Local Tropopause Research Articles

Evidence for ice particles in the tropical stratosphere from in-situ measurements

Abstract. In-situ ice crystal size distribution measurements are presented within the tropical troposphere and lower stratosphere. The measurements were performed using a combination of a Forward Scattering Spectrometer Probe (FSSP-100) and a Cloud Imaging Probe (CIP), which were installed on the Russian high altitude research aircraft M55 "Geophysica" during the SCOUT-O3 campaign in Darwin, Australia. One of the objectives of the campaign was to characterise the Hector convective system, which appears on an almost daily basis during the pre-monsoon season over the Tiwi Islands, north of Darwin. In total 90 encounters with ice clouds, between 10 and 19 km altitude were selected from the dataset and were analysed. Six of these encounters were observed in the lower stratosphere, up to 1.4 km above the local tropopause. Concurrent lidar measurements on board "Geophysica" indicate that these ice clouds were a result of overshooting convection. Large ice crystals, with a maximum dimension up to 400 μm, were observed in the stratosphere. The stratospheric ice clouds included an ice water content ranging from 7.7×10−5 to 8.5×10−4 g m−3 and were observed at ambient relative humidities (with respect to ice) between 75 and 157%. Three modal lognormal size distributions were fitted to the average size distributions for different potential temperature intervals, showing that the shape of the size distribution of the stratospheric ice clouds are similar to those observed in the upper troposphere. In the tropical troposphere the effective radius of the ice cloud particles decreases from 100 μm at about 10 km altitude, to 3 μm at the tropopause, while the ice water content decreases from 0.04 to 10−5 g m−3. No clear trend in the number concentration was observed with altitude, due to the thin and inhomogeneous characteristics of the observed cirrus clouds. The ice water content calculated from the observed ice crystal size distribution is compared to the ice water content derived from two hygrometer instruments. This independent measurement of the ice water content agrees within the combined uncertainty of the instruments for ice water contents exceeding 3×10−4g m−3. Stratospheric residence times, calculated based on gravitational settling, and evaporation rates show that the ice crystals observed in the stratosphere over the Hector storm system had a high potential of humidifying the stratosphere locally. Utilizing total aerosol number concentration measurements from a four channel condensation particle counter during two separate campaigns, it can be shown that the fraction of ice particles to the number of aerosol particles remaining ranges from 1:300 to 1:30 000 for tropical upper tropospheric ice clouds with ambient temperatures below −75°C.

A Madden–Julian Oscillation‐triggered record ozone minimum over the Tibetan Plateau in December 2003 and its association with stratospheric “low‐ozone pockets”

A record minimum in total column ozone over the Tibetan Plateau in December 2003 is analyzed using both satellite observations and a chemical transport model (MOZART‐3). Ozone profiles observed by the MIPAS and GOMOS over the ozone minimum region (25°–35°N, 85°–110°E) show pronounced ozone reduction in both the upper troposphere and lower stratosphere (UTLS) and the middle stratosphere regions. MOZART‐3 simulation indicates that ozone reduction in the UTLS region is generally caused by uplift of the local tropopause and northward transport of tropical ozone‐poor air associated with an anomalous anticyclone in the upper troposphere. MIPAS observation suggests that the displacement of “low‐ozone pockets” is responsible for ozone decline in the middle‐stratospheric region and contributes to 29–46% of the total column ozone reduction. Further analysis shows that the upper‐tropospheric anticyclonic anomaly is closely associated with the Rossby‐wave triggered by the eastward propagating Madden–Julian Oscillation convective heating.

The AD 1362 Öræfajökull eruption, S.E. Iceland: Physical volcanology and volatile release

The explosive rhyolitic eruption of Öræfajökull volcano, Iceland, in AD 1362 is described and interpreted based on the sequence of pyroclastic fall and flow deposits at 10 proximal locations around the south side of the volcano. Öræfajökull is an ice-clad stratovolcano in south central Iceland which has an ice-filled caldera (4–5 km diameter) of uncertain origin. The main phase of the eruption took place over a few days in June and proceeded in three main phases that produced widely dispersed fallout deposits and a pyroclastic flow deposit. An initial phase of phreatomagmatic eruptive activity produced a volumetrically minor, coarse ash fall deposit (unit A) with a bi-lobate dispersal. This was followed by a second phreatomagmatic, possibly phreatoplinian, phase that deposited more fine ash beds (unit B), dispersed to the SSE. Phases A and B were followed by an intense, climactic Plinian phase that lasted ∼ 8–12 h and produced unit C, a coarse-lapilli, pumice-clast-dominated fall deposit in the proximal region. At the end of Plinian activity, pyroclastic flows formed a poorly-sorted deposit, unit D, presently of very limited thickness and exposed distribution. Much of Eastern Iceland is covered with a very fine distal ash layer, dispersed to the NE. This was probably deposited from an umbrella cloud and is the distal representation of the Plinian fallout. A total bulk fall deposit volume of ∼ 2.3 km 3 is calculated (∼ 1.2 km 3 DRE). Pyroclastic flow deposit volumes have been crudely estimated to be < 0.1 km 3. Maximum clast size data interpreted by 1-D models suggests an eruption column ∼ 30 km high and mass discharge rates of ∼ 10 8 kg s − 1 . Ash fall may have taken place from heights around 15 km, above the local tropopause (∼ 10 km), with coarser clasts dispersed below that under a different wind regime. Analyses of glass inclusions and matrix glasses suggest that the syn-eruptive SO 2 release was only ∼ 1 Mt. This result is supported by published Greenland ice-core acidity peak data that also suggest very minor sulphate deposition and thus SO 2 release. The small sulphur release reflects the low sulphur solubility in the 1362 rhyolitic melt. The low tropopause over Iceland and the 30-km-high eruption column certainly led to stratospheric injection of gas and ash but little sulphate aerosol was generated. Moreover, pre-eruptive and degassed halogen concentrations (Cl, F) indicate that these volatiles were not efficiently released during the eruption. Besides the local pyroclastic flow (and related lahar) hazard, the impact of the Öræfajökull 1362 eruption was perhaps restricted to widespread ash fall across Eastern Iceland and parts of northern Europe.

Quantifying the impact of the North American monsoon and deep midlatitude convection on the subtropical lowermost stratosphere using in situ measurements

The chemical composition of the lowermost stratosphere exhibits both spatial and temporal variability depending upon the relative strength of (1) isentropic transport from the tropical tropopause layer (TTL), (2) diabatic descent from the midlatitude and northern midlatitude stratosphere followed by equatorward isentropic transport, and (3) diabatic ascent from the troposphere through convection. In situ measurements made in the lowermost stratosphere over Florida illustrate the additional impact of equatorward flow around the monsoon anticyclone. This flow carries, along with older stratospheric air, the distinct signature of deep midlatitude convection. We use simultaneous in situ measurements of water vapor (H2O), ozone (O3), total odd nitrogen (NOy), carbon dioxide (CO2), and carbon monoxide (CO) in the framework of a simple box model to quantify the composition of the air sampled in the lowermost stratosphere during the mission on the basis of tracer mixing ratios ascribed to the source regions for these transport pathways. The results show that in the summer, convection has a significant impact on the composition of air in the lowermost stratosphere, being the dominant source of water vapor up to the 380 K isentrope. The implications of these results extend from the potential for heterogeneous ozone loss resulting from the increased frequency and lifetime of cirrus near the local tropopause, to air with increased water vapor that as part of the equatorward flow associated with the North American monsoon can become part of the general circulation.

A numerical study of tropical cross-tropopause transport by convective overshoots

Abstract. Observations obtained during the Tropical Convection, Cirrus and Nitrogen Oxides (TROCCINOX) golden day have revealed the presence of ice particles up to 410 K (18.2 km) 2 km above the local tropopause. The case was investigated using a three-dimensional quadruply nested non-hydrostatic simulation and Meteosat Second Generation (MSG) observations. The simulation reproduced the measurements along the flight track fairly well. A reasonable agreement with MSG observations was also achieved: the 10.8-μm brightness temperature (BT) minimum of 187 K was reproduced (a value 6 K colder than the environmental cold-point temperature) as was the positive BT difference between the 6.2- and 10.8-μm bands, an overshoot signature. The simulation produced several overshooting plumes up to 410 K yielding an upward transport of water vapour of a few tons per second across the tropical tropopause. The estimated mass flux agrees with those derived from over tracer budgets, indicating that convection transports mass across the tropopause.

Temperature climatology and trend estimates in the UTLS region as observed over a southern subtropical site, Durban, South Africa

Abstract. Temperature trends in the UTLS region are under-reported, particularly in the Southern Hemisphere, and yet temperature is one of the most important indicators of changes in dynamical and radiative processes in the atmosphere. Here radiosonde data from Durban, South Africa (30.0° S, 30.9° E) over the period 1980 to 2001 (22 years) between 250 and 20 hPa are used to derive a mean temperature climatology and to determine trends. The seasonal cycle at the 250-hPa level is anti-correlated with the seasonal cycles at the 150-hPa and 100-hPa heights. The 100-hPa level (local tropopause) exhibits a minimum temperature in late summer and a maximum in winter, and closely corresponds to previous results for tropical regions. Based on a Fourier analysis, both the annual cycle (AO) and the semi-annual cycle (SAO) are dominant, although the former is about 4 times stronger. The AO is strongest at the 100-hPa height. A trend analysis reveals a cooling trend at almost all heights in the UTLS region, with a maximum cooling rate of 1.09±0.41 K per decade, at 70-hPa. Cooling rates are in good agreement with other studies and are slightly higher in summer than in winter.

Measurements of NO, NO&amp;lt;sub&amp;gt;y&amp;lt;/sub&amp;gt;, N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O, and O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; during SPURT: implications for transport and chemistry in the lowermost stratosphere

Abstract. We present measurements of NO, NOy, O3, and N2O within the lowermost stratosphere (LMS) over Europe obtained during the SPURT project. The measurements cover all seasons between November 2001 and July 2003. They span a broad band of latitudes from 30° N to 75° N and a potential temperature range from 290 to 380 K. The measurements represent a comprehensive data set of these tracers and reveal atmospheric transport processes that influence tracer distributions in the LMS. Median mixing ratios of stratospheric tracers in equivalent latitude-potential temperature coordinates show a clear seasonal cycle related to the Brewer-Dobson circulation, with highest values in spring and lowest values in autumn. Vertical tracer profiles show strong gradients at the extratropical tropopause, suggesting that vertical (cross-isentropic) mixing is reduced above the tropopause. Pronounced meridional gradients in the tracer mixing ratios are found on potential temperature surfaces in the LMS. This suggests strongly reduced mixing along isentropes. Concurrent large gradients in static stability in the vertical direction, and of PV in the meridional direction, suggest the presence of a mixing barrier. Seasonal cycles were found in the correlation slopes ΔO3/ΔN2O and ΔNOy/ΔN2O well above the tropopause. Absolute slope values are smallest in spring indicating chemically aged stratospheric air originating from high altitudes and latitudes. Larger values were measured in summer and autumn suggesting that a substantial fraction of air takes a "short-cut" from the tropical tropopause region into the extratropical LMS. The seasonal change in the composition of the LMS has direct implications for the ozone chemistry in this region. Comparisons of measured NO with the critical NO value at which net ozone production changes from negative to positive, imply ozone production up to 20 K above the local tropopause in spring, up to 30 K in summer, and up to 40 K in autumn. Above these heights, and in winter, net ozone production is negative.

Cirrus Classification at Midlatitude from Systematic Lidar Observations

Abstract Systematic cirrus lidar measurements performed in the south of France during 2000 are analyzed statistically to search for cloud classes. The classes are based on cloud characteristics (cloud thickness, light backscattering efficiency, and its variance), cloud absolute geometric height, cloud height relative to the tropopause, and the temperature at the cloud level. The successive use of principal component analysis, cluster methods, and linear discriminant analysis allows the identification of four cirrus classes. Almost all the cirrus detections correspond to three classes with similar proportion of the total cirrus detected (around 30%). The absolute geometric height and the thickness are found to be the main discriminant variables. The first cirrus class corresponds to thin clouds above the local tropopause (absolute geometric height: 11.5 km), or at least around the tropopause, while another class corresponds also to thin clouds but at a lower altitude range in the troposphere (absolute geometric height: 8.6 km). The third class corresponds to thick clouds (thickness of 3.2 km) located below the tropopause, in an altitude range between the two first classes (absolute geometric height: 9.8 km). As expected, the high-altitude cirrus class is characterized with the lowest mean temperature. It is noted that the temperature is closely related to the altitude and so the role of temperature in the cirrus classes cannot be disentangled from the role of the altitude.

Seasonal cycles and variability of O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; and H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O in the UT/LMS during SPURT

Abstract. Airborne high resolution in situ measurements of a large set of trace gases including ozone (O3) and total water (H2O) in the upper troposphere and the lowermost stratosphere (UT/LMS) have been performed above Europe within the SPURT project. SPURT provides an extensive data coverage of the UT/LMS in each season within the time period between November 2001 and July 2003. In the LMS a distinct spring maximum and autumn minimum is observed in O3, whereas its annual cycle in the UT is shifted by 2–3 months later towards the end of the year. The more variable H2O measurements reveal a maximum during summer and a minimum during autumn/winter with no phase shift between the two atmospheric compartments. For a comprehensive insight into trace gas composition and variability in the UT/LMS several statistical methods are applied using chemical, thermal and dynamical vertical coordinates. In particular, 2-dimensional probability distribution functions serve as a tool to transform localised aircraft data to a more comprehensive view of the probed atmospheric region. It appears that both trace gases, O3 and H2O, reveal the most compact arrangement and are best correlated in the view of potential vorticity (PV) and distance to the local tropopause, indicating an advanced mixing state on these surfaces. Thus, strong gradients of PV seem to act as a transport barrier both in the vertical and the horizontal direction. The alignment of trace gas isopleths reflects the existence of a year-round extra-tropical tropopause transition layer. The SPURT measurements reveal that this layer is mainly affected by stratospheric air during winter/spring and by tropospheric air during autumn/summer. Normalised mixing entropy values for O3 and H2O in the LMS appear to be maximal during spring and summer, respectively, indicating highest variability of these trace gases during the respective seasons.

Indications of thin cirrus clouds in the stratosphere at mid-latitudes

Abstract. This study is devoted to the possible presence of cirrus clouds in the stratosphere. Three months of lidar data collected in the south of France (44° N) for detection of stratospheric cirrus are carefully analyzed. Most of the cirrus clouds appear to be located in the troposphere below the dynamical tropopause even when the cloud top is close to the thermal tropopause. Ten cirrus cases are found to be unambiguously located above the local dynamical tropopause according to high-resolution PV advection calculations. The highest cloud detected above the local tropopause (nearly 3 km above) is observed inside air masses that originate from the sub-tropical regions and are then transported rapidly to mid-latitudes through isentropic transport. The details of the air mass history is described with a 3-D trajectory model. The back-plumes indicate that the air mass, moist with respect to typical stratospheric air, was transported from the subtropical troposphere to the lowermost stratosphere in 4 days before detection above France. A continuous cooling of 5–10° along the trajectory took place during its transit. This cooling could have been partly responsible for the thin cirrus layer detected.

Formation of ice supersaturation by mesoscale gravity waves

Abstract. We investigate the formation and evolution of an ice-supersaturated region (ISSR) that was detected by means of an operational radiosonde sounding launched from the meteorological station of Lindenberg on 21 March 2000, 00:00 UTC. The supersaturated layer was situated below the local tropopause, between 320 and 408 hPa altitude. Our investigation uses satellite imagery from METEOSAT and the Advanced Very High Resolution Radiometer (AVHRR) and analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF). Mesoscale simulations reveal that the ISSR was formed by a temporary vertical uplift of upper tropospheric air parcels by 20 to 40 hPa in 1 to 2 h. This resulted in a significant local increase of the specific humidity by the moisture transport from below. The ascent was triggered by the superposition of two internal gravity waves, a mountain wave induced by flow past the Erzgebirge and Riesengebirge south of Lindenberg, and an inertial gravity wave excited by the anti-cyclonically curved jet stream over the Baltic Sea. The wave-induced ISSR was rather thick with a depth of about 2 km. The wave-induced upward motion causing the supersaturation also triggered the formation of a cirrus cloud. METEOSAT imagery shows that the cirrus cloud got optically thick within two hours. During this period another longer lasting thin but extended cirrus existed just beneath the tropopause. The wave-induced ISSR disappeared after about half a day in accordance with the decaying wave activity.

Multilinear regression analysis of the 65‐year Tromsø total ozone series

The 65‐year total ozone record from Tromsø, Norway (69.7°N, 18.9°E), has been analyzed using multiple linear regression. Parameters included are modified linear depletion term, Northern Hemisphere teleconnection patterns, local temperature at the 30‐mbar level, stratospheric aerosols, Quasi‐Biennial Oscillation (QBO), and solar activity. Because of the availability of most input parameters, the analysis was limited to the period 1950–2001. All multiple correlation coefficients found are above 0.7, most even larger than 0.8. The series reveals a linear depletion term of about 9% decrease/decade in March/April since the late 1970s, decreasing to few percent in summer. The depletion term is insensitive to the envelope function (linear versus equivalent effective stratospheric chlorine), but highly sensitive to the modification term, forcing the contribution to zero in years with early vortex breakup. Aerosol‐induced depletion is found throughout late winter and spring, while the QBO is of importance in summer. Midstratospheric conditions, parameterized by the 30‐mbar level temperature, are dominant throughout the winter half year from September to February. Teleconnection patterns reveal a complex contribution, with no single pattern being persistently dominant over several months. The Polar‐Eurasian pattern is most prominent in midwinter, the North Atlantic Oscillation pattern in March. In May, the East Atlantic Jet pattern is dominant, while in June the Pacific Transition pattern is the most prominent parameter in the analysis. This variability of teleconnection pattern contributions is probably caused by the changing influence of air masses on the local tropopause altitude, with a dominant influence of North Atlantic and Arctic patterns in winter/spring, and of lower‐latitude patterns in summer and early autumn.

Tropical and extratropical tropospheric air in the lowermost stratosphere over Europe: A CO‐based budget

We present a new approach to calculate a trace gas budget for the lowermost stratosphere based on airborne CO measurements. The measurements were obtained during the SPURT‐project (SPURT in German: Trace gas transport in the tropopause region) and covered the tropopause region over Europe from 35°–75°N during different seasons. Using CO as a tracer for tropospheric air we quantified the contribution of three source regions to the composition of the lowermost stratosphere. Influence from the extratropical troposphere mainly contributes to the mixing layer above the local tropopause, where it decreases rapidly to less than 25% within a potential temperature range of 25 K above the local tropopause. With increasing distance from the tropopause mixing layer, the tropospheric fraction is dominated by influence from the tropics. During winter and spring the latter contributes about 35% to the chemical composition of the lowermost stratosphere over Europe, increasing to about 55% in summer and autumn.

Seasonality and extent of extratropical TST derived from in-situ CO measurements during SPURT

Abstract. We present airborne in-situ trace gas measurements which were performed on eight campaigns between November 2001 and July 2003 during the SPURT-project (SPURenstofftransport in der Tropopausenregion, trace gas transport in the tropopause region). The measurements on a quasi regular basis allowed an overview of the seasonal variations of the trace gas distribution in the tropopause region over Europe from 35°-75°N to investigate the influence of transport and mixing across the extratropical tropopause on the lowermost stratosphere. From the correlation of CO and O3 irreversible mixing of tropospheric air into the lowermost stratosphere is identified. The CO distribution indicates that transport and subsequent mixing of tropospheric air across the extratropical tropopause predominantly affects a layer, which closely follows the shape of the local tropopause. In addition, the seasonal cycle of CO2 illustrates the strong coupling of that layer to the extratropical troposphere. Both, horizontal gradients of CO on isentropes as well as the CO-O3-distribution in the lowermost stratosphere reveal that the influence of quasi-horizontal transport and subsequent mixing weakens with distance from the local tropopause. The mixing layer extends to about 25 K in potential temperature above the local tropopause exhibiting only a weak seasonality. However, at large distances from the tropopause a significant influence of tropospheric air is still evident. The relation between N2O and CO2 indicates that a significant contribution of air originating from the tropical tropopause contributes to the background air in the extratropical lowermost stratosphere.

Warming of the Arctic lower stratosphere by light absorbing particles

Recent measurements of light absorbing particles in the Arctic lower stratosphere show significantly higher mass concentrations of black carbon than were measured in 1992. The difference is primarily a result of measurements with a more quantitative and accurate technique than was previously used. We attribute the large amount of light absorbing material to transport from lower latitude, tropospheric sources rather than increases in aircraft emissions. The calculated heating rate in this aerosol layer, as compared to an atmosphere consisting of only gases, increases by 12% during the winter. This is a result of light absorption by the particles and could perturb the altitude of the local tropopause and affect tropospheric/stratospheric exchange processes.

Rayleigh lidar temperature measurements in the upper troposphere and lower stratosphere

A single-wavelength Rayleigh lidar system has been used to measure the temperatures in the upper troposphere and lower stratosphere in the night in the altitude range from about 8 to 30 km . The temperature derivation is based on an inversion algorithm of the pure Rayleigh backscatter. Calculations include the derivation of the air molecular concentration by an iterative method and the backscattered signals corrected by the background aerosol, which is now found to be low and stable. The uncertainties in estimating the temperature using this method are discussed in detail. The temperature profiles and the tropopause characteristics derived by using the lidar measurements are compared with the radiosonde data. Good agreement is found between these two measurements revealing the potential of this method. The comparison with radiosonde data shows that the lidar measured tropopause temperature is lower by 0.8±1.5 K and the tropopause height is higher by 0.45±0.8 km than the radiosonde measurements. The climatology of local tropopause (24.57°N,121.13°E) is briefly discussed in terms of a double tropopause formation and seasonal variations of the tropopause height and temperature.

Detection of gaseous oxygenated hydrocarbons in upper tropospheric and lower stratospheric aircraft borne experiments

Atmospheric oxygenated hydrocarbons (OHCs), including acetone, were measured at altitudes between 8.5 and 12.5km using an aircraft-based chemical ionization mass spectrometer (CIMS) instrument equipped with an ion trap (IT) mass spectrometer. The IT-CIMS instrument offers a high mass resolution. It also provided a unique opportunity to carry out fragment ion investigations of mass selected ions, which greatly improved our ability to identify trace gases. Furthermore, in the IT, water molecules are “boiled off” from hydrated ions which simplifies core ion identification and greatly reduces the risk of masking of product ions by reagent ions. Acetone could be unambiguously identified in the upper troposphere (UT)/lower stratosphere (LS) by a combination of isotope and fragment ion investigations. A protonated molecule B with mass m=73 could for the first time be detected in the UT/LS by CIMS due to the absence of the otherwise masking reagent ion H+(H2O)4 (m=73). The atmospheric volume mixing ratio (VMR) of the molecule B (m=72) is about 150pptv in the UT and decreases very steeply above the local tropopause indicating that B has a shorter lifetime than acetone. In the UT, the abundance ratio of B and acetone was found to be around 0.3. Additional observed molecules have mass 86 and 100.

An alternative mechanism explaining the hygropause formation in tropical regions

Radiosonde data obtained during CEPEX and TOGA‐COARE experiments show that a very simple mechanism can explain the existence, in tropical regions, of a local hygropause with a very low water vapor mixing ratio, less than 2 ppmv. This mechanism explains also why this hygropause is generally located well above the average tropopause in the lower stratosphere, and why it can temporarily coincide with the local tropopause. This interpretation is based on the existence, during periods of intense convective phenomena, of an uplift of the tropopause inducing a cooling of its temperature. We indeed find in these circumstances, when temperature and water vapor measurements are simultaneously available, very low water vapor values at the same level as the temperature minimum. After the end of the intense convection episode, the minimum temperature level is reestablished at a lower altitude while the hygropause remains positioned in the lower stratosphere but with a low value for the water vapor mixing ratio.

Tracer correlations in the northern high latitude lowermost stratosphere: Influence of cross‐tropopause mass exchange

We present an analysis of trace gas correlations in the lowermost stratosphere. In‐situ aircraft measurements of CO, N2O, NOy and O3, obtained during the STREAM 1997 winter campaign, have been used to investigate the role of cross‐tropopause mass exchange on tracer‐tracer relations. At altitudes several kilometers above the local tropopause, undisturbed stratospheric air was found with NOy/NOy* ratios close to unity, NOy/O3 about 0.003–0.006 and CO mixing ratios as low as 20 ppbv (NOy* is a proxy for total reactive nitrogen derived from NOy–N2O relations measured in the stratosphere). Mixing of tropospheric air into the lowermost stratosphere has been identified by enhanced ratios of NOy/NOy* and NOy/O3, and from scatter plots of CO versus O3. The enhanced NOy/O3 ratio in the lowermost stratospheric mixing zone points to a reduced efficiency of O3 formation from aircraft NOx emissions.

Water vapor measurements from the troposphere to the lowermost stratosphere: Some signatures of troposphere to stratosphere exchanges

In situ water vapor measurements performed in the upper troposphere and the lower stratosphere, from an aircraft, in the North Atlantic region around 50°N, are analyzed. It is shown that the water vapor content observed in the region located slightly above the local tropopause is consistent with the transport from troposphere to stratosphere at middle and high latitudes. It appears that there is a seasonal variation with a higher water vapor content during summer. The source of this variation could be due to the difference in the relative contribution of the troposphere to stratosphere isentropic and convective transport as well as transport from the upper levels of the stratosphere due to the general circulation.