Long-term Fog Variation And Its Impact Factors Over East China

Fog is a kind of hazardous low-visibility weather phenomenon.The long-term variation of fog includes two aspects,fog frequency(fog days)and fog lifetime(fog duration).Previous studies have demonstrated that long-term fog variation is controlled by three impact factors,climate change,urbanization(refers to urban heat island and dry island induced by land use change)and aerosols.During the past 5 decades,the significant climate change,rapid urbanization and deteriorated aerosol pollution have taken place in East China.Fog,which can be viewed as near-surface cloud,may have strong responses to these factors and undergo notable temporal variation.However,few studies have disentangled their complex influences on fog variation.Therefore,this thesis aims to quantitatively separate the contributions of three factors to fog variation and identify the dominant impact factor.It helps to facilitate our understanding on how human activities affect natural environment,cloud/fog physics and aerosol-cloud interactions and provide theoretical basis for reducing the hazards of fog weather.The long-term variation of fog days and haze days over three major city clusters of East China(the Beijing-Tianjin-Hebei Region,the Yangtze River Delta Region and the Pearl River Delta Region)are revealed by ground meteorological observation data from 1960 to 2012.Haze days increase markedly,while fog days increase first and then decrease with the turning point occurring at 1980?1990s.The turning point occurs 5?15 years earlier in large cities than in small cities.Furthermore,the respective contributions of climate change,urbanization and aerosols are quantitatively calculated by a statistical method based on multivariate regression.During the initial urban development stage(1960-1985),the increase in fog days is dominantly contributed by the promoting effects of increasing aerosols(45?85%).During the fast urban development stage(1986-2012),the promoting effect of aerosols weakens,while the inhibiting effect of urbanization strengthens and becomes the dominant factor(53?60%).During the overall period(1960-2012),aerosols have positive effects on fog(20?40%),and the contribution of urbanization and aerosols is at least 1.6 times higher than that of climate change.The above long-term observation indicates that the suppressing effect of urban heat land and dry island effect is stronger than the promoting effect of aerosols.To explore the physical mechanisms and relative extent of urbanization and aerosol effects on fog,a dense fog event is simulated by an online-coupled model,WRF-Chem.Numerical experiments show that urbanization suppresses low-level fog,delays its formation,advances its dissipation and decreases liquid water content(LWC).However,the urbanization-induced updrafts could promote upper-level fog by facilitating adiabatic cooling and water vapor convergence.Aerosols have a general promoting effect on fog,which increases LWC,produces more fog droplets and decreases droplet radius.Further experiments indicate that the current aerosol pollution level in China could always promote fog.When the effects of urbanization and aerosols are combined,the weak aerosol effect is overweighted by the much stronger urbanization effect.Budget analysis on LWC shows that among microphysical,boundary layer and advection processes,urban development(urbanization and aerosols)have the largest influence on microphysical process(52.1%).Among various microphysical processes,urban development has the largest influence on condensation/evaporation process(72.7%).The trend of fog duration is analyzed by using manually-recorded weather phenomenon data from 1960 to 2010.More than 90% of stations witness an increase in fog duration,and the increase in fog duration is mainly reflected by the delay in fog dissipation at daytime.By comparing background and non-background stations,we find that the increase in fog duration is caused by the increase in aerosol pollution.The promoting effect of aerosols is contributed by aerosol-radiation interaction(ARI;also aerosol direct effect)and aerosol-cloud interaction(ACI;also aerosol indirect effect).Therefore,the separate effects of ARI and ACI on fog lifetime are quantitatively revealed by WRF-Chem simulations.In a case study,ACI advances fog formation,delays fog dissipation and increases fog duration by about one hour.In contrast,ARI effect is quite weak,which increases fog duration only by 3 min.It is because that the more but smaller fog droplets produced under polluted conditions can inhibit sedimentation and favors fog maintenance.In addition,fog droplets have a much stronger extinction ability than unactivated aerosols,so fog droplets can block substantial solar radiation and lead to delayed dissipation.Further experiments show that ACI effect on fog duration overweighs ARI effect under different moisture conditions,pollution levels and sensitive experiment settings.The above results not only comprehensively reveals the long-term fog variation feature over East China,but also illustrate the mechanisms of how the three impact factors affect fog and evaluate their contributions.Accordingly,we can infer the future fog change in urban regions of China.Supposing that urban areas keep expanding and air quality keeps deteriorating,fog frequency will decrease but fog duration will increase.Therefore,we should effectively control the emission of aerosols to shorten fog duration and reduce the hazards of fog weather.