Stratosphere-troposphere Exchange Research Articles

Investigating stratospheric changes between 2009 and 2018 with halogenated trace gas data from aircraft, AirCores, and a global model focusing on CFC-11

Abstract. We present new observations of trace gases in the stratosphere based on a cost-effective sampling technique that can access much higher altitudes than aircraft. The further development of this method now provides detection of species with abundances in the parts per trillion (ppt) range and below. We obtain mixing ratios for six gases (CFC-11, CFC-12, HCFC-22, H-1211, H-1301, and SF6), all of which are important for understanding stratospheric ozone depletion and circulation. After demonstrating the quality of the data through comparisons with ground-based records and aircraft-based observations, we combine them with the latter to demonstrate its potential. We first compare the data with results from a global model driven by three widely used meteorological reanalyses. Secondly, we focus on CFC-11 as recent evidence has indicated renewed atmospheric emissions of that species relevant on a global scale. Because the stratosphere represents the main sink region for CFC-11, potential changes in stratospheric circulation and troposphere–stratosphere exchange fluxes have been identified as the largest source of uncertainty for the accurate quantification of such emissions. Our observations span over a decade (up until 2018) and therefore cover the period of the slowdown of CFC-11 global mixing ratio decreases measured at the Earth's surface. The spatial and temporal coverage of the observations is insufficient for a global quantitative analysis, but we do find some trends that are in contrast with expectations, indicating that the stratosphere may have contributed to the slower concentration decline in recent years. Further investigating the reanalysis-driven model data, we find that the dynamical changes in the stratosphere required to explain the apparent change in tropospheric CFC-11 emissions after 2013 are possible but with a very high uncertainty range. This is partly caused by the high variability of mass flux from the stratosphere to the troposphere, especially at timescales of a few years, and partly by large differences between runs driven by different reanalysis products, none of which agree with our observations well enough for such a quantitative analysis.

Response of Ozone to a Gravity Wave Process in the UTLS Region Over the Tibetan Plateau

In this paper, ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts and the Weather Research and Forecasting (WRF) mesoscale model are used to compare and simulate a topographic gravity wave event that occurred over the Tibetan Plateau between 18:00 on 3 May 2014 and 00:00 on 4 May 2014. The stratosphere–troposphere exchange (STE) process and its detailed characteristics during the gravity wave event are presented. The response of the upper troposphere and lower stratosphere (UTLS) ozone to the gravity wave is discovered from the perspective of the gravity wave dynamic process. The results show that the gravity wave structure between 90° E and 100° E had a westward tilt with height. From 19:00 to 21:00 the gravity waves decayed and from 22:00 to 23:00 their shape gradually blurred. Meanwhile, there was a response of the ozone mixing ratio in the UTLS over the Tibetan Plateau to the gravity waves breaking. After 21:00, this wave process caused the STE, as air from the stratosphere with a greater ozone concentration was injected into the upper troposphere, leading to rapidly increased ozone concentration there.

Variations in Ozone Concentration over the Mid-Latitude Region Revealed by Ozonesonde Observations in Pohang, South Korea

Ozone absorbs harmful UV rays at high elevations but acts as a pollutant gas in the lower atmosphere. It is necessary to monitor both the vertical profile and the total column ozone. In this study, variations in the ozone concentration of Pohang were divided into three vertical layers: the stratospheric layer (STL), the second ozone peak layer (SOPL), and the tropospheric layer (TRL). Our results indicated that the ozone concentration in the STL, SOPL, TRL, and total column ozone increased by 0.45%, 2.64%, 5.26%, and 1.07% decade−1, respectively. The increase in the SOPL during springtime indicates that stratosphere–troposphere exchange is accelerating, while the increase during summertime appears to have been influenced by the lower layers. The growth of tropospheric ozone concentration is the result of both increased ozone precursors from industrialization in East Asia and the influx of stratospheric ozone. Our results reaffirmed the trend of ozone concentration in mid-latitudes of the northern hemisphere from vertical profiles in Pohang and, in particular, suggests that the recent changes of ozone in this region need to be carefully monitored.

First in situ UV profile across the UTLS accompanied by ozone measurement over the Tibetan Plateau

Ultraviolet radiation (UV) and ozone can greatly affect human health and the Earth's ecological environment. By deploying a UV radiometer aboard a stratospheric balloon released at Qaidam (QDM) during the Asian summer monsoon (ASM) period in 2019, we provided in situ measurement of the UV profiles from the surface to the upper troposphere and lower stratosphere over the Tibetan Plateau (TP), China, for the first time. Based on two in situ UV profiles accompanied by four ozonesonde measurements, this study exhibited detailed variations of downwelling UV and vertical ozone distributions over the TP during the ASM period. The UV differences between the surface and stratospheric balloon flight altitudes were 16.7, 15.8, 12.6 and 18.0 Wm−2 during the four ozonesonde launches. Due to the diurnal variations in photochemical production and the stratosphere-troposphere exchange, the integrated ozone columns below 30 km ranged from 184.4 to 221.6 DU from four ozonesonde measurements. A positive correlation between UV attenuation and ozone column was exhibited under low cloud cover and clear sky conditions. The results of this study are expected to improve our understanding of UV and ozone properties, as well as their potential effects on terrestrial ecosystems and living environments over this significant plateau.

Regionally Varying Assessments of Upper-Level Tropical Width in Reanalyses and CMIP5 Models Using a Tropopause Break Metric

AbstractChanges in tropical width can have important consequences in sectors including ecosystems, agriculture, and health. Observations suggest tropical expansion over the past 30 years although studies have not agreed on the magnitude of this change. Climate model projections have also indicated an expansion and show similar uncertainty in its magnitude. This study utilizes an objective, longitudinally varying, tropopause break method to define the extent of the tropics at upper levels. The location of the tropopause break is associated with enhanced stratosphere–troposphere exchange and thus its structure influences the chemical composition of the stratosphere. The method shows regional variations in the width of the upper-level tropics in the past and future. Four modern reanalyses show significant contraction of the tropics over the eastern Pacific between 1981 and 2015, and slight but significant expansion in other regions. The east Pacific narrowing contributes to zonal mean narrowing, contradicting prior work, and is attributed to the use of monthly and zonal mean data in prior studies. Six global climate models perform well in representing the climatological location of the tropical boundary. Future projections show a spread in the width trend (from ~0.5° decade−1 of narrowing to ~0.4° decade−1 of widening), with a narrowing projected across the east Pacific and Northern Hemisphere Americas. This study illustrates that this objective tropopause break method that uses instantaneous data and does not require zonal averaging is appropriate for identifying upper-level tropical width trends and the break location is connected with local and regional changes in precipitation.

Stratospheric impact on the Northern Hemisphere winter and spring ozone interannual variability in the troposphere

Abstract. In this study we use ozone and stratospheric ozone tracer simulations from the high-resolution (0.5∘×0.5∘) Goddard Earth Observing System, Version 5 (GEOS-5), in a replay mode to study the impact of stratospheric ozone on tropospheric ozone interannual variability (IAV). We use these simulations in conjunction with ozonesonde measurements from 1990 to 2016 during the winter and spring seasons. The simulations include a stratospheric ozone tracer (StratO3) to aid in the evaluation of the impact of stratospheric ozone IAV on the IAV of tropospheric ozone at different altitudes and locations. The model is in good agreement with the observed interannual variation in tropospheric ozone, except for the post-Pinatubo period (1992–1994) over the region of North America. Ozonesonde data show a negative ozone anomaly in 1992–1994 following the Pinatubo eruption, with recovery thereafter. The simulated anomaly is only half the magnitude of that observed. Our analysis suggests that the simulated stratosphere–troposphere exchange (STE) flux deduced from the analysis might be too strong over the North American (50–70∘ N) region after the Mt. Pinatubo eruption in the early 1990s, masking the impact of lower stratospheric ozone concentration on tropospheric ozone. European ozonesonde measurements show a similar but weaker ozone depletion after the Mt. Pinatubo eruption, which is fully reproduced by the model. Analysis based on the stratospheric ozone tracer identifies differences in strength and vertical extent of stratospheric ozone impact on the tropospheric ozone interannual variation (IAV) between North America and Europe. Over North American stations, the StratO3 IAV has a significant impact on tropospheric ozone from the upper to lower troposphere and explains about 60 % and 66 % of the simulated ozone IAV at 400 hPa and ∼11 % and 34 % at 700 hPa in winter and spring, respectively. Over European stations, the influence is limited to the middle to upper troposphere and becomes much smaller at 700 hPa. The Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), assimilated fields exhibit strong longitudinal variations over Northern Hemisphere (NH) mid-high latitudes, with lower tropopause height and lower geopotential height over North America than over Europe. These variations associated with the relevant variations in the location of tropospheric jet flows are responsible for the longitudinal differences in the stratospheric ozone impact, with stronger effects over North America than over Europe.

Stratosphere–Troposphere Exchange and O3 Variability in the Lower Stratosphere and Upper Troposphere over the Irene SHADOZ Site, South Africa

This study aims to investigate the Stratosphere-Troposphere Exchange (STE) events and ozone changes over Irene (25.5° S, 28.1° E). Twelve years of ozonesondes data (2000–2007, 2012–2015) from Irene station operating in the framework of the Southern Hemisphere Additional Ozonesodes (SHADOZ) was used to study the troposphere (0–16 km) and stratosphere (17–28 km) ozone (O3) vertical profiles. Ozone profiles were grouped into three categories (2000–2003, 2004–2007 and 2012–2015) and average composites were calculated for each category. Fifteen O3 enhancement events were identified over the study period. These events were observed in all seasons (one event in summer, four events in autumn, five events in winter and five events in spring); however, they predominantly occur in winter and spring. The STE events presented here are observed to be influenced by the Southern Hemisphere polar vortex. To strengthen the investigation into STE events, advected potential vorticity maps were used, which were assimilated using Modélisation Isentrope du transport Méso–échelle de l’Ozone Stratosphérique par Advection (MIMOSA) model for the 350 K (~12–13 km) isentropic level. These maps indicated transport of high latitude air masses responsible for the reduction of the O3 mole fractions at the lower stratosphere over Irene which coincides with the enhancement of ozone in the upper troposphere. In general, the stratosphere is dominated by higher Modern Retrospective Analysis for Research Application (MERRA-2) potential vorticity (PV) values compared to the troposphere. However, during the STE events, higher PV values from the stratosphere were observed to intrude the troposphere. Ozone decline was observed from 12 km to 24 km with the highest decline occurring from 14 km to 18 km. An average decrease of 6.0% and 9.1% was calculated from 12 to 24 km in 2004–2007 and 2012–2015 respectively, when compared with 2000–2003 average composite. The observed decline occurred in the upper troposphere and lower stratosphere with winter and spring showing more decline compared with summer and autumn.

Direct Observations of Atmospheric Transport and Stratosphere-Troposphere Exchange from High-Precision Carbon Dioxide and Carbon Monoxide Profile Measurements

The balloon-borne Aircore campaign was conducted in Inner Mongolia, China, on June 13 and 14 2018, which detected carbon dioxide (CO2) and carbon monoxide (CO) profiles from surface to 24 km, showing strong positive and negative correlations between 8 km and 10 km on 13 and 14 June, respectively. Backward trajectories, meteorological analyses, and CO2 horizontal distributions were combined to interpret this phenomenon. The results indicated that the source region experienced a stratospheric intrusion and exhibited a large horizontal CO2 gradient; namely, lower CO concentrations corresponded to higher CO2 concentrations and vice versa. The laminar structure with multiple origins resulted in the highly negative correlation between CO2 and CO in the upper troposphere on 14 June. The contribution of stratospheric air mass to the upper troposphere and that of tropospheric air mass to the lower stratosphere were 26.7% and 24.3%, respectively, based on a mass balance approach. Another interesting phenomenon is that CO2 and CO concentrations increased substantially at approximately 8 km on 13 June. An analysis based on the backward trajectory implied that the air mass possibly came from anthropogenic sources. The slope of CO2/CO representing the anthropogenic sources was 87.3 ppm ppm−1. In addition, the CO2 profile showed that there was a large CO2 gradient of 4 ppm km−1 within the boundary layer on 13 June, and this gradient disappeared on 14 June.

Observational evidence of moistening the lowermost stratosphere via isentropic mixing across the subtropical jet

Abstract. Isentropic mixing across and above the subtropical jet is a significant mechanism for stratosphere–troposphere exchange. In this work, we show new observational evidence on the role of this process in moistening the lowermost stratosphere. The new measurement, obtained from the Spatial Heterodyne Observations of Water (SHOW) instrument during a demonstration flight on the NASA's ER-2 high-altitude research aircraft, captured an event of poleward water vapour transport, including a fine-scale (vertically <∼1 km) moist filament above the local tropopause in a high-spatial-resolution two-dimensional cross section of the water vapour distribution. Analysis of these measurements combined with ERA5 reanalysis data reveals that this poleward mixing of air with enhanced water vapour occurred in the region of a double tropopause following a large Rossby wave-breaking event. These new observations highlight the importance of high-resolution measurements in resolving processes that are important to the lowermost-stratosphere water vapour budget.

Calculation of the Vertical Velocity in the Asian Summer Monsoon Anticyclone Region Using the Thermodynamic Method With in situ and Satellite Data

The correct calculation of the vertical velocity of the Asian Summer Monsoon anticyclone (ASMA) region is helpful to know more accurately the ozone stratosphere-troposphere exchange, so as to explore the variation of ozone in the ASMA region. Therefore, the vertical velocity over the ASMA in June, July, August and September 2012 and 2016 is calculated by the thermodynamic method, which may avoid the deviations from the kinematics method using the mass continuity equation. In order to improve the accuracy, we used high-resolution heating rate datasets obtained via the Beijing Climate Center Radiation Transfer Model based on in situ observations and revised satellite data from MLS/AIRS. The vertical velocity calculated by the thermodynamic method (VT) is then compared with the data from ERA-Interim (VERA-I). In the daytime, values of VT were similar to VERA-I, and were dominated by ascending motion, although VT showed descending motion at the western edge of the ASMA below 100 hPa. The intensity of VT was slightly smaller than that of VERA-I at lower levels (200 hPa –100 hPa) over the ASMA region and significantly weaker above 100 hPa. In the night-time, situation was more complex. Both VT and VERA-I showed the convergence of vertical wind at 150 hPa, and the divergence at 80 hPa, but VT had a smaller standard deviation. Besides, VT showed descending in the western and northern ASMA, but VERA-I only descended in the west. The descending motion in the west, seen in both VT and VERA-I, is produced by the heating difference between Qinghai-Tibet Plateau and Iranian Plateau. The difference of the two vertical velocities in the northern ASMA may indicate the different understandings of the local Hadley Circulation and local Brewer-Dobson Circulation.

Atmospheric deposition of 7Be, 210Pb in Xining, a typical city on the Qinghai-Tibet Plateau, China

First continuous observations of 7Be and 210Pb monthly total depositional fluxes were performed from December 2014 to November 2017 in Xining, a typical city on the Qinghai-Tibet Plateau, China. The depositional fluxes showed a clear seasonal variation, with peak depositional fluxes in summer for 7Be and in summer or fall for 210Pb, and minimum values in winter for both 7Be and 210Pb. These are ascribed to the seasonal variability in precipitation and the stratosphere-troposphere exchange of atmosphere. The annual depositional fluxes were 1555 Bq m−2 year−1 for 7Be and 284 Bq m−2 year−1 for 210Pb. The annual volume-weighted activities and depositional fluxes of 7Be increased with altitude.

Influence of the south-north bias of the South Asia High on the water vapor distribution in the upper troposphere-lower stratosphere of the Asian monsoon region

Based on the ERA-40 reanalysis data provided by the European Interim Numerical Forecast Centre from 1958 to 2002, the influence of the north-south bias of the South Asia High(SAH) on the water vapor distribution in the upper troposphere-lower stratosphere of the Asian monsoon region is analysed. It is shown that: (1) The north-south bias of SAH has little influence on the position of the 200 hPa water vapor high value centre, which mainly affects its intensity; and it has a great influence on the position and intensity of the 100 hPa water vapor high value centre; It has little effect on the water vapor distribution at 70 hPa. (2) In the south years of SAH, the anti-cyclone is abnormally weak, the vertical ascending motion is relatively insubstantial, and the water vapor transported to the upper troposphere is less than the north years. Therefore, the water vapor high value in the south years is smaller than that in the north years. (3) Abnormal temperature distribution of 200 hPa in anticyclone is consistent with the abnormal distribution of water vapor, and the warm centre is conducive to the formation of high water vapor. It is shown that the distribution of water vapor in the anticyclone range is caused by the interaction of temperature field and circulation field. This study has a certain reference meaning for understanding the stratospheric-tropospheric exchange of the Asian monsoon region.

Tritium as a hydrological tracer in Mediterranean precipitation events

Abstract. Climate models are in need of improved constraints for water vapor transport in the atmosphere, and tritium can serve as a powerful tracer in the hydrological cycle. Although the general principles of tritium distribution and transfer processes within and between the various hydrological compartments are known, variation on short timescales and aspects of altitude dependence are still under debate. To address questions regarding tritium sources, sinks, and transfer processes, the sampling of individual precipitation events in Corte on the island of Corsica in the Mediterranean Sea was performed between April 2017 and April 2018. Tritium concentrations of 46 event samples were compared to their moisture origin and corresponding air mass history. Air mass back-trajectories were generated from the novel high-resolution ERA5 dataset from the ECMWF (European Centre for Medium-Range Weather Forecasts). Geographical source regions with similar tritium concentrations were predefined using generally known tritium distribution patterns, such as the “continental effect”, and from data records derived at long-term measurement stations of tritium in precipitation across the working area. Our model-derived source region tritium concentrations agreed well with annual mean station values. Moisture that originated from continental Europe and the Atlantic Ocean was most distinct regarding tritium concentrations with values up to 8.8 TU (tritium units) and near 0 TU, respectively. The seasonality of tritium values ranged from 1.6 TU in January to 10.1 TU in May, and they exhibited well-known elevated concentrations in spring and early summer due to increased stratosphere–troposphere exchange. However, this pattern was interrupted by extreme events. The average altitude of trajectories was correlated with the tritium concentrations in precipitation, especially in spring and early summer and if outlier values of extreme tritium concentrations were excluded. However, in combination with the trajectory information, these outlier values proved to be valuable for improving the comprehension of tritium movement in the atmosphere. Our work shows how event-based tritium research can advance the understanding of its distribution in the atmosphere.

A Rossby wave breaking-induced enhancement in the tropospheric ozone over the Central Himalayan region

The high-altitude regions in the Himalayas are prone to high ozone concentrations frequently resulting from diverse dynamical and transport mechanisms. Here, we report an unusual enhancement in the surface and tropospheric ozone concentrations over the central Himalayan region from ground-based and space-borne measurements in the month of December 2010. The surface ozone levels (~80 ppbv) on 18–19 December 2010 is observed to be two-fold higher relative to the seasonal average (December-January-February) of about 40–50 ppbv in the central Himalayan region. The space-borne measurements from Tropospheric Emission Spectrometer and Ozone Monitoring Instrument onboard Aqua satellite also show higher values in the tropospheric column ozone over this region. The satellite observations indicate an increase in tropopause temperature of about 5 °C and decrease in tropopause altitude about 1 km during 18–19 December 2010 resulting in the occurrence of tropopause fold facilitating the stratospheric-tropospheric exchange processes over the study region. The plausible reason for the occurrence of tropopause fold and subsequent enhancement of tropospheric and surface ozone is found to be associated with the breaking Rossby waves in the upper troposphere. The wave breaking leads to the advection of high-PV (potential vorticity) air, with magnitudes of about 3–4 PVU, towards the central Himalayan region from high-latitudes. The vertical component of PV advection also shows a deep stratospheric intrusion of high-PV air into the troposphere. The isentropic transport of ozone across the folding tropopause due to the wave breaking is clearly depicted from the satellite and reanalysis datasets. Therefore, the present study has strong implications of upper tropospheric wave dynamics to the tropospheric and surface ozone over the Himalayan regions having complex topography.

Seasonal Features and a Case Study of Tropopause Folds over the Tibetan Plateau

Tropopause fold is the primary mechanism for stratosphere-troposphere exchange (STE) at the midlatitudes. Investigation of the features of tropopause folds over the Tibetan Plateau (TP) is important since the TP is a hotspot in global STE. In this study, we investigated seasonal features of the tropopause fold events over the TP using the 40-year ERA-Interim reanalysis data. The development of a tropopause folding case is specifically examined. The results show that shallow tropopause folds occur mostly in spring, while medium and deep folds occur mostly in winter. The multiyear mean monthly frequency of shallow tropopause folds over the TP reaches its maximum value of about 7% in May and then decreases gradually to its minimum value of 1% in August and increases again since September. Deep folds rarely occur in summer and autumn. Both the seasonal cycle and seasonal distribution of total tropopause folds over the TP are dominated by shallow folds. The relative high-frequency areas of medium and deep folds are located over the southern edge of the TP. The westerly jet movement controls the displacement of the high-frequency folding region over the TP. The region of high-frequency tropopause folds is located in the southern portion of the plateau in spring and moves northward in summer. The jet migrates back to the south in autumn and is located along about 30°N in winter, and the region where folds occur most frequently also shifts southward correspondingly. A medium fold event that occurred on 29 December 2018 is used to demonstrate the evolution of a tropopause fold case over the TP in winter; that is, the folding structure moves from west to east, the tropopause pressure is greater than 320 hPa over the folding region, while it is about 200 hPa in the surrounding areas, and the stratospheric air with high potential vorticity (PV) is transported from the high latitudes to the plateau by meridional winds. A trajectory model result verifies the transport pathway of the air parcels during the intrusion event.

Geographical distribution of extreme deep and intense convective storms on Earth

Vigorous convective storms can not only produce severe destructive weather events and lead to huge damages, but also play a vital role in stratosphere–troposphere exchange (STE). Based on 16-yr Tropical Rainfall Measuring Mission (TRMM) observational data, global geographical distribution of extremely deep and intense convective storms has been investigated. The results show that convective storms are mainly distributed in the tropics, with the largest storm event density located in tropical Africa, tropical Americas and the Maritime Continent. However, compared to convective storms, the most intense and deepest convective storms present a distinctly different geographical distribution pattern, which is true even if the same convective storm samples are used. Overall, tropical Africa is the only region in the world where the distributions of deep and intense convective storms overlap. Other than that, convective storms in the Maritime Continent region are characterized by large convective depth but weak convective intensity, while the opposite is true in subtropical land regions. The top 1% deep convective storms occur most frequently in boreal winter (48%) and spring (36%), and are mainly distributed in the Maritime Continent and tropical Africa. Those most intense convective storms show a weak seasonal variation in quantity while their distributions are significantly different from season to season. Further analysis reveals that convective properties of storms in different regions are distinctly different, and even different in the same region such as tropical Africa but in different seasons. This result suggests that although the formation of convective storm requires sufficient water vapor supply and instable atmospheric conditions, deep convection needs more water vapor supply to develop compared to intense storms. • The geographic distributions of deep and intense convective storms exhibit distinctly different patterns. • Convective storms are strong in convective depth in maritime continent, but strong in convective intensity in subtropical continent. • Deep convection is more likely to occur in a more humid environment than intense storms.

Update and evaluation of the ozone dry deposition in Oslo CTM3 v1.0

Abstract. High concentrations of ozone in ambient air are hazardous not only to humans but to the ecosystem in general. The impact of ozone damage on vegetation and agricultural plants in combination with advancing climate change may affect food security in the future. While the future scenarios in themselves are uncertain, there are limiting factors constraining the accuracy of surface ozone modeling also at present: the distribution and amount of ozone precursors and ozone-depleting substances, the stratosphere–troposphere exchange, as well as scavenging processes. Removal of any substance through gravitational settling or by uptake by plants and soil is referred to as dry deposition. The process of dry deposition is important for predicting surface ozone concentrations and understanding the observed amount and increase of tropospheric background ozone. The conceptual dry deposition velocities are calculated following a resistance-analogous approach, wherein aerodynamic, quasi-laminar, and canopy resistance are key components, but these are hard to measure explicitly. We present an update of the dry deposition scheme implemented in Oslo CTM3. We change from a purely empirical dry deposition parameterization to a more process-based one which takes the state of the atmosphere and vegetation into account. We examine the sensitivity of the scheme to various parameters, e.g., the stomatal conductance-based description of the canopy resistance and the choice of ozone surface resistance, and evaluate the resulting modeled ozone dry deposition with respect to observations and multi-model studies. Individual dry deposition velocities are now available for each land surface type and agree generally well with observations. We also estimate the impact on the modeled ozone concentrations at the surface. We show that the global annual total ozone dry deposition decreases with respect to the previous model version (−37 %), leading to an increase in surface ozone of more than 100 % in some regions. While high sensitivity to changes in dry deposition to vegetation is found in the tropics and the Northern Hemisphere, the largest impact on global scales is associated with the choice of prescribed ozone surface resistance over the ocean and deserts.

Evidence of small-scale quasi-isentropic mixing in ridges of extratropical baroclinic waves

Abstract. Stratosphere–troposphere exchange within extratropical cyclones provides the potential for anthropogenic and natural surface emissions to rapidly reach the stratosphere as well as for ozone from the stratosphere to penetrate deep into the troposphere, even down into the boundary layer. The efficiency of this process directly influences the surface climate, the chemistry in the stratosphere, the chemical composition of the extratropical transition layer, and surface pollution levels. Here, we present evidence for a mixing process within extratropical cyclones which has gained only a small amount of attention so far and which fosters the transport of tropospheric air masses into the stratosphere in ridges of baroclinic waves. We analyzed airborne measurement data from a research flight of the WISE (Wave-driven ISentropic Exchange) campaign over the North Atlantic in autumn 2017, supported by forecasts from a numerical weather prediction model and trajectory calculations. Further detailed process understanding is obtained from experiments of idealized baroclinic life cycles. The major outcome of this analysis is that air masses mix in the region of the tropopause and potentially enter the stratosphere in ridges of baroclinic waves at the anticyclonic side of the jet without changing their potential temperature drastically. This quasi-isentropic exchange occurs above the outflow of warm conveyor belts, in regions which exhibit enhanced static stability in the lower stratosphere and a Kelvin–Helmholtz instability across the tropopause. The enhanced static stability is related to radiative cooling below the tropopause and the presence of small-scale waves. The Kelvin–Helmholtz instability is related to vertical shear of the horizontal wind associated with small-scale waves at the upper edge of the jet stream. The instability leads to the occurrence of turbulence and consequent mixing of trace gases in the tropopause region. While the overall relevance of this process has yet to be assessed, it has the potential to significantly modify the chemical composition of the extratropical transition layer in the lowermost stratosphere in regions which have previously gained a small amount of attention in terms of mixing in baroclinic waves.

IAP-AACM v1.0: a global to regional evaluation of the atmospheric chemistry model in CAS-ESM

Abstract. In this study, a full description and comprehensive evaluation of a global–regional nested model, the Aerosol and Atmospheric Chemistry Model of the Institute of Atmospheric Physics (IAP-AACM), is presented for the first time. Not only are the global budgets and distribution explored, but comparisons of the nested simulation over China against multiple datasets are investigated, which benefit from access to Chinese air quality monitoring data from 2013 to the present and the “Model Inter-Comparison Study for Asia” project. The model results and analysis can help reduce uncertainties and aid with understanding model diversity with respect to assessing global and regional aerosol effects on climate and human health, especially over East Asia and areas affected by East Asia. For the global simulation, the 1-year simulation for 2014 shows that the IAP-AACM is within the range of other models. Overall, it reasonably reproduced spatial distributions and seasonal variations of trace gases and aerosols in both surface concentrations and column burdens (mostly within a factor of 2). The model captured spatial variation for carbon monoxide well with a slight underestimation over ocean, which implicates the uncertainty of the ocean source. The simulation also matched the seasonal cycle of ozone well except for the continents in the Northern Hemisphere, which was partly due to the lack of stratospheric–tropospheric exchange. For aerosols, the simulation of fine-mode particulate matter (PM2.5) matched observations well. The simulation of primary aerosols (normalized mean biases, NMBs, are within ±0.64) is better than that of secondary aerosols (NMB values are greater than 1.0 in some regions). For the nested regional simulation, the IAP-AACM shows the superiority of higher-resolution simulation using the nested domain over East Asia. The model reproduced variation of sulfur dioxide (SO2), nitrogen dioxide (NO2), and PM2.5 accurately in typical cities, with correlation coefficients (R) above 0.5 and NMBs within ±0.5. Compared with the global simulation, the nested simulation exhibits an improved ability to capture the high temporal and spatial variability over China. In particular, the R values for SO2, NO2 and PM2.5 are increased by ∼0.15, ∼0.2, and ∼0.25 respectively in the nested grid. Based on the evaluation and analysis, future model improvements are suggested.

Tropospheric mixing and parametrization of unresolved convective updrafts as implemented in the Chemical Lagrangian Model of the Stratosphere (CLaMS v2.0)

Abstract. Inaccurate representation of mixing in chemistry transport models, mainly suffering from an excessive numerical diffusion, strongly influences the quantitative estimates of the stratosphere–troposphere exchange (STE). The Lagrangian view of transport offers an alternative to exploit the numerical diffusion for parametrization of the physical mixing. Here, we follow this concept and discuss how to extend the representation of tropospheric transport in the Chemical Lagrangian Model of the Stratosphere (CLaMS). Although the current transport scheme in CLaMS (v1.0) shows a good ability to represent transport of tracers in the stably stratified stratosphere (Pommrich et al., 2014, and the references therein), there are deficiencies in the representation of the effects of convective uplift and mixing due to weak vertical stability in the troposphere. We show how the CLaMS transport scheme was modified by including additional tropospheric mixing and vertical transport due to unresolved convective updrafts by parametrizing these processes in terms of the dry and moist Brunt–Väisälä frequencies. The regions with enhanced convective updrafts in the novel CLaMS simulation covering the 2005–2008 period coincide with regions of enhanced convection as diagnosed from the satellite observations of the outgoing longwave radiation (OLR). We analyze how well this approach improves the CLaMS representation of CO2 in the upper troposphere and lower stratosphere, in particular the propagation of the CO2 seasonal cycle from the planetary boundary layer (PBL) into the lower stratosphere. The CO2 values in the PBL are specified by the CarbonTracker data set (version CT2013B), and the Comprehensive Observation Network for TRace gases by AIrLiner (CONTRAIL) observations are used to validate the model. The proposed extension of tropospheric transport increases the influence of the PBL in the middle and upper troposphere and at the same time impacts the STE. The effect on mean age away from the troposphere in the deep stratosphere is weak.