol distribution was mainly caused by lower-tropospheric meridional wind anomalies related to EASM. Compared with the weakest monsoon years, the strongest monsoon years experienced southerly wind anomalies, which enabled more aerosols to be transported northward and resulted in a convergence of aerosols over the northern sub-region. In addition, the wet deposition of aerosols reduced (enhanced) the aerosol concentrations in the northern (southern) sub-region during the strongest monsoon years compared with the weakest monsoon years, which partly offset the impact of the lower southerly winds on the aerosol distribution over East Asia.

Role of microphysical parameterizations with droplet relative dispersion in IAP AGCM 4.1

Previous studies have shown that accurate descriptions of the cloud droplet effective radius (Re) and the autoconversion process of cloud droplets to raindrops (Ar) can effectively improve simulated clouds and surface precipitation, and reduce the uncertainty of aerosol indirect effects in GCMs. In this paper, we implement cloud microphysical schemes including two-moment Ar and Re considering relative dispersion of the cloud droplet size distribution into version 4.1 of the Institute of Atmospheric Physics’s atmospheric GCM (IAP AGCM 4.1), which is the atmospheric component of the Chinese Academy of Sciences’ Earth System Model. Analysis of the effects of different schemes shows that the newly implemented schemes can improve both the simulated shortwave and longwave cloud radiative forcings, as compared to the standard scheme, in IAP AGCM 4.1. The new schemes also effectively enhance the large-scale precipitation, especially over low latitudes, although the influences of total precipitation are insignificant for different schemes. Further studies show that similar results can be found with the Community Atmosphere Model, version 5.1.摘 要前人的研究结果指出云滴有效半径和云水自动转化过程的精确参数化可以有效的提高云和降水的模拟, 同时也可以减少模式给出的气溶胶间接效应的不确定性. 本研究在 IAP AGCM 4.1 中耦合了考虑云滴谱离散度的云滴有效半径和双参数云水自动转化过程的参数化方案. 研究结果显示, 该新云微物理方案可以明显的提高云的短波辐射和长波辐射的模拟. 另外, 新方案可以有效的增加模式的大尺度降水, 特别是低纬度大尺度降水. 进一步的结果表明, 耦合新方案的 CAM5.1 同样也可以更好模拟云的辐射强迫.