Research On The Two-way Feedback Mechanism Between The Radiative Effect Of Dust Aerosol And Boundary Layer Proces

Mineral dust is the most abundant component in terms of atmospheric absorptive aerosols from natural emissions,contributing to more than half of the global aerosol loading.Dust radiative effect imposes pronounced perturbations in the exchange of substances and energy through regulating the surface and atmospheric radiation balance,thereby dictating the development and turbulence characteristics of the planetary boundary layer(PBL).As a key region of human activities,PBL structures in turn alter the temporal-spatial distribution and optical properties of aerosols.The interactions,especially for the physical mechanisms,between dust direct radiative forcing and PBL processes are still poorly understood because of limited observational constraints and high-resolution numerical simulations.Based on series of observational datasets and synthesized aerosol optical parameters from publications(84publications for SSA values and 40 publications for ASY values at 550 nm wavelength),this study conducts thousands of single-column sensitivity experiments and three-dimensional simulations of a typical dust storm event to explore the impacts of aerosol vertical distribution and optical properties on daytime and nighttime PBL development.Thermodynamic mechanisms from these sensitivity experiments are further evaluated in the real-case simulations over Tarim Basin to examine their general applicability among the realistic atmospheric pollution conditions.Parallel experiments are also applied to reveal the modulation effect of advection processes on radiative budgets and dust-PBL interactions.Furthermore,this study divides the PBL variations associated with total radiative effect into instant,legacy and nonlinear parts and then identifies the individual contribution of these three components in the subsequent dust processes.This study also highlighted the importance of the two-way feedback mechanisms of dust radiative effect and PBL evolution in improving air quality prediction and providing foresighted strategic policies.The main conclusions are as follows:(1)The comprehensive review of aerosol optical parameters worldwide in the past three decades(1988-2021)indicates that dust aerosols can be characterized by either high absorbing efficiency or strong scattering ability,as well as significant forward-scattering performance.Based on the observational thresholds mentioned-above,this study designs thousands of experiments of aerosol layer height,optical property and background aerosol extinction coefficient and then integrates them into several representative scenarios of daytime and nighttime PBL structures.Aerosols consistently suppresses the daytime convective boundary layer development in all scenarios.The strongest inhibition effect occurs when absorbing aerosols are below but near PBL top.The nocturnal scenarios are more complex because absorptive and significant forward-scattering aerosols in the low-level atmosphere(Low＿promotion,Low＿pro scenario)promote the PBL growth.However,the suppression effects are detected in both of scattering and weak forward-scattering aerosols at low altitudes(Low＿suppression,Low＿sup scenario)and upper-level aerosols(High scenario).(2)In the daytime,dust particles attenuate the incident solar radiation through scattering and absorbing effects,which reduces the shortwave flux reaching the ground,thereby causing the cooling effect within PBL.Aerosol layers also result in positive radiative forcing and warming effect by absorbing shortwave radiation and then reheating the atmosphere.A tradeoff between the cooling and warming effects is represented as net heating rate.Dust plumes induce cooling effect dominating warming effect,generate negative net heating in all of the daytime experiments,and eventually suppresses the PBL evolution characterized by declined PBL height,lower mixed-layer potential temperature,thicker entrainment zone and altered neutral point heights.At night,suspended aerosol layers,as mainly composed of coarse particles,inevitably prohibit outgoing terrestrial radiation and re-emit infrared radiative forcing,which thus increase the surface net radiation with warming effect and cause atmospheric longwave radiation deficit with cooling effect simultaneously.For the Low＿pro scenario,diminished potential temperature gradients related to positive net heating rate contribute to the weaker local static stability and stronger nocturnal intermittent turbulence.While in Low＿sup and High scenarios,the negative net heating rate together with larger potential temperature gradients leads to the enhancement of stable stratification and the weakening of intermittent turbulence.(3)The simulation results of a typical dust storm episode from April 27 to May 1,2015 over Tarim Basin show that dust-induced shortwave radiation variations prevail in the daytime,while longwave radiation adjustments due to dust aerosols dominate at night.Such inverse surface and atmospheric radiative forcing reallocation gives rise to alternating variations in surface heat flux,and thereby opposite daytime and nighttime changes in surface temperature and PBL development in response to dust aerosol accumulation and distribution.During this dust event,nocturnal dust particles are accumulated near the surface,and their optical parameter ranges consist with the Low＿pro scenario.Daytime PBL suppression and nighttime PBL promotion jointly verify the applicability of representative scenarios and thermodynamic mechanisms from single-column experiments into realistic cases.It is noteworthy that significant changes in daytime and nighttime PBL heights are not entirely coincident with the largest dust loading and radiation flux changes.This study further points out that the spatial heterogeneity between dustload,radiative perturbations and PBL height variations is concerned with the modulation effect of advection processes.When advection heating rate is included,the cooling effect within daytime PBL and the heating effect above PBL enhance resistance in both upwelling from PBL and downwelling from the free atmosphere.This resistance further weakens entrainment processes through decreasing horizontal and vertical advection and ultimately amplifies the suppression effect on daytime PBL development.In the nighttime,dust aerosols lead to a warmer and more unstable near-surface atmospheric layer,and increase the low-level wind speed accordingly.The combined effect of higher temperature and wind speed magnifies the advection heating and net heating rate,resulting in stronger nocturnal intermittent turbulence near the surface that facilitates the impacts of dust radiative effect on nocturnal PBL growth.This study demonstrates that entrainment and advection processes are dominant mechanisms conducive to dust-PBL interactions.The amplification effect compensates for the application limitations of sensitivity results into the actual pollution episodes,and eventually brings more comprehensive insights into the impacts of dust radiative effect on PBL structures.(4)The division of instant,legacy and nonlinear effects of aerosol radiative forcing on PBL characteristics is proposed in this study to estimate their relevant contributions on subsequent dust processes.Daytime legacy effect plays the dominant role and induces an asymmetric pattern in dust loading anomalies over Tarim Basin,with the high-value centers(central regions)showing increased total concentration,while the marginal regions registered pronounced decreases.Instant and nonlinear effects imposes negligible influences in both regions.The nocturnal instant and legacy effects also cause the asymmetry of dustload variations,which is opposite to the daytime distribution.The legacy impact explains most of dustload responses and the nonlinear effect is more prominent in the nighttime than daytime.This study indicates that dust radiative effect modifies the deposition fluxes by adjusting the daytime and nighttime meteorological fields(PBL structures),and ultimately triggers the asymmetric changes in column-integrated concentration of suspending dust aerosols.In the daytime,the legacy effect varies the vertical distributions of net heating rate within mixing layer and entrainment zone.Distinct net heating rate distributions further differ the PBL heights,turbulence eddy motions and entrainment circulation of the central and marginal regions.In addition,net heating rate regulates the geostrophic wind and thermal wind speeds as a result of variable gradients of temperature and pressure.Therefore,the dust deposition fluxes capture opposite responses in the central and marginal areas,which eventually determines the asymmetric pattern of daytime dust loading.The nocturnal instant and legacy effects exhibit the similar physical processes.The synergistic effects of nocturnal atmospheric temperature and pressure distributions also alter the geostrophic wind and thermal wind,and then lead to the reverse variations of dust deposition rate in contrast with the daytime,thereby resulting in the asymmetric characteristics at night.