Trends in the Different Grades of Precipitation over South China during 1960–2010 and the Possible Link with Anthropogenic Aerosols

Using observed daily precipitation data to classify five levels of rainy days by strength in South China （SC）,with an emphasis on the Pearl River Delta （PRD） region,the spatiotemporal variation of different grades of precipitation during the period 1960-2010 was analyzed and the possible link with anthropogenic aerosols examined.Statistical analysis showed that drizzle and small precipitation has significantly decreased,whereas medium to heavy precipitation has increased slightly over the past 50 years （although not statistically significant）.Further data analysis suggested that the decline in drizzle and small precipitation probably has a strong link to increased concentrations of anthropogenic aerosols produced by large-scale human activities related to the rapid socioeconomic development of the PRD region.These aerosols may also have led to the obvious decreasing trend in horizontal visibility and sunshine duration in SC,which is statistically significant according to the t-test.

Impact of Anthropogenic Aerosols on Summer Precipitation in the Beijing–Tianjin–Hebei Urban Agglomeration in China:Regional Climate Modeling Using WRF-Chem

The WRF model with chemistry （WRF-Chem） was employed to simulate the impacts of anthropogenic aerosols on summer precipitation over the Beijing-Tianjin-Hebei urban agglomeration in China. With the aid of a high-resolution gridded inventory of anthropogenic emissions of trace gases and aerosols, we conducted relatively long-term regional simulations, considering direct, semi-direct and indirect effects of the aerosols. Comparing the results of sensitivity experiments with and without emissions, it was found that anthropogenic aerosols tended to enhance summer precipitation over the metropolitan areas. Domain-averaged rainfall was increased throughout the day, except for the time around noon. Aerosols shifted the precipitation probability distribution from light or moderate to extreme rain. Further analysis showed that the anthropogenic aerosol radiative forcing had a cooling effect at the land surface, but a warming effect in the atmosphere. However, enhanced convective strength and updrafts accompanied by water vapor increases and cyclone-like wind shear anomalies were found in the urban areas. These responses may originate from cloud microphysical effects of aerosols on convection, which were identified as the primary cause for the summer rainfall enhancement.

Direct Radiative Forcing of Anthropogenic Aerosols over Oceans from Satellite Observations

Anthropogenic aerosols play an important role in the atmospheric energy balance. Anthropogenic aerosol optical depth （AOD） and its accompanying shortwave radiative forcing （RF） are usually simulated by nu- merical models. Recently, with the development of space-borne instruments and sophisticated retrieval algorithms, it has become possible to estimate aerosol radiative forcing based on satellite observations. In this study, we have estimated shortwave direct radiative forcing due to anthropogenic aerosols over oceans in all-sky conditions by combining clouds and the Single Scanner Footprint data of the Clouds and Earth’s Radiant Energy System （CERES/SSF） experiment, which provide measurements of upward shortwave fluxes at the top of atmosphere, with Moderate Resolution Imaging Spectroradiometer （MODIS） aerosol and cloud products. We found that globally averaged aerosol radiative forcing over oceans in the clear-sky conditions and all-sky conditions were -1.03±0.48 W m-2 and -0.34 ±0.16 W m-2, respectively. Direct radiative forcing by anthropogenic aerosols shows large regional and seasonal variations. In some regions and in particular seasons, the magnitude of direct forcing by anthropogenic aerosols can be comparable to the forcing of greenhouse gases. However, it shows that aerosols caused the cooling effect, rather than warming effect from global scale, which is different from greenhouse gases.

Comments on "Direct Radiative Forcing of Anthropogenic Aerosols over Oceans from Satellite Observation"

Previous observational studies have estimated anthropogenic aerosol direct radiative forcing over oceans without due consideration of cloudy-sky aerosols. However, when interaction between clouds and aerosols located below or above the cloud level is taken into account, the aerosol direct radiative forcing is larger by as much as 5 W m-2 in most mid-latitude regions in the Northern Hemisphere.

Numerical Study of the Effect of Anthropogenic Aerosols on Spring Persistent Rain over Eastern China

The eff ect of anthropogenic aerosols on the spring persistent rain （SPR） over eastern China is investigated by using a high-resolution Community Atmosphere Model version 5.1 （CAM5.1）. The results show that the SPR starts later due to anthropogenic aerosols, with a shortened duration and reduced rainfall amount. A reduction in air temperature over the low latitudes in East Asia is linked to anthropogenic aerosols;so is a weakened southwesterly on the north side of the subtropical high. Meanwhile, air temperature increases signifi cantly over the high latitudes. This north-south asymmetrical thermal eff ect acts to reduce the meridional temperature gradient, weakening the upper-level westerly jet over East Asia and the vertical motion over southeastern China. As a result, the SPR is reduced and has a much shorter duration. The indirect eff ect of anthropogenic aerosols also plays an important role in changing the SPR. Cloud droplet number concentration increases due to anthropogenic aerosols acting as cloud condensation nuclei, leading to a reduction in cloud eff ective radius over eastern China and a reduced precipitation effi ciency there.

Influences of the Internal Mixing of Anthropogenic Aerosols on Global Aridity Change

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. Simula-tions 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 anthropo-genie aerosols since the pre-industrial are -1.02 and -1.68 W m^-2 for PIM and EM schemes, respectively. The vari-ables related to aridity such as global mean temperature, net radiation flux at the surface, and the potential evapora-tion 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 hu-midification 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 allevi-ated in China, but intensified in the Arabian Peninsula and East Africa in the PIM scheme.

Impacts of greenhouse gases and anthropogenic aerosols changes on surface air temperature in East Asia under different post-pandemic period emission scenarios

In order to know how surface air temperature(SAT)changes in East Asia under different emission scenarios after the COVID-19 outbreak,we investigated the impacts of greenhouse gases(GHGs)and anthropogenic aerosols changes on SAT in East Asia by using the aerosol-climate coupled model BCC-AGCM 2.0_CUACE/Aero,combining with the post-pandemic emission scenarios proposed by Covid multi-Earth system model intercomparison project(CovidMIP scenarios for short,including fossil-fueled recovery,moderate green stimulus,strong green stimulus,hereinafter as FFF,MGG,SGG,respectively).We assessed the impacts of changes in GHGs and anthropogenic aerosols together and separately on SAT in East Asia and its typical subregions during 2020-2050.The results show that by mid-21st-century,SAT in East Asia will increase by 0.81±0.083°C under Baseline(same as SSP2-4.5 scenario,i.e.,SAT difference between 2045-2050 and 2020-2025),and there will be more intense warming in all the three scenarios in East Asia,in which the largest SAT difference(SAT-d)compared to Baseline is 0.33±0.11°C under SGG and the smallest SAT-d is 0.07±0.14°C under FFF.To further explore the mechanism of these SAT-d,we analyzed the trend of surface longwave and shortwave net radiation flux driven by GHGs and anthropogenic aerosols there.It is found that in early period(2020-2035),the role of aerosol changes is bigger than that of GHG changes in dominating SAT-d,particularly sulfate,whose reduction will become the main contributor to SAT-d by affecting the net solar flux at surface.In later period(2036-2050),because of GHGs’longer atmospheric lifetime than aerosols,the role of decreasing GHGs concentrations will determine the drop in SAT-d through affecting the net longwave flux at surface.

Simulation of the Ecosystem Productivity Responses to Aerosol Diffuse Radiation Fertilization Effects over the Pan-Arctic during 2001–19

The pan-Arctic is confronted with air pollution transported from lower latitudes.Observations have shown that aerosols help increase plant photosynthesis through the diffuse radiation fertilization effects(DRFEs).While such DRFEs have been explored at low to middle latitudes,the aerosol impacts on pan-Arctic ecosystems and the contributions by anthropogenic and natural emission sources remain less quantified.Here,we perform regional simulations at 0.2o×0.2ousing a well-validated vegetation model(Yale Interactive terrestrial Biosphere,YIBs)in combination with multi-source of observations to quantify the impacts of aerosol DRFEs on the net primary productivity(NPP)in the pan-Arctic during 2001-19.Results show that aerosol DRFEs increase pan-Arctic NPP by 2.19 Pg C(12.8%)yr^(-1)under clear-sky conditions,in which natural and anthropogenic sources contribute to 8.9% and 3.9%,respectively.Under all-sky conditions,such DRFEs are largely dampened by cloud to only 0.26 Pg C(1.24%)yr^(-1),with contributions of 0.65% by natural and 0.59% by anthropogenic species.Natural aerosols cause a positive NPP trend of 0.022% yr^(-1)following the increased fire activities in the pan-Arctic.In contrast,anthropogenic aerosols induce a negative trend of-0.01% yr^(-1)due to reduced emissions from the middle latitudes.Such trends in aerosol DRFEs show a turning point in the year of 2007 with more positive NPP trends by natural aerosols but negative NPP trends by anthropogenic aerosols thereafter.Though affected by modeling uncertainties,this study suggests a likely increasing impact of aerosols on terrestrial ecosystems in the pan-Arctic under global warming.

Optical Properties and Source Analysis of Aerosols over a Desert Area in Dunhuang,Northwest China

Aerosol observational data for 2012 obtained from Dunhuang Station of CARE-China（Campaign on Atmospheric Aerosol Research Network of China） were analyzed to achieve in-depth knowledge of aerosol optical properties over Dunhuang region. The results showed that the annual average aerosol optical depth（AOD） at 500 nm was 0.32 ± 0.06, and the ？ngstr？m exponent（α） was 0.73 ± 0.27. Aerosol optical properties revealed significant seasonal characteristics. Frequent sandstorms in MAM（March–April–May） resulted in the seasonal maximum AOD, 0.41 ± 0.04, and a relatively smaller αvalue, 0.44 ± 0.04. The tourism seasons, JJA（June–July–August） and SON（September–October–November） coincide with serious emissions of small anthropogenic aerosols. While in DJF（December–January–February）, the composition of the atmosphere was a mixture of dust particles and polluted aerosols released by domestic heating; the average AOD and αwere 0.29 ± 0.02 and 0.66 ± 0.17, respectively. Different air masses exhibited different degrees of influence on the aerosol concentration over Dunhuang in different seasons. During MAM, ranges of AOD（0.11–1.18） and α（0.06–0.82） were the largest under the dust influence of northwest-short-distance air mass in the four trajectories. Urban aerosols transported by northwest-short-distance air mass accounted for a very large proportion in JJA and the mixed aerosols observed in SON were mainly conveyed by air masses from the west. In DJF, the similar ranges of AOD and α under the three air mass demonstrated the analogous diffusion effects on regional pollutants over Dunhuang.

Impacts of Internally and Externally Mixed Anthropogenic Sulfate and Carbonaceous Aerosols on East Asian Climate

A coupled regional climate and aerosol-chemistry model, RIEMS 2.0 （Regional Integrated Environmental Model System for Asia）, in which anthropogenic sulfate, black carbon, and organic carbon were assumed to be externally mixed （EM）, internally mixed （IM） or partially internally mixed （IEM）, was used to simulate the impacts of these anthropogenic aerosols on East Asian climate for the entire year of 2006. The distributions of aerosol mass concentration, radiative forcing and hence the surface air temperature and precipitation variations under three mixing assumptions of aerosols were analyzed. The results indicated that the mass concentration of sulfate was sensitive to mixing assumptions, but carbonaceous aerosols were much less sensitive to the mixing types. Modeled results were compared with observations in a variety of sites in East Asia. It was found that the simulated concentrations of sulfate and carbonaceous aerosols were in accord with the observations in terms of magnitude. The simulated aerosol concentrations in IM case were closest to observation results. The regional average column burdens of sulfate, black carbon, and organic carbon, if internally mixed, were 11.49, 0.47, and 2.17 mg m 2 , respectively. The radiative forcing of anthropogenic aerosols at the top of the atmosphere increased from –1.27 （EM） to –1.97 W m 2 （IM） while the normalized radiative forcing （NRF） decreased from –0.145 （EM） to –0.139 W mg 1 （IM）. The radiative forcing and NRF were –1.82 W m 2 and –0.141 W mg 1 for IEM, respectively. The surface air temperature changes over the domain due to the anthropogenic sulfate and carbonaceous aerosols were –0.067, –0.078, and –0.072 K, with maxima of –0.47, –0.50, and –0.49 K, for EM, IM, and IEM, respectively. Meanwhile, the annual precipitation variations were –8.0 （EM）, –20.6 （IM）, and –21.9 mm （IEM）, with maxima of 148, 122, and 102 mm, respectively, indicating that the climate effects were stronger if the sulfate and carbonaceous aerosols were internally mixed.

Abrupt Summer Warming and Changes in Temperature Extremes over Northeast Asia Since the Mid-1990s: Drivers and Physical Processes

This study investigated the drivers and physical processes for the abrupt decadal summer surface warming and increases in hot temperature extremes that occurred over Northeast Asia in the mid-1990s. Observations indicate an abrupt increase in summer mean surface air temperature （SAT） over Northeast Asia since the mid-1990s. Accompanying this abrupt surface wanning, significant changes in some temperature extremes, characterized by increases in summer mean daily maximum temperature （Tmax）, daily minimum temperature （Train）, annual hottest day temperature （TXx）, and annual warmest night temperature （TNx） were observed. There were also increases in the frequency of summer days （SU） and tropical nights （TR）. Atmospheric general circulation model experiments forced by changes in sea surface temperature （SST）/sea ice extent （SIE）, anthropogenic greenhouse gas （GHG） concentrations, and anthropogenic aerosol （AA） forcing, relative to the period 1964- 93, reproduced the general patterns of observed summer mean SAT changes and associated changes in temperature extremes, although the abrupt decrease in precipitation since the mid-1990s was not simulated. Additional model experiments with different forcings indicated that changes in SST/SIE explained 76% of the area-averaged summer mean surface warming signal over Northeast Asia, while the direct impact of changes in GHG and AA explained the remaining 24% of the surface warming signal. Analysis of physical processes indicated that the direct impact of the changes in AA （through aerosol- radiation and aerosol-cloud interactions）, mainly related to the reduction of AA precursor emissions over Europe, played a dominant role in the increase in TXx and a similarly important role as SST/SIE changes in the increase in the frequency of SU over Northeast Asia via AA-induced coupled atmosphere-land surface and cloud feedbacks, rather than through a direct impact of AA changes on cloud condensation nuclei. The modelling results also imply that the abrupt summer surface warming and increases in hot temperature extremes over Northeast Asia since the mid-1990s will probably sustain in the next few decades as GHG concentrations continue to increase and AA precursor emissions over both North America and Europe continue to decrease.

Preface to the Special Issue on the Program of “Carbon Budget and Relevant Issues”-A Strategic Scientific Pioneering Program of the Chinese Academy of Sciences

Global warming has been one of the biggest issues faced by the world in recent decades.It is closely related to anthropogenic emissions of greenhouse gases（GHGs）—mainly CO_2,CH_4 and N_2O—and the effects of reducing emissions and increasing the carbon fixation capability.

Anthropogenic influence on extreme Meiyu rainfall in 2020 and its future risk

Eastern China experienced excessive Meiyu rainfall in the summer of 2020,with a long rainy season and frequent extreme rainfall events.Extreme rainfall occurred on daily to monthly time scales.In particular,persistent heavy rainfall events occurred;e.g.,the maximum accumulated rainfall over four consecutive weeks(Rx28day)in the lower reaches of the Yangtze River was 94%greater than climatology,breaking the observational record since 1961.With ongoing anthropogenic climate change,it is vital to understand the anthropogenic influence on this extreme rainfall event and its driving mechanisms.In this study,based on multi-model simulations under different external forcings that participate in the Detection and Attribution Model Intercomparison Project(DAMIP)in the Coupled Model Intercomparison Project-phase 6(CMIP6),we show that anthropogenic forcing has reduced the probability of the Rx28day extreme rainfall as that in observations in the lower reaches of the Yangtze River in 2020,by 46%(22–62%).Specifically,greenhouse gas(GHG)emissions have increased the probability by 44%as a result of atmospheric warming and moistening.However,this effect was offset by anthropogenic aerosols,which reduced the probability by 73%by reducing atmospheric moisture and weakening the East Asian summer monsoon circulation.With the continuous emissions of GHGs and reductions in aerosols in the future,similar persistent heavy rainfall events are projected to occur more frequently.A higher occurrence probability is expected under higher emission scenarios,which is estimated to be 4.6,13.6 and 27.7 times that in the present day under the SSP1-2.6,SSP2-4.5,and SSP5-8.5 emission scenarios,respectively,by the end of the 21st century.Thus,efficient mitigation measures will help to reduce the impacts related to extreme rainfall.

CMIP6和CMIP5中哈德雷环流变化趋势的比较

近些年来,哈德雷环流变宽和副热带干旱区域向极地区域移动受到了气候变化领域高度的关注.本文使用第六次国际耦合气候模式比较计划(CMIP6)的模拟结果,分析了哈德雷环流宽度和强度的变化趋势,并且与CMIP5模拟结果进行了对比.分析表明,在CMIP6中包含了全部辐射强迫因子的历史模拟试验中,哈德雷环流在1970~2014年间平均每10年变宽0.13°±0.02°,与CMIP5的结果基本相当.关于哈德雷环流的强度,CMIP6给出的北支环流强度减弱比CMIP5大,而在南支环流则有变弱的加强趋势.CMIP6单个辐射强迫模拟试验表明,温室气体的增加导致南北半支哈德雷环流均变宽和变弱,而人类活动导致的气溶胶增加和平流层臭氧的变化使得南支哈德雷环流变弱的趋势加强.CMIP6未来模拟试验结果表明,哈德雷环流变宽和变弱的趋势均随辐射强迫的增强而加强.