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The Tibetan Plateau (TP) with a large landmass serves as an obstacle that hinders westerly flows and alters climate downwind. Here, we investigate the TP influence on the magnitude and spatial distribution of wintertime fine particulate matter (PM2.5) concentrations downwind and associated underlying mechanisms. Based on simulations using an Earth system model, we show that the removal of the TP would reduce surface PM2.5 concentrations by −30.4% in the Sichuan basin (SC) and by −12.4% in the North China Plain (NCP), but increase the concentrations by 18.1% in eastern China (EC), suggesting that the TP could naturally intensify PM2.5 pollution in SC and NCP. If the TP were absent, more meridional circulations would turn into zonal ones and the East Asian winter monsoon would become weaker. There would be less precipitation and lower humidity over SC and EC in the south, while the opposite occurs over NCP in the north. Consequently, the changes in circulations would result in a net outflow of PM2.5 from SC and NCP, but a net inflow of PM2.5 to EC. In response to the spatial changes in precipitation, wet deposition would decrease in SC and EC but increase in NCP. PM2.5 production would reduce in SC and EC but amplify in NCP, following the changes in humidity. In magnitude, the changes in transport and wet deposition would be dominant in SC and NCP, while in EC, transport, wet deposition, and chemical production would be equally important. This study illustrates significant and heterogeneous impacts of the TP on air quality downwind.

Stratospheric ozone transported to the troposphere is estimated to account for 5 %–15 % of the tropospheric ozone sources. However, the chances of intruded stratospheric ozone reaching the surface are low. Here, we report an event of a strong surface ozone surge of stratospheric origin in the North China Plain (NCP, 34–40∘ N, 114–121∘ E) during the night of 31 July 2021. The hourly measurements reveal surface ozone concentrations of up to 80–90 ppbv at several cities over the NCP from 23:00 LST (Local Standard time, = UTC +8 h) on 31 July to 06:00 LST on 1 August 2021. The ozone enhancement was 40–50 ppbv higher than the corresponding monthly mean. A high-frequency surface measurement indicates that this ozone surge occurred abruptly, with an increase reaching 40–50 ppbv within 10 min. A concurrent decline in surface carbon monoxide (CO) concentrations suggests that this surface ozone surge might have resulted from the downward transport of a stratospheric ozone-rich and CO-poor air mass. This is further confirmed by the vertical evolutions of humidity and ozone profiles based on radiosonde and satellite data respectively. Such an event of stratospheric impact on surface ozone is rarely documented in view of its magnitude, coverage, and duration. We find that this surface ozone surge was induced by a combined effect of dying Typhoon In-fa and shallow local mesoscale convective systems (MCSs) that facilitated transport of stratospheric ozone to the surface. This finding is based on analysis of meteorological reanalysis and radiosonde data, combined with high-resolution Weather Research and Forecasting (WRF) simulation and backward trajectory analysis using the FLEXible PARTicle (FLEXPART) particle dispersion model. Although Typhoon In-fa on the synoptic scale was at its dissipation stage when it passed through the NCP, it could still bring down a stratospheric dry and ozone-rich air mass. As a result, the stratospheric air mass descended to the middle-to-low troposphere over the NCP before the MCSs formed. With the pre-existing stratospheric air mass, the convective downdrafts of the MCSs facilitated the final descent of stratospheric air mass to the surface. Significant surface ozone enhancement occurred in the convective downdraft regions during the development and propagation of the MCSs. This study underscores the substantial roles of weak convection in transporting stratospheric ozone to the lower troposphere and even to the surface, which has important implications for air quality and climate change.

The European Space Agency Climate Change Initiative Land Cover data (ESA CCI-LC, from 1992 to 2015) is introduced to the National Center for Atmospheric Research Community Earth System Model version 1.2.1 (NCAR CESM1.2.1). In comparison with the original land surface data in the Community Land Model version 4 (ORG), the new data features notable land use and land cover change (LULCC) with increased forests over northeastern Asia and Alaska by decreasing shrublands and grasslands. Overestimated bare land cover over the Tibetan Plateau (TP) and the Rocky Mountains in the ORG are corrected with the replacements by grasslands and shrublands respectively in the new data. The model simulation results show that with the introduction of the ESA CCI-LC, the simulated surface albedo, surface net radiation flux, sensible and latent heat fluxes are relatively improved over the regions where significant LULCC exists, such as northeastern Asia, Alaska, the TP, and Australia. Surface air temperature, precipitation, and atmospheric circulation are improved in boreal winter but degraded in summer. The winter warming over northeastern Asia results from increased longwave downwelling flux and adiabatic heating while the notable winter cooling over Alaska is attributed to strong cold advection followed by reduced longwave downwelling flux. LULCC alters precipitation by influencing water vapor content. In winter, LULCC affects atmospheric circulation via modulating baroclinicity while in summer, it influences land-sea thermal contrast, thus affecting the intensity of East Asian summer monsoon. LULCC also alters the simulated dust burden.

Stratospheric ozone transported to the troposphere is estimated to account for 5–10 % of the tropospheric ozone sources. However, chances for intruded stratospheric ozone to reach the surface are low. Here, we report an event of strong surface ozone surge with stratospheric origins in the North China Plain (NCP, 34° N–40° N, 114° E–121° E) at night of 31 July 2021. The hourly measurements revealed that surface ozone concentrations were up to 80–90 ppbv at several cities over the NCP from 23:00 on 31 July 1 to 6:00 on 01 August, 2021, which was 40–50 ppbv higher than the corresponding monthly mean. A high-frequency surface measurement indicates that this ozone surge occurred abruptly and reached 40–50 ppbv within ~10 minutes. A concurrent decline in surface carbon monoxide (CO) concentrations suggests that this surface ozone surge resulted from downward transport of stratospheric ozone-rich and CO-poor airmass. This is further confirmed by the vertical evolutions of humidity and ozone profiles at night, based on radiosonde and satellite data, respectively. Such an event of stratospheric impact on surface ozone is rarely documented in terms of its magnitude, covering areas, abruptness, and duration. We find that this surface ozone surge was induced by a combined effect of a dying typhoon In-fa and shallow local mesoscale convective systems (MCS) that facilitated the transport of stratospheric ozone to the surface. This finding is based on analysis of meteorological reanalysis and radiosonde data, combining with high-resolution FLEXPART-WRF modeling. (WRF: Weather Research and Forecasting, FLEXPART: Flexible Lagrangian particle dispersion model). Although the synoptic-scale typhoon In-fa was in dissipation stage when it passed through the NCP, it could still bring down stratospheric dry and ozone-rich airmass. As a result, the stratospheric airmass descended to the middle-to-low troposphere over the NCP before the MCS formed. With the pre-existed stratospheric airmass, the convective downdrafts of the MCS facilitated the final descending of stratospheric airmass to the surface. Significant surface ozone enhancement occurred in the convective downdraft regions during the development and propagation of the MCS. This study underscores the non-negligible roles of dying typhoons and shallow convection in the transport of stratospheric ozone to the troposphere and even the surface, which have important implications for air quality, tropospheric ozone budget, and climate change.

Based on an ensemble of 17 typhoons that made landfall between 2014 and 2018, we investigate the positive and negative influences of typhoons on tropospheric ozone over southern China. With respect to the proximity of typhoon centres and the typhoon developmental stages, we find that surface ozone is enhanced when typhoons are 400–1500 km away during the initial stages of development (e.g. from 1 d before to 1 d after typhoon genesis). The positive ozone anomalies reach 10–20 ppbv above the background ozone level on average. The maximum enhancement of surface ozone appears at a radial distance of 1100–1300 km from the typhoon centre during these initial stages. As the typhoons approach southern China, the influences of these systems switch to reducing ozone and, hence, lead to negative ozone anomalies of 6–9 ppbv. Exploring the linkages between ozone variations and typhoon-induced meteorological evolution, we find that increasing temperature and weak winds in the atmospheric boundary layer (ABL) and dominating downward motions promote ozone production and accumulation over the outskirts of typhoons during typhoon initial stages, whereas deteriorating weather, accompanied by dropping temperature, wind gales and convective activity, reduces the production and accumulation of surface ozone when typhoons are making landfall. We further examine the impacts of typhoons on tropospheric ozone profiles vertically, especially the influences of typhoon-induced stratospheric intrusions on lower troposphere and surface ozone. Based on temporally dense ozone profile observations, we find two high-ozone regions, located in the ABL and the middle to upper troposphere respectively, during different typhoon stages. On average, the high-ozone region in the ABL has a maximum ozone enhancement of 10–12 ppbv at 1–1.5 km altitude during the initial typhoon stages. In the high-ozone region in the middle to upper troposphere, ozone enhancement persists over a longer period with a maximum ozone enhancement of ∼ 10 ppbv at 7–8 km altitude shortly after typhoon genesis; this value increases to ∼ 30 ppbv near 12 km altitude when typhoons reach their maximum intensity. When typhoons make landfall, negative ozone anomalies appear and extend upward with a maximum ozone reduction of 14–18 ppbv at 5 km altitude and 20–25 ppbv at 11 km altitude. Although the overall tropospheric ozone is usually reduced during typhoon landfall, we find that five of eight typhoon samples induced ozone-rich air with a stratospheric origin above 4 km altitude; moreover, in three typhoon cases, the ozone-rich air intrusions can sink to the ABL. This suggests that the typhoon-induced stratospheric intrusions play an important role in surface ozone enhancement.

In this study, we use an ensemble of 17 landfalling typhoons over 2014–2018 to investigate the positive and negative influences of typhoons on tropospheric ozone over southern China. Referring to the proximity to typhoons and typhoon developmental stages, we found that surface ozone is enhanced when typhoons are 400–1500 km away during the initial stages of typhoons (e.g., from 1 day before and to 1 day after typhoon genesis). The positive ozone anomaly averagely reaches 10–20 ppbv at the daytime and 9 ppbv at nighttime compared with the background ozone level. Particularly, surface ozone at radial distances of 1100–1300 km is most significantly enhanced during these initial stages. As the typhoons approach and land in southern China, the influences of typhoons change from enhancing to reducing ozone. Surface ozone concentrations decrease with a negative ozone anomaly ranging between -12 % ~ -17 % relative to the background ozone level. We explore the physical linkages between typhoons, meteorological conditions and ozone variations. Results show that during typhoon initial stages, the increasing temperature and weak winds in the atmospheric boundary layer (ABL) and dominating downward motions promote ozone production and accumulation over the outskirts of typhoons. While the deteriorating weather accompanied by dropping temperature, wind gales and convective activity reduces the production and accumulation of surface ozone when typhoons are making landfalling. Variations of tropospheric ozone profiles during the differential developmental stages of landfalling typhoons are further examined to quantify the influences of typhoon-induced stratospheric intrusions on lower troposphere and surface ozone. Using temporally dense ozone vertical observations, we found two regions of high ozone concentrations separately located in the ABL and the middle-to-upper troposphere under the influences of typhoons. Averagely, the ozone enhancement maximizes around 10–12 ppbv at 1–1.5 km altitude at the typhoon initial stages. The ozone enhancement persists over a longer period in the middle-to-upper troposphere with a positive ozone anomaly of 10 ppbv at 7–8 km altitude shortly after typhoon genesis, and 30 ppbv near 12 km altitude when typhoons reach their maximum intensity. When typhoons are landing, a negative ozone anomaly appears and extends upward with a maximum ozone reduction of 14–18 ppbv at 5 km altitude and 20–25 ppbv at 11 km altitude. Though the overall tropospheric ozone is usually reduced during typhoon landfalling, we quantify that five of eight typhoon samples deduce ozone-rich air with the stratospheric origin (80 ppbv) above 4 km altitude, and in 3 typhoon cases the ozone-rich air intrusions (60 ppbv) can sink to the ABL. This suggests that the typhoon-induced stratospheric ozone-rich air intrusions play an important role in surface ozone enhancement.

It is an old question whether tropospheric water vapor at different levels changes consistently in response to the enhanced greenhouse gas in the atmosphere. Earlier studies using older versions of climate models and available data revealed a significant difference between models and observations. Water vapor changes in the interior of the tropical troposphere have been found to be more strongly coupled to changes at the surface in climate models than in observations. We reexamine this issue using four leading CMIP5 models (CCSM4, HadGEM2-A, GFDL-CM3 and MPI-ESM-MR) and more updated observational datasets (ERA-Interim and NCEP reanalysis). Focusing on the Tropics, we have calculated the correlations between interannual variation of specific humidity in all levels of the troposphere with that at the surface. It is found that the previously noted biases in the strength of the coupling between water vapor changes in the interior of the troposphere and those at the surface still exist in the updated models—the change in the tropical averaged tropospheric water vapor is more strongly correlated with the change in the surface, especially in the middle troposphere. It is argued that the vertical profile of water vapor correlations in observations is more consistent with the “hot tower” concept for tropical convections. Zonal mean correlation results and those from the moisture regime sorting method are consistent with each other, both of which indicate the role of deep convection as a mechanism to couple the middle tropospheric water vapor and that in the surface and that an inaccurate representation of deep convection as a possible cause for the discrepancies between models and observations in the coupling between middle tropospheric water vapor and those at the surface.

As key variables in general circulation models, precipitation and moisture in four leading models from CMIP5 (phase 5 of the Coupled Model Intercomparison Project) are analyzed, with a focus on four tropical oceanic regions. It is found that precipitation in these models is overestimated in most areas. However, moisture bias has large intermodel differences. The model biases in precipitation and moisture are further examined in conjunction with large-scale circulation by regime-sorting analysis. Results show that all models consistently overestimate the frequency of occurrence of strong upward motion regimes and peak descending regimes of 500-hPa vertical velocity [Formula: see text]. In a given [Formula: see text] regime, models produce too much precipitation compared to observation and reanalysis. But for moisture, their biases differ from model to model and also from level to level. Furthermore, error causes are revealed through decomposing contribution biases into dynamic and thermodynamic components. For precipitation, the contribution errors in strong upward motion regimes are attributed to the overly frequent [Formula: see text]. In the weak upward motion regime, the biases in the dependence of precipitation on [Formula: see text] and the [Formula: see text] probability density function (PDF) make comparable contributions, but often of opposite signs. On the other hand, the biases in column-integrated water vapor contribution are mainly due to errors in the frequency of occurrence of [Formula: see text], while thermodynamic components contribute little. These findings suggest that errors in the frequency of [Formula: see text] occurrence are a significant cause of biases in the precipitation and moisture simulation.