| As we know that tropospheric aerosols and atmospheric boundary layer(ABL)play an important role in global climate change and extreme weather,the interaction between aerosol and ABL has been an area of intense research interest in atmospheric sciences.Aerosol-radiation-ABL interaction modifies the energy budget of the earth-atmosphere system,and perturbs the consequent evolution of the ABL,which can alter the evolution of atmospheric chemical species,indicating a two-way feedback between aerosols and ABL.The rapid expansion of urban areas and increases in human activity over the Yangtze River Delta(YRD)region,China,have several impacts on urban environments such as air pollution,high-frequency meteorological disaster,etc.,so it is meaningful and necessary to investigate the effect of aerosols on radiation and the aerosol-ABL interaction over the YRD.In this study,observation analysis and numerical simulation have been applied into the examination of the initial condition uncertainty and spatio-temporal characteristics of aerosols,as well as aerosol-radiation-ABL interactions over the YRD.In order to explore an aerosol-radiation-ABL interaction,some necessary improvements on the Weather Research and Forecasting/Chemistry(WRF/Chem)model have been adopted in this thesis.Taking into account of the rapid urbanization over the YRD,urban land-use data have been updated by MODIS land cover product.At the same time,a diurnal cycle function has been applied to create an emission inventory for describing the temporal evolution of aerosol more correctly.To probe into the impact of uncertainty related to initial aerosol,the PM10 aerosol data assimilation(DA)module has been added into the National Centers for Environmental Prediction(NCEP)Gridpoint Statistical Interpolation(GSI)system.To determine the interaction between aerosol and ABL,aerosol-radiation and aerosol-cloud interactions have been isolated completely from aerosol-cloud-radiation interactions and some diagnosis(e.g.instantaneous/effective radiative forcing,heating/cooling rate,etc.)has been added into the WRF/Chem model.In addition,to understand the mechanisms of response,some modules about process analysis(e.g.temperature evolution,particle formation,etc.)has been implemented within the WRF/Chem model,including such processes as advection,turbulence,convection,radiation,emission,etc.The main discoveries and conclusions are as follows:First,a systematic observation study on the aerosol-radiation-ABL interactions has been implemented.The composite analysis of the multi-polluted cases shows that the dirty days have caused a decrease of more than 25%in the net shortwave radiation intensity(Nsw)and no significant difference for Nsw between urban and suburban areas.Sensible/latent heat fluxes at surface become smaller during the dirty days,and Bowen ration has a little change,which depends on the land-use type.Accordingly,the near-surface temperature is lower during the dirty days than that on clean days,However,the temperature decrease is not consistent between urban and suburban areas,weakening urban heat island(UHI)intensity with a decrease of about 0.14?.Due to the changes of the ABL thermal structure,atmospheric stratification becomes more stable,weakening downward horizontal momentum flux up to 26%,with a consequence of lower near-surface wind speeds and UHI circulation.The result also shows that dirty days are often evoked under the condition with weaker wind,higher temperature and relative humidity,leading to higher aerosol eoncentrations.During the biomass-burning day,the temperature decrease gradually decreases with height up to about 680 m,and the decrease is higher in suburban areas,resulting in the stronger UHI intensity with an increase of about 1?2?.Secondly,to improve the accuracy of aerosol simulation,the capability of assimilating surface PM10 observations has been developed within the NCEP GSI three-dimensional variational(3DVAR)DA system.It provides aerosol analyses for the Goddard Chemistry Aerosol Radiation and Transport aerosol scheme within the WRF/Chem model.Control and assimilation experiments were performed for June 2011 over China to explore in detail the impact of assimilating surface PM10.In the assimilation experiment,analyses were produced every 6 hour to adjust the mass concentrations of different aerosol species.The statistical results from two parallel experiments demonstrate that the assimilation of surface PM10 observations can significantly reduce the uncertainty of initial aerosol fields and effectively improve the subsequent aerosol forecasts for at least 12 hour.However,the benefit from the assimilation of PM10 diminishes rapidly within the forecast range.Process analysis for PM10 formation indicates that the rapidly diminishing DA impact on aerosol forecasts,especially in early forecast hours,is dominated by vertical mixing with an additional contribution from advection.Thirdly,the WRF/Chem model has been utilized to simulate aerosol loading in Jan.and Jul.,2010 for testing model performance.The result shows that aerosol loading mainly locates in the urban areas and its downwind areas,with average surface-level PM2.5 concentration of 96.32 ?g m-3 in Jan.and 48.50 ?g m-3 in Jul.,respectively.Unspeciated aerosols of PM2.5 accounts for the largest proportion(27.6%?45.6%),nitrate take the second place(18.6%?29.5%),and black carbon comes to the least(3.2%?8.6%).There are same seasonal characteristics for aerosol surface concentration,aerosol column burden and aerosol optical thickness(AOD),which is high in winter and low in summer,while single scattering albedo(SSA)of opposite seasonal changes,mainly related to absorbing aerosols(e.g.black carbon)loading.The simulated results accords well with ground-based observations and satellite products for PM10 concentration,PM2.5 species,550 nm AOD,and 470 nm SSA.Fourthly,numerical simulation study on the aerosol-radiation-ABL interactions has been performed.The effective radiative forcing(ERF)due to aerosol-radiation interactions with internal mixture at the top of the atmosphere(TOA)is-0.04 W m-2 in Jan.and-2.22 W m-2 in Jul.,respectively,and the ERF at surface is-16.05 and-8.91 W m-2.The surface dimming induces a mean reduction of both sensible(H,10.25 and 3.04 W m-2,respectively)and latent(LE,2.51 and 4.16 W m-2,respectively)heat fluxes.The reduction in H dominates in the compensation of reduction in net radiation flux over north YRD and in winter,and vice versa for the reduction in LE,mainly depending on soil moisture.The change of energy budget within the earth-atmosphere system results in a cooling of 2-m air with an average of 0.41 and 0.09 K,respectively.And then the air temperature response(?T)shifts from negative to positive in the vertical as altitude increases.The process analysis in the evolution of air temperature indicates that ?T below the middle of ABL is dominated by shortwave heating rate,advection heating rate,and turbulent cooling rate with an additional contribution from longwave cooling rate,and AT above the top of ABL by shortwave heating rate and advection cooling rate.The reduction in turbulent kinetic energy(TKE)extends to more than 1.6 km in the vertical,which causes a reduction of ABL height up to 27.65 and 8.13 m.Aerosol-radiation interaction also reduces 10-m wind speeds by stabilizing the air and weakening downward horizontal momentum flux.In the vertical,the wind response suffers from negative to positive,and to negative.With the stronger atmospheric stratification and lower ABL height,more aerosol mass is trapped near the surface,a relative increase up to 6.56%and 12.02%.In summary,it can be concluded that the aerosol-radiation interactions decrease the amount of solar energy reaching the ground,a surface dimming resulting in a radiative cooling near the surface.However,absorbing aerosols within ABL can increase the solar heating and air temperature.This trend can cause the stronger atmospheric stability and the lower ABL height,hence reducing the atmospheric turbulence and near-surface wind speeds,which further leads to more accumulation of fine particulate matter and exacerbation of air pollution.Therefore,aerosol-radiation interaction causes a change of the ABL thermal and dynamic structure in the horizontal and vertical,and further rises up the deterioration of air quality,indicating a positive feedback mechanism between aerosol,radiation and ABL. |
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