Tropopause Region Research Articles

Stability of the Hydrogen Trioxy Radical via Infrared Action Spectroscopy

The hydrogen trioxy radical (HO3) has been proposed as an intermediate in several important chemical reactions and relaxation processes involving OH in the atmosphere. In this work, the gas-phase infrared action spectrum of HO3 is obtained in the OH overtone region, along with the product state distribution of the OH fragment following dissociation. The highest observed OH product channel sets an upper limit for the HO-O2 binding energy of 6.12 kcal mol(-1). The experimental stability of HO3 and derived equilibrium constant imply that up to 66% of atmospheric OH may be converted into HO3 in the tropopause region.

A set of diagnostics for evaluating chemistry‐climate models in the extratropical tropopause region

Three related diagnostics are used to evaluate the representation of chemical transport processes in the extratropical upper troposphere and lower stratosphere (UTLS) by chemistry‐transport and chemistry‐climate models. The diagnostics are based on in situ observations of ozone, carbon monoxide, water vapor profiles (obtained on board the NASA ER‐2 research aircraft, near 65°N and during 1997), and their interrelationships in the UTLS. The first diagnostic compares the observed and modeled UTLS trace gas profiles in a relative altitude coordinate. The second one compares the observed and modeled UTLS tracer relationships. The third one compares the observed and modeled thickness of the tropopause transition layer. Together, they characterize the model's ability to reproduce the observed chemical distribution in the UTLS region and chemical transition across the extratropical tropopause. These are key indicators of whether the contributions of dynamics and chemistry to this region are correctly represented in the models. These diagnostics are used to evaluate the performance of an NCAR chemistry‐transport model (CTM), MOZART‐3, and a chemistry‐climate model (CCM), WACCM3. Results from four model runs with different meteorological fields and grid resolution are examined. Overall, the NCAR models show qualitative agreement with the observations in the location of the chemical transition across the extratropical tropopause. Quantitatively, there are significant differences between the modeled and the observed chemical distributions. Both the meteorological field and grid resolutions are contributing factors to the differences.

Instantaneous fluctuations of temperature and moisture in the upper troposphere and tropopause region. Part 1: Probability densities and their variability

Using nine years (1995--2003) of MOZAIC temperature and humidity data, we analyse the statistics of instantaneous fluctuations of temperature, relative and absolute humidity with various spatial resolutions. We determine the probability density functions (p.d.f.s), their low order moments up to kurtosis, and study how these quantities vary with spatial resolution and with the background mean relative humidity. Seasonal and geographical variations are considered. Bivariate distributions of joint fluctuations of temperature and relative humidity are presented as well. These investigations are thought to provide an observational basis for the validation of statistical cloud schemes for large--scale models. For instance, we can show that temperature fluctuations cannot be neglected in such a scheme and that the bivariate distributions of simultaneous fluctuations of temperature and humidity have to be taken into account.

Aircraft measurements of asymmetric temperature microstructure causing azimuth variations of VHF radar echo power

VHF wind-profiling radars often measure a decrease of echo power with zenith angle, which can be explained from in situ measurements of horizontal layering or anisotropy of metre-scale temperature structure in the atmosphere. There can also be an azimuthal variation of echo power, which is increased in an azimuth opposite to the vertical shear vector of horizontal wind. This paper checks if the azimuth variation can also be linked to in situ observations of temperature structure, using aircraft flights in the tropopause region near a VHF radar. At heights where VHF radar measures wind shear and aspect sensitivity, there can be an asymmetry in the probability distribution of horizontal gradient of potential temperature, for horizontal scale of e.g. hundreds of metres. The asymmetry is often of opposite sign for up-shear and down-shear flights, and less when VHF echoes are isotropic instead of aspect sensitive. The range of horizontal scales with asymmetry can be used to distinguish e.g. sheared anisotropic turbulence and Kelvin–Helmholtz instability as causes of azimuthal VHF echo power variations.

Accuracy of tropospheric and stratospheric water vapor measurements by the cryogenic frost point hygrometer: Instrumental details and observations

The cryogenic frost point hygrometer (CFH), currently built at the University of Colorado, is a new balloon borne hygrometer, which is capable of continuously measuring water vapor between the surface and the middle stratosphere. The design is loosely based on the old NOAA/CMDL frost point hygrometer, with improved accuracy and a number of significant new features that overcome some limitations of the older instrument. The measurement uncertainty of the new instrument depends on altitude and ranges between less than 4% in the tropical lower troposphere to no more than 10% in the middle stratosphere at 28 km. In the tropopause region the uncertainty is less than 9%. This instrument is used routinely at several sites for validation of satellite measurements and process studies in the upper troposphere and lower stratosphere region. It has proved to be particularly well suited for dehydration observations in the tropical upper troposphere, because the effects of cloud contamination have been significantly reduced. Results of this instrument are compared with the old NOAA/CMDL hygrometer, the Russian Fluorescent Lyman Alpha Stratospheric Hygrometer, the Vaisala RS92, the AURA/MLS satellite instrument, a cloud lidar, the NOAA/CSD frost point hygrometer and the Harvard Lyman‐alpha hygrometer, both of the later instruments flown on board the NASA WB‐57F high‐altitude research aircraft. These comparisons demonstrate the level of accuracy of tropospheric and stratospheric water vapor measurements made by this instrument and point to areas where more research and development are needed.

Carbonaceous material in aerosol particles in the lower stratosphere and tropopause region

The Particle Analysis by Laser Mass Spectrometry (PALMS) instrument has measured the composition of single particles in the lower stratosphere. The average fraction of carbonaceous material in the stratospheric particles decreased rapidly above the tropopause. The decrease in the average carbon content was mostly associated with a sharp increase in the bottom 2 km of the stratosphere in the frequency of fairly pure sulfate particles both with or without meteoric metals. The low potassium content of the fairly pure sulfate particles is used to show that they were formed in the stratosphere and were not tropospheric particles that had lost organics because of oxidation. Of the tropospheric carbonaceous‐sulfate particles found in the stratosphere, the mass spectra had similar patterns from the upper troposphere to the maximum altitude sampled, about 19 km. A reduction in the carbon to sulfate ratio in tropospheric particles was only apparent above about 440 K potential temperature. This implies that carbon compounds can remain for months in particles larger than about 300 nm. Despite the slow rate, these data do not exclude rapid heterogeneous reactions of organics in the particles with OH or other radicals. There was no evidence of significant transfer of semivolatile organics between particles in the stratosphere. However, particles that originated in the stratosphere acquired small amounts of carbon when they were transported to the tropopause.

Tropical cirrus clouds near cold point tropopause under ice supersaturated conditions observed by lidar and balloon‐borne cryogenic frost point hygrometer

Simultaneous vertical profiles of cirrus cloud backscattering and frost point temperature were obtained for the first time in the tropopause region over Bandung, Indonesia, (6.9°S, 107.6°E). These profiles were measured by ground‐based lidar and by balloon‐borne Cryogenic Frost point Hygrometer (CFH) sondes. Supersaturation up to several ten percent was observed by the CFH just below the cold point tropopause at the altitude where a cirrus cloud was observed by lidar. The water vapor mixing ratio decreased slightly at the altitude of the cirrus cloud, suggesting that this decrease was caused by uptake in the cirrus cloud and that the water vapor reduction corresponds to the lower limit of the cloud water content of the observed cirrus cloud. Theoretical calculations of the scattering parameters for the observed cirrus cloud particles and estimations of the time constants for sedimentation and for condensational growth indicate that particles size range is between 4 μm and 30 μm.

Tropical tropopause climatology as observed with radio occultation measurements from CHAMP compared to ECMWF and NCEP analyses

A temperature climatology from radio occultation measurements of the CHAllenging Minisatellite Payload (CHAMP) satellite, comprising five years of measurements from September 2001 to August 2006, was analyzed in the tropical (15°S–15°N) tropopause region. Validation against operational analyses of the European Centre for Medium‐Range Weather Forecasts (ECMWF) showed excellent overall agreement except near the tropical tropopause, where systematic differences generally amount to −1 K to −2 K. Validation against tropopause temperatures from the U.S. National Centers for Environmental Prediction (NCEP) reanalyses showed NCEP deviations of about +2 K to +4 K. The ECMWF deviations can be attributed to the lower vertical resolution and weaker representation of atmospheric wave activity in ECMWF analyses. This evidence is confirmed by improved data from February 2006 onwards, where an enhancement of the ECMWF analyses became effective. Initial inspection of extreme tropical tropopause profiles provided evidence that extremely cold tropopause temperatures can reach −100°C.

An Adaptive Vertical Coordinate Formulation for a Nonhydrostatic Model with Flux-Form Equations

Abstract The concept of adaptive vertical coordinates is used to upgrade conventional terrain-following σ coordinates to arbitrary hybrid coordinates. Compared with previous approaches for implementing adaptive coordinates, the method presented here combines unrestricted applicability to nonhydrostatic models with the capability to integrate the atmospheric equations in flux form. The coordinate is based on a three-dimensional field carrying the vertical position of the coordinate surfaces, which is made time dependent by introducing a prognostic equation. As a specific example, the adaptive coordinate is used to emulate a hybrid isentropic system. Idealized tests in which the coordinate surfaces are artificially moved reveal that the ensuing spurious motions are small enough to be negligible in realistic applications. Mountain wave tests demonstrate that the hybrid coordinate remains numerically stable under strong forcing. However, the model layer distribution established with the hybrid isentropic coordinate is not optimal for representing the dynamics of breaking gravity waves because the vertical distance between the model levels tends to be too large in the wave breaking region. On the other hand, real case studies demonstrate that the hybrid coordinate significantly improves the representation of the tropopause because of enhanced vertical resolution in the tropopause region.

Solar and anthropogenic forcing of tropical hydrology

Holocene climate proxies suggest substantial correlations between tropical meteorology and solar variations, but these have thus far not been explained. Using a coupled ocean‐atmosphere‐composition model forced by sustained multi‐decadal irradiance increases, we show that greater tropical temperatures alter the hydrologic cycle, enhancing the climatological precipitation maxima in the tropics while drying the subtropical subsidence regions. The shift is enhanced by tropopause region ozone increases, and the model captures the pattern inferred from paleoclimate records. The physical process we describe likely affected past civilizations, including the Maya, Moche, and Ancestral Puebloans who experienced drought coincident with increased irradiance during the late medieval (∼900–1250). Similarly, decreased irradiance may have affected cultures via a weakened monsoon during the Little Ice Age (∼1400–1750). Projections of 21st‐century climate change yield hydrologic cycle changes via similar processes, suggesting a strong likelihood of increased subtropical drought as climate warms.

Inertia gravity waves in the upper troposphere during the MaCWAVE winter campaign – Part II: Radar investigations and modelling studies

Abstract. We present an experimental and modelling study of a strong gravity wave event in the upper troposphere/lower stratosphere near the Scandinavian mountain ridge. Continuous VHF radar measurements during the MaCWAVE rocket and ground-based measurement campaign were performed at the Norwegian Andoya Rocket Range (ARR) near Andenes (69.3° N, 16° E) in January 2003. Detailed gravity wave investigations based on PSU/NCAR Fifth-Generation Mesoscale Model (MM5) data have been used for comparison with experimentally obtained results. The model data show the presence of a mountain wave and of an inertia gravity wave generated by a jet streak near the tropopause region. Temporal and spatial dependencies of jet induced inertia gravity waves with dominant observed periods of about 13 h and vertical wavelengths of ~4.5–5 km are investigated with wavelet transform applied on radar measurements and model data. The jet induced wave packet is observed to move upstream and downward in the upper troposphere. The model data agree with the experimentally obtained results fairly well. Possible reasons for the observed differences, e.g. in the time of maximum of the wave activity, are discussed. Finally, the vertical fluxes of horizontal momentum are estimated with different methods and provide similar amplitudes. We found indications that the derived positive vertical flux of the horizontal momentum corresponds to the obtained parameters of the jet-induced inertia gravity wave, but only at the periods and heights of the strongest wave activity.

Impact of H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O coating and ice crystal size on radiative properties of sub-visible cirrus

Abstract. Recent laboratory experiments showed that at conditions resembling those near the tropopause region, small ice particles can be coated by a liquid H2SO4/H2O over-layer formed after the freezing of diluted sulfuric acid/water aerosol drops. Here, idealized radiative transfer tests are conducted to evaluate the impact that such an over-layer would have on the radiative effects produced by sub-visible cirrus clouds (SVCs). Spherical particle shape is assumed to keep the problem tractable. The calculations show that the over-layer increases both the shortwave (SW) and longwave (LW) cloud radiative effects (CRE), but the impact is small: ~0.02 W m−2, or even less, for the total (LW+SW) CRE at the top of the atmosphere. For the smallest ice particles, for which the over-layer is thickest, the fractional change in CRE can, however, reach ~20% for the SW CRE and over 50% for the LW CRE. The dependence of LW and SW CRE on particle size is also studied in the paper. Calculations for spherical and spheroidal uncoated ice particles show that even for high, optically thin cirrus, the total CRE can be negative, if the diameter of the particles is smaller than about 3–4 μm. Apart from the SVCs, this result could be relevant for contrail cirrus clouds, which are believed to consist of large numbers of very small ice particles.

Balloon-borne observations of high aerosol concentrations near the summertime tropopause over the Tibetan Plateau

Vertical profiles of aerosols (radii ≥ 0.15 μm) were measured using a balloon-borne optical particle counter at Lhasa (29.4°N, 91.1°E, 3650 m a.s.l.), China in 1999. The measurements display occurrences of relatively high number concentrations (0.7–0.8 particles cm − 3 ) of sub-micron size aerosols with radii = 0.15–0.6 μm near the tropopause region (between about 130 and 70 hPa) in the Asian summer monsoon period. Size distributions of the observed aerosols are clearly different from those of cirrus clouds. The high number concentrations occurred inside the Tibetan anticyclone, suggesting that these were triggered by deep convection over the Tibetan Plateau. Convective transportation of lower tropospheric materials may affect atmospheric constituents near the tropopause region. In addition, the effects of adiabatic cooling and hydration associated with deep convection may also trigger enlargement of pre-existing and/or vertically-transported aqueous solution droplets (e.g., liquid sulfate particles) near the tropopause region. Enhancement of the aerosols should be considered in assessing the global/regional climate system and the geochemical cycle.

Simulations of preindustrial, present-day, and 2100 conditions in the NASA GISS composition and climate model G-PUCCINI

Abstract. A model of atmospheric composition and climate has been developed at the NASA Goddard Institute for Space Studies (GISS) that includes composition seamlessly from the surface to the lower mesosphere. The model is able to capture many features of the observed magnitude, distribution, and seasonal cycle of trace species. The simulation is especially realistic in the troposphere. In the stratosphere, high latitude regions show substantial biases during period when transport governs the distribution as meridional mixing is too rapid in this model version. In other regions, including the extrapolar tropopause region that dominates radiative forcing (RF) by ozone, stratospheric gases are generally well-simulated. The model's stratosphere-troposphere exchange (STE) agrees well with values inferred from observations for both the global mean flux and the ratio of Northern (NH) to Southern Hemisphere (SH) downward fluxes. Simulations of preindustrial (PI) to present-day (PD) changes show tropospheric ozone burden increases of 11% while the stratospheric burden decreases by 18%. The resulting tropopause RF values are −0.06 W/m2 from stratospheric ozone and 0.40 W/m2 from tropospheric ozone. Global mean mass-weighted OH decreases by 16% from the PI to the PD. STE of ozone also decreased substantially during this time, by 14%. Comparison of the PD with a simulation using 1979 pre-ozone hole conditions for the stratosphere shows a much larger downward flux of ozone into the troposphere in 1979, resulting in a substantially greater tropospheric ozone burden than that seen in the PD run. This implies that reduced STE due to stratospheric ozone depletion may have offset as much as 2/3 of the tropospheric ozone burden increase from PI to PD. However, the model overestimates the downward flux of ozone at high Southern latitudes, so this estimate is likely an upper limit. In the future, the tropospheric ozone burden increases by 101% in 2100 for the A2 scenario including both emissions and climate changes. The primary reason is enhanced STE, which increases by 124% (168% in the SH extratropics, and 114% in the NH extratropics). Climate plays a minimal role in the SH increases, but contributes 38% in the NH. Chemistry and dry deposition both change so as to reduce tropospheric ozone, partially in compensation for the enhanced STE, but the increased ozone influx dominates the burden changes. The net RF due to projected ozone changes is 0.8 W/m2 for A2. The influence of climate change alone is −0.2 W/m2, making it a substantial contributor to the net RF. The tropospheric oxidation capacity increases seven percent in the full A2 simulation, and 36% due to A2 climate change alone.

Dehydration in the northern hemisphere mid-latitude tropopause region observed during STREAM 1998

Measurements in the vicinity of the polar jet stream during the STREAM 1998 campaign in Timmins, Canada, show that during the flight on 15 July, a deep intrusion of stratospheric air into the tropo ...

Design and Calibration of a Multi-Channel Aerosol Sampler for Tropopause Region Studies from the CARIBIC Platform

An aircraft-based, multi-channel aerosol sampler for studies of the upper troposphere and lowermost stratosphere from the CARIBIC (Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container) platform was designed and calibrated. The sampler operates with an impaction technique at a flow rate of 10.4 lpm and consists of sixteen sampling channels. Samples are collected in a time sequence. Each channel contains two sample types that are used for quantitative measurement of concentrations, using particle-induced X-ray emission (PIXE), and single particle analysis with electron microscopy. The minimum detection limits for PIXE analysis after 1.5 h sampling are, for example, 2.0, 0.14, and 0.02 ng/m3 STP (standard temperature and pressure) for sulfur, potassium, and nickel. Calibration included penetration studies of a cyclone arrangement used to define the upper size limit in the sampling to 2.0 μ m diameter and the collection efficiency of the impactor. Both components of the sampling system showed penetration and collection efficiency close to 100%, respectively, in the particle size range of interest. The impactor cut-off was found to be dependent on the ratio of the impactor upstream-to-downstream pressure for ratios well below the critical pressure drop (i.e., the pressure where the jet reaches sonic velocity) being 0.15 μ m and 0.08 μ m for ratios 0.41 and 0.2.

Impact of aircraft NO<sub>x</sub> emissions on the atmosphere – tradeoffs to reduce the impact

Abstract. Within the EU-project TRADEOFF, the impact of NOx (=NO+NO2) emissions from subsonic aviation upon the chemical composition of the atmosphere has been calculated with focus on changes in reactive nitrogen and ozone. We apply a 3-D chemical transport model that includes comprehensive chemistry for both the troposphere and the stratosphere and uses various aircraft emission scenarios developed during TRADEOFF for the year 2000. The environmental effects of enhanced air traffic along polar routes and of possible changes in cruising altitude are investigated, taking into account effects of flight route changes on fuel consumption and emissions. In a reference case including both civil and military aircraft the model predicts aircraft-induced maximum increases of zonal-mean NOy (=total reactive nitrogen) between 156 pptv (August) and 322 pptv (May) in the tropopause region of the Northern Hemisphere. Resulting maximum increases in zonal-mean ozone vary between 3.1 ppbv in September and 7.7 ppbv in June. Enhanced use of polar routes implies substantially larger zonal-mean ozone increases in high Northern latitudes during summer, while the effect is negligible in winter. Lowering the flight altitude leads to smaller ozone increases in the lower stratosphere and upper troposphere, and to larger ozone increases at altitudes below. Regarding total ozone change, the degree of cancellation between these two effects depends on latitude and season, but annually and globally averaged the contribution from higher altitudes dominates, mainly due to washout of NOy in the troposphere, which weakens the tropospheric increase. Raising flight altitudes increases the ozone burden both in the troposphere and the lower stratosphere, primarily due to a more efficient accumulation of pollutants in the stratosphere.

Measurements of NO, NO<sub>y</sub>, N<sub>2</sub>O, and O<sub>3</sub> 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.

Fine‐scale structure of the extratropical tropopause region

A vertically high resolved climatology of the thermal and wind structure of the extratropical tropopause region is presented. The climatology is based on data from 80 U.S. radiosonde stations covering the period 1998–2002. Time averages for each radiosonde station are computed using the tropopause as a common reference level for all vertical profiles within the mean. A strong inversion at the tropopause in the mean vertical temperature gradient is uncovered; that is, temperature strongly increases with altitude within the lowermost stratosphere. This tropopause inversion layer exists on average throughout the investigated extratropics (about 30°N to 70°N). Accordingly, the static stability parameter shows considerably enhanced values within the lowermost extratropical stratosphere compared to typical extratropical stratospheric values further aloft. Conventional averages are not able to capture the tropopause inversion layer. Mean profiles of the horizontal wind show behavior qualitatively corresponding to thermal wind balance. Winter and summer exhibit distinctly different climate states in the extratropical tropopause region. An approximated potential vorticity is considered and found to be close to well mixed within the troposphere as well as within the tropopause inversion layer. This suggests the view of the tropopause inversion layer as representing a dynamically active atmospheric layer. Some potential implications are discussed.

Highly resolved observations of trace gases in the lowermost stratosphere and upper troposphere from the Spurt project: an overview

Abstract. During SPURT (Spurenstofftransport in der Tropopausenregion, trace gas transport in the tropopause region) we performed measurements of a wide range of trace gases with different lifetimes and sink/source characteristics in the northern hemispheric upper troposphere (UT) and lowermost stratosphere (LMS). A large number of in-situ instruments were deployed on board a Learjet 35A, flying at altitudes up to 13.7 km, at times reaching to nearly 380 K potential temperature. Eight measurement campaigns (consisting of a total of 36 flights), distributed over all seasons and typically covering latitudes between 35° N and 75° N in the European longitude sector (10° W–20° E), were performed. Here we present an overview of the project, describing the instrumentation, the encountered meteorological situations during the campaigns and the data set available from SPURT. Measurements were obtained for N2O, CH4, CO, CO2, CFC12, H2, SF6, NO, NOy, O3 and H2O. We illustrate the strength of this new data set by showing mean distributions of the mixing ratios of selected trace gases, using a potential temperature-equivalent latitude coordinate system. The observations reveal that the LMS is most stratospheric in character during spring, with the highest mixing ratios of O3 and NOy and the lowest mixing ratios of N2O and SF6. The lowest mixing ratios of NOy and O3 are observed during autumn, together with the highest mixing ratios of N2O and SF6 indicating a strong tropospheric influence. For H2O, however, the maximum concentrations in the LMS are found during summer, suggesting unique (temperature- and convection-controlled) conditions for this molecule during transport across the tropopause. The SPURT data set is presently the most accurate and complete data set for many trace species in the LMS, and its main value is the simultaneous measurement of a suite of trace gases having different lifetimes and physical-chemical histories. It is thus very well suited for studies of atmospheric transport, for model validation, and for investigations of seasonal changes in the UT/LMS, as demonstrated in accompanying and elsewhere published studies.