Showing papers on "Tropopause published in 2004"

The Life Cycle of the Northern Hemisphere Sudden Stratospheric Warmings

Abstract: Motivated by recent evidence of strong stratospheric‐tropospheric coupling during the Northern Hemisphere winter, this study examines the evolution of the atmospheric flow and wave fluxes at levels throughout the stratosphere and troposphere during the composite life cycle of a sudden stratospheric warming. The composite comprises 39 major and minor warming events using 44 years of NCEP‐NCAR reanalysis data. The incipient stage of the life cycle is characterized by preconditioning of the stratospheric zonal flow and anomalous, quasistationary wavenumber-1 forcing in both the stratosphere and troposphere. As the life cycle intensifies, planetary wave driving gives rise to weakening of the stratospheric polar vortex and downward propagation of the attendant easterly wind and positive temperature anomalies. When these anomalies reach the tropopause, the life cycle is marked by momentum flux and mean meridional circulation anomalies at tropospheric levels that are consistent with the negative phase of the Northern Hemisphere annular mode. The anomalous momentum fluxes are largest over the Atlantic half of the hemisphere and are associated primarily with waves of wavenumber 3 and higher.

Upward Wave Activity Flux as a Precursor to Extreme Stratospheric Events and Subsequent Anomalous Surface Weather Regimes

Abstract: It has recently been shown that extreme stratospheric events (ESEs) are followed by surface weather anomalies (for up to 60 days), suggesting that stratospheric variability might be used to extend weather prediction beyond current time scales. In this paper, attention is drawn away from the stratosphere to demonstrate that the originating point of ESEs is located in the troposphere. First, it is shown that anomalously strong eddy heat fluxes at 100 hPa nearly always precede weak vortex events, and conversely, anomalously weak eddy heat fluxes precede strong vortex events, consistent with wave–mean flow interaction theory. This finding clarifies the dynamical nature of ESEs and suggests that a major source of stratospheric variability (and thus predictability) is located in the troposphere below and not in the stratosphere itself. Second, it is shown that the daily time series of eddy heat flux found at 100 hPa and integrated over the prior 40 days, exhibit a remarkably high anticorrelation (−0.8)...

Interactive chemistry in the Laboratoire de Météorologie Dynamique general circulation model: Description and background tropospheric chemistry evaluation

Abstract: We provide a description and evaluation of LMDz-INCA, which couples the Laboratoire de Meteorologie Dynamique general circulation model (LMDz) and the Interaction with Chemistry and Aerosols (INCA) model. In this first version of the model a CH 4 ANO x ACOAO 3 chemical scheme representative of the background chemistry of the troposphere is considered. We derive rapid interhemispheric exchange times of 1.13-1.38 years and 0.70-0.82 years, based on surface and pressure-weighted mixing ratios of inert tracers, respectively. The general patterns of the nitrogen deposition are correctly reproduced by the model. However, scavenging processes remain a major source of uncertainty in current models, with convective precipitation playing a key role in the global distribution of soluble species. The global and annual mean methane (7.9 years) and methylchloroform (4.6 years) chemical lifetimes suggest that OH is too high by about 19-25% in the model. This disagreement with previous estimates is attributed to the missing nonmethane hydrocarbons in this version of the model. The model simulates quite satisfactorily the distribution and seasonal cycle of CO at most stations. At several tropical sites and in the Northern Hemisphere during summer, the OH overestimate leads, however, to a too intense CO chemical destruction. LMDz-INCA reproduces fairly well the distribution of ozone throughout most of the troposphere. A main disagreement appears in the Northern Hemisphere upper troposphere during summer, due to a too high tropopause in the GCM. When the GCM winds are relaxed toward assimilated meteorology, a much higher variability is obtained for ozone in the upper troposphere, reflecting more frequent stratospheric intrusions. The stratospheric influx of ozone increases from 523 Tg/yr in the base case simulation to 783 Tg/yr in the nudged version.

CHAMP and SAC-C atmospheric occultation results and intercomparisons

Abstract: [1] The German Challenging Minisatellite Payload (CHAMP) and Argentine Satelite de Aplicaciones Cientificas-C (SAC-C) Earth science missions, launched in 2000, carry a new generation of Global Positioning System (GPS) receivers for radio occultation sounding of the ionosphere and neutral atmosphere. Though the occultation concept for obtaining profiles of atmospheric temperature, pressure, and moisture was proven in 1995 with GPS/MET, concurrent measurements from CHAMP and SAC-C present the first opportunity for a preliminary evaluation of three central claims: (1) GPS soundings are effectively free of instrumental bias and drift; (2) individual temperature profiles are accurate to <0.5 K between ∼5 and 20 km; and (3) averaged profiles for climate studies can be accurate to <0.1 K. These properties imply that a weak climate trend can be monitored and detected in less than a decade and studied by different instruments at different times with no external calibration. While this detection cannot by itself tell us the source of the climate change, whether natural and anthropogenic, this detection is a prerequisite to answer the more difficult problem of understanding the cause of change. In this paper, these three claims are evaluated by comparing nearby CHAMP and SAC-C profiles. Of nearly 130,000 profiles examined, 212 pairs occurring within 30 min and 200 km of one another were found. Profile pairs agree to <0.86 K (68% confidence interval) and to within 0.1 K in the mean between 5 and 15 km altitude, after removing the expected variability of the atmosphere. If the errors in CHAMP and SAC-C are assumed to be uncorrelated, this implies that individual profiles are precise to <0.6 K between 5 and 15 km. Individual comparisons show closest agreement near the tropopause and display finer resolution than and substantially different temperatures from numerical weather model analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF). Comparisons between CHAMP and SAC-C largely indicate precision; however, several features observed in common, especially near the tropopause, tend also to indicate accuracy. Limitations of previous experiments (e.g., GPS/MET) in probing the lower troposphere have significantly improved with CHAMP and SAC-C, with the majority of profiles (60%) descending to the lowest 0.5 km. This is expected to increase to 90–95% with future system improvements. However, the N-bias problem encountered in GPS/MET is also present in CHAMP and SAC-C, and it is expected to be much reduced once open loop tracking is implemented. Examples are selected to illustrate lower tropospheric sensing, including detection of the planetary boundary layer height. For the first time, such performance is achieved with GPS Antispoofing encryption on. Daily occultations currently number ∼350–400; this is expected to reach over 1000 in the near future, rivaling the number of semidaily radiosonde launches. With several new missions in planning, this may increase tenfold in the next 3–8 years, making GPS sounding a potentially significant input to numerical weather prediction and climate research.

Transport and freeze‐drying in the tropical tropopause layer

Abstract: [1] We use a Lagrangian, one-dimensional cloud model to simulate ice cloud formation and dehydration along trajectories in the tropical tropopause layer (TTL). Time-height curtains of temperature along the trajectory paths are extracted from meteorological analyses. The temperatures are adjusted near the tropopause such that the spatial average cold point temperature matches tropical radiosonde measurements. Temperature perturbations due to Kelvin waves, Rossby gravity waves, and high-frequency gravity waves are superimposed. The cloud model tracks the growth and sedimentation of individual ice crystals. Ice number densities in the cloud simulations without waves range from 1 cm−3) and smaller crystals (1–10 μm radius), resulting in less sedimentation but still effective dehydration overall. Inclusion of wave-driven temperature oscillations decreases the final TTL H2O mixing ratio somewhat primarily because the wave perturbations decrease the tropical average cold point tropopause temperature by ∼0.75 K. Ultimately, air rising through the TTL is effectively dehydrated with or without wave perturbations. In general, the model results suggest that the final water vapor mixing ratios are primarily controlled by the minimum temperatures encountered by parcels and that they are relatively insensitive to factors such as the wave-driven temperature variability, the supersaturation threshold for ice nucleation, and the rate of ascent through the tropopause layer. However, the frequency and geographical distribution of cloud formation is very sensitive to these parameters. On average, the clouds dehydrate the air along trajectories down to mixing ratios ∼10–40% higher than the temperature minimum saturation mixing ratio. The simulations predict efficient freeze-drying of air by cloud formation within the TTL: For the December–January 1995/1996 period simulated the average final H2O mixing ratios at the tropopause (370–380 K potential temperature) range from 2.5 to 3.2 ppmv. These values are somewhat lower than the estimates of the stratospheric water vapor entry value from satellite and in situ measurements; hence an additional source of water (such as injection by deep convection) may be required to explain the observed tropical tropopause humidity.

Interannual changes of stratospheric water vapor and correlations with tropical tropopause temperatures

Abstract: Interannual variations of stratospheric water vapor over 1992‐2003 are studied using Halogen Occultation Experiment (HALOE) satellite measurements. Interannual anomalies in water vapor with an approximate 2-yr periodicity are evident near the tropical tropopause, and these propagate vertically and latitudinally with the mean stratospheric transport circulation (in a manner analogous to the seasonal ‘‘tape recorder’’). Unusually low water vapor anomalies are observed in the lower stratosphere for 2001‐03. These interannual anomalies are also observed in Arctic lower-stratospheric water vapor measurements by the Polar Ozone and Aerosol Measurement (POAM) satellite instrument during 1998‐2003. Comparisons of the HALOE data with balloon measurements of lower-stratospheric water vapor at Boulder, Colorado (408N), show partial agreement for seasonal and interannual changes during 1992‐2002, but decadal increases observed in the balloon measurements for this period are not observed in HALOE data. Interannual changes in HALOE water vapor are well correlated with anomalies in tropical tropopause temperatures. The approximate 2-yr periodicity is attributable to tropopause

Definitions and sharpness of the extratropical tropopause: A trace gas perspective

Abstract: [1] Definitions of the extratropical tropopause are examined from the perspective of chemical composition. Fine-scale measurements of temperature, ozone, carbon monoxide, and water vapor from approximately 70 aircraft flights, with ascending and descending tropopause crossings near 40°N and 65°N, are used in this analysis. Using the relationship of the stratospheric tracer O3 and the tropospheric tracer CO, we address the issues of tropopause sharpness and where the transitions from troposphere to stratosphere occur in terms of the chemical composition. Tracer relationships indicate that mixing of stratospheric and tropospheric air masses occurs in the vicinity of the tropopause to form a transition layer. Statistically, this transition layer is centered on the thermal tropopause. Furthermore, we show that the transition is much sharper near 65°N (a region away from the subtropical jet) but spans a larger altitude range near 40°N (in the vicinity of the subtropical jet). This latter feature is consistent with enhanced stratosphere-troposphere exchange and mixing activity near the tropopause break.

Perspicacious indicators of atmospheric blocking

Abstract: [1] A novel dynamically-based approach is introduced to identify, describe and diagnose atmospheric blocking events. The approach is based upon the potential vorticity perspective and takes into account the three-dimensional structure of the phenomenon. It is argued that the essence of a blocking anomaly is located in the upper troposphere, just below the tropopause. The associated novel blocking indicators are derived from two-dimensional fields at 6-hourly temporal resolution, and provide information on the spatial scale, shape, amplitude and movement of blocks. A northern hemisphere winter (DJF) climatology for the ERA15 period (1979–1993) is presented and comments are made on the relationship between the indicators and previous blocking indices.

Tropical troposphere‐to‐stratosphere transport inferred from trajectory calculations

Abstract: [1] We present an analysis of trajectory calculations in the tropical tropopause layer (TTL) based on European Centre for Medium-Range Weather Forecasts (ECMWF) analysis wind and temperature fields. Over 500,000 forward and backward trajectories were calculated for January/February and July/August 2001. We analyze the pathways between 340 K and 400 K potential temperature (θ) of those trajectories involved in troposphere-to-stratosphere transport (TST). Even though trajectory calculations in this region may suffer from deficiencies in the underlying vertical wind field, they incorporate not only slow radiative ascent but also effects of deep convection, zonal and meridional transport, and their regional variability. From the trajectory calculations we derive a mean residence time of air parcels in the TTL, which shows a maximum at θ ≈360 K of ∼13 days for a change in potential temperature of ±10 K. The analysis of trajectory pathways reveals that approximately 80% of the trajectories ascending into the stratosphere enter the TTL over the western Pacific. Upon further ascent, they typically travel ∼5000–10,000 km before they arrive at the location where they assume minimum water mixing ratios. These pathways show regional and seasonal patterns and are largely controlled by the upper level circulation of the Asian-Australian monsoon, the northern hemispherical subtropical jet and the equatorial easterly jet from South Asia to Africa. As a consequence of the interplay of these meteorological systems, about 70% of TST trajectories assume their minimum water mixing ratio over the western Pacific, which shows also a global minimum in tropopause temperatures. Average water mixing ratios of air after TST are χH2O = 1.6 ppmv for January/February and χH2O = 3.6 ppmv for July/August 2001. Mixing of stratospherically young air, which just underwent TST, with older air masses entering the lower tropical stratosphere sideways yields an estimate of χH2O = 2.3 ppmv for January/February and χH2O = 3.7 ppmv for July/August for air at θ = 400 K, which compares favorably with satellite observations. Our analysis emphasizes the importance of particular pathways for tropical TST, with the western Pacific being the dominant source of stratospheric air in general and being the place, in particular, where ∼70% of tropical TST assumes its final water mixing ratio.

Impact of monsoon circulations on the upper troposphere and lower stratosphere

Abstract: [1] Observations from in situ aircraft and output from a global chemical transport model are used to better understand the impact of Northern Hemisphere summer monsoon circulations on stratosphere-troposphere exchange. A long-term model climatology resembles the satellite climatology of water vapor and ozone in the monsoon regions. A simulation with observed winds is able to reproduce individual transport events observed from aircraft, and these events are used to infer that large-scale motions and monsoon circulations can explain the global correlations of ozone and water vapor around the tropopause. A detailed analysis of model fluxes of water vapor and ozone indicates that the Asian monsoon circulation may contribute 75% of the total net upward water vapor flux in the tropics at tropopause levels from July to September. Some of this air may enter the tropical stratosphere and bypass the tropical tropopause altogether.

The tropopause and the thermal stratification in the extratropics of a dry atmosphere

Abstract: A dynamical constraint on the extratropical tropopause height and thermal stratification is derived by considerations of entropy fluxes, or isentropic mass fluxes, and their different magnitudes in the troposphere and stratosphere. The dynamical constraint is based on a relation between isentropic mass fluxes and eddy fluxes of potential vorticity and surface potential temperature and on diffusive eddy flux closures. It takes baroclinic eddy fluxes as central for determining the extratropical tropopause height and thermal stratification and relates the tropopause potential temperature approximately linearly to the surface potential temperature and its gradient. Simulations with an idealized GCM point to the possibility of an extratropical climate in which baroclinic eddy fluxes maintain a statically stable thermal stratification and, in interaction with large-scale diabatic processes, lead to the formation of a sharp tropopause. The simulations show that the extratropical tropopause height and thermal stratification are set locally by extratropical processes and do not depend on tropical processes and that, across a wide range of atmospheric circulations, the dynamical constraint describes the relation between tropopause and surface potential temperatures well. An analysis of observational data shows that the dynamical constraint, derived for an idealized dry atmosphere, can account for interannual variations of the tropopause height and thermal stratification in the extratropics of the earth's atmosphere. The dynamical constraint implies that if baroclinic eddies determine the tropopause height and thermal stratification, an atmosphere organizes itself into a state in which nonlinear interactions among eddies are inhibited. The inhibition of nonlinear eddy–eddy interactions offers an explanation for the historic successes of linear and weakly nonlinear models of large-scale extratropical dynamics.

Radiation balance of the tropical tropopause layer

Abstract: [1] The radiation balance of the tropical tropopause layer (TTL) is examined using several different radiation codes with standard profiles compiled from observations in the tropics assuming clear sky conditions. These codes include detailed radiative transfer models and simplified codes for global climate models. The importance of the various radiatively active gases are examined. Water vapor is the most important contributor to the TTL radiation balance, but carbon dioxide and ozone also play a role. Differences in radiative heating between radiation models are mostly due to treatments of shortwave radiation. Differences between models below the TTL are due to different treatments of water vapor continuum absorption. The level of zero clear sky radiative heating, a level important for understanding the transport of air into the stratosphere, is generally found near 15 km, 125 hPa and 200°K (360 K potential temperature), consistent with previous work. Changes in time and space can modify this level by ±500 m, and individual profiles vary from these averages by ±400 m (1σ). Increases in water vapor in the TTL would tend to increase the altitude of the level at which the net heating is zero, while increases in carbon dioxide or ozone would tend to decrease this level. Clouds in the TTL tend to increase the level due to enhancements in longwave cooling above clouds. The implications for transport are discussed.

Seasonal variation of methane, water vapor, and nitrogen oxides near the tropopause: Satellite observations and model simulations

Abstract: [1] Seasonal variations of several trace constituents near the tropopause are analyzed based on satellite measurements, and results are compared to a recent numerical model simulation. We examine methane, water vapor, and nitrogen oxides (NOx) derived from Halogen Occultation Experiment (HALOE) satellite observations; these species have strong gradients near the tropopause, so that their seasonality is indicative of stratosphere-troposphere exchange (STE) and circulation in the near-tropopause region. Model results are from the Model for Ozone and Related Chemical Tracers (MOZART) stratosphere-troposphere chemical transport model (CTM). Results show overall good agreement between observations and model simulations for methane and water vapor, whereas nitrogen oxides near the tropopause are much lower in the model than suggested by HALOE data. The latter difference is probably related to the lightning and convective parameterizations incorporated in MOZART, which produce NOx maxima not near the tropopause, but in the upper troposphere. Constituent seasonal variations highlight the imporatance of the Northern Hemisphere (NH) summer monsoons as regions for transport into the lowermost stratosphere. In MOZART, there is clear evidence that air from the monsoon region is transported into the tropics and entrained into the upward Brewer-Dobson circulation, bypassing the tropical tropopause.

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 O 3 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 CO 2 illustrates the strong coupling of that layer to the extratropical troposphere. Both, horizontal gradients of CO on isentropes as well as the CO-O 3 -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 N 2 O and CO 2 indicates that a significant contribution of air originating from the tropical tropopause contributes to the background air in the extratropical lowermost stratosphere.

Identification of anthropogenic climate change using a second-generation reanalysis

Abstract: [1] Changes in the height of the tropopause provide a sensitive indicator of human effects on climate. A previous attempt to identify human effects on tropopause height relied on information from ‘first-generation’ reanalyses of past weather observations. Climate data from these initial model-based reanalyses have well-documented deficiencies, raising concerns regarding the robustness of earlier detection work that employed these data. Here we address these concerns using information from the new second-generation ERA-40 reanalysis. Over 1979 to 2001, tropopause height increases by nearly 200 m in ERA-40, partly due to tropospheric warming. The spatial pattern of height increase is consistent with climate model predictions of the expected response to anthropogenic influences alone, significantly strengthening earlier detection results. Atmospheric temperature changes in two different satellite data sets are more highly correlated with changes in ERA-40 than with those in a first-generation reanalysis, illustrating the improved quality of temperature information in ERA-40. Our results provide support for claims that human activities have warmed the troposphere and cooled the lower stratosphere over the last several decades of the 20th century, and that both of these changes in atmospheric temperature have contributed to an overall increase in tropopause height. INDEX TERMS: 0350 Atmospheric Composition and Structure: Pressure, density, and temperature; 0370 Atmospheric Composition and Structure: Volcanic effects (8409); 1620 Global Change: Climate dynamics (3309); 1640 Global Change: Remote sensing; KEYWORDS: climate change, detection, reanalysis

In‐situ observations of mid‐latitude forest fire plumes deep in the stratosphere

Abstract: We observed a plume of air highly enriched in carbon monoxide and particles in the stratosphere at altitudes up to 15.8 km. It can be unambiguously attributed to North American forest fires. This plume demonstrates an extratropical direct transport path from the planetary boundary layer several kilometers deep into the stratosphere, which is not fully captured by large-scale atmospheric transport models. This process indicates that the stratospheric ozone layer could be sensitive to changes in forest burning associated with climatic warming.

Geographical distribution and interseasonal variability of tropical deep convection: UARS MLS observations and analyses

Abstract: Tropical deep convection and its dynamical effect on the tropopause and stratosphere are investigated using a suite of data from the Upper Atmospheric Research Satellite (UARS) Microwave Limb Sounder (MLS), including upper tropospheric humidity, cloud radiance, and gravity wave measurements.

Ice supersaturations exceeding 100% at the cold tropical tropopause: implications for cirrus formation and dehydration

Abstract: Recent in situ measurements at tropical tropopause temperatures as low as 187 K indicate supersaturations with respect to ice exceeding 100% with little or no ice present. In contrast, models used to simulate cloud formation near the tropopause assume a supersaturation threshold for ice nucleation of about 65% based on laboratory measurements of aqueous aerosol freezing. The high supersaturations reported here, along with cloud simulations assuming a plausible range of temperature histories in the sampled air mass, indicate that the vast majority of aerosols in the air sampled on this flight must have had supersaturation thresholds for ice nucleation exceeding 100% (i.e. near liquid water saturation at these temperatures). Possible explanations for this high threshold are that (1) the expressions used for calculating vapor pressure over supercooled water at low temperatures give values are at least 20% too low, (2) organic films on the aerosol surfaces reduce their accommodation coefficient for uptake of water, resulting in aerosols with more concentrated solutions when moderate-rapid cooling occurs and correspondingly inhibited homogeneous freezing, and (3) if surface freezing dominates, organic coatings may increase the surface energy of the ice embryo/vapor interface resulting in suppressed ice nucleation. Simulations of in situ cloud formation in the tropical tropopause layer (TTL) throughout the tropics indicate that if decreased accommodation coefficients and resulting high thresholds for ice nucleation prevailed throughout the tropics, then the calculated occurrence frequency and areal coverage of TTL cirrus would be significantly suppressed. However, the simulations also show that even if in situ TTL cirrus form only over a very small fraction of the tropics in the western Pacific, enough air passes through them due to rapid horizontal transport such that they can still effectively freeze-dry air entering the stratosphere. The TTL cirrus simulations show that even if very large supersaturations are required for ice nucleation, these large supersaturations should occur very rarely.

IMPACT, the LLNL 3‐D global atmospheric chemical transport model for the combined troposphere and stratosphere: Model description and analysis of ozone and other trace gases

Abstract: [1] We present a global chemical transport model called the Integrated Massively Parallel Atmospheric Chemical Transport (IMPACT) model. This model treats chemical and physical processes in the troposphere, the stratosphere, and the climatically critical tropopause region, allowing for physically based simulations of past, present, and future ozone and its precursors. The model is driven by meteorological fields from general circulation models (GCMs) or assimilated fields representing particular time periods. It includes anthropogenic and natural emissions, advective and convective transport, vertical diffusion, dry deposition, wet scavenging, and photochemistry. Simulations presented here use meteorological fields from the National Center for Atmospheric Research (NCAR) Middle Atmospheric Community Climate Model, Version 3 (MACCM3). IMPACT simulations of radon/lead are compared to observed vertical profiles and seasonal cycles. IMPACT results for a full chemistry simulation, with approximately 100 chemical species and 300 reactions representative of a mid-1990s atmosphere, are presented. The results are compared with surface, satellite, and ozonesonde observations. The model calculates a total annual flux from the stratosphere of 663 Tg O3/year, and a net in situ tropospheric photochemical source (that is, production minus loss) of 161 Tg O3/year, with 826 Tg O3/year dry deposited. NOx is overpredicted in the lower midlatitude stratosphere, perhaps because model aerosol surface densities are lower than actual values or the NOx to NOy conversion rate is underpredicted. Analysis of the free radical budget shows that ozone and NOy abundances are simulated satisfactorily, as are HOx catalytic cycles and total production and removal rates for ozone.

The Long-Term Coupling between Column Ozone and Tropopause Properties

Abstract: The observational data of the vertical temperature distribution and column ozone, obtained from 10 main stations in the Northern Hemisphere, are analyzed in order to explore the tropopause variations in conjunction with the dynamical variability in column ozone. From the analysis presented, it is evident that the summer distribution of the frequency of occurrence of the tropopause over Greece, apart from its main maximum (around 12 km), is also characterized by a secondary one around 16 km. It is proposed that this elevated maximum possibly originates from the height variation of the tropopause from 12 to 16 km depending on whether the Athens station is located below the cyclonic shear side or below the anticyclonic shear side of the subtropical jet stream. It is also suggested that the transport in the upper troposphere and lower stratosphere that originated in the equatorial region forces the appearance of the multiple tropopauses above Greece. Furthermore, the observational analysis of the ver...

Warming of the Arctic lower stratosphere by light absorbing particles

Abstract: [1] 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.

Tropical tropopause parameters derived from GPS radio occultation measurements with CHAMP

Abstract: [1] The temperature structure in the tropical upper troposphere and lower stratosphere (UTLS) region is discussed based on Global Positioning System (GPS) radio occultation (RO) data from the German CHAMP (CHAllenging Minisatellite Payload) satellite mission. Several climatologies for tropopause parameters based on radiosonde data and model analyses have been published in recent years. Both data sources suffer either from low global coverage or poor vertical resolution. The GPS RO technique, on the other hand, is characterized by global coverage, high vertical resolution, all-weather viewing, and long-term stability. CHAMP RO data are available since February 2001. Since May 2001, up to 200 high- resolution temperature profiles per day are available. The temperature bias between CHAMP temperature profiles and radiosonde data as well as ECMWF analyses is less than 0.5 K between 300–30 hPa. On the basis of the May 2001 to November 2003 data set of CHAMP RO data the structure and temporal and spatial variability of the tropical tropopause based on several tropopause definitions (thermal and cold-point tropopause) are discussed. This includes an overview of the global tropopause characteristics, the discussion of the annual cycle and the latitudinal-longitudinal structure of the tropical tropopause. In the CHAMP RO temperature data, clear evidence of the stratospheric quasi-biennial oscillation (QBO) was found. The goal of this study is to show the potential of GPS RO for global monitoring of the temperature demonstrated exemplarily for the tropical UTLS region and based on the first 31 months (as of November 2003) of CHAMP RO data.

Convective Influence on the Heat Balance of the Tropical Tropopause Layer: A Cloud-Resolving Model Study

Abstract: The tropical tropopause layer (TTL), and in particular the cold point tropopause, has been previously suggested as a feature decoupled from convection. Using a cloud-resolving model, the authors demonstrate that convection, in fact, has a cooling effect in the TTL that significantly affects its thermal structure. In particular, the cold point is found to be strongly tied to the convective cooling maximum. The authors interpret these as natural features of an entrainment layer such as the TTL. The recognition that the cold point tropopause is strongly tied to, rather than decoupled from, convection suggests that dehydration processes at the cold point cannot be assumed as gradual and the effect of convection may not be ignored.

Stratosphere‐troposphere exchange of mass and ozone

Abstract: [1] This study examines the relationship between the extratropical cross-tropopause fluxes of mass and ozone. The adiabatic and diabatic components of the net fluxes are also compared. The rate of change of mass in the lowermost stratosphere and the flux across the 380 K isentropic surface are used to determine the net tropopause mass flux in the framework of a global circulation model. The diabatic mass flux is calculated from the heating rate at the tropopause, and the adiabatic component is determined by the difference of the net and diabatic fluxes. Consistent ozone fields are obtained by driving the Goddard Chemistry and Transport Model with meteorological output of the global circulation model for the same years. The ozone flux is determined by convolving the mass flux and ozone mixing ratio. The results show the following: (1) The seasonal cycle of the ozone mixing ratio is out of phase with the transport cycle leading to a temporal offset of the mass and ozone fluxes; (2) the downward net diabatic flux of mass and ozone occurs primarily at middle latitudes while the adiabatic mass flux is dominated by troposphere-to-stratosphere transport at higher latitudes; and (3) the Southern Hemisphere stratospheric vortex is more effective at blocking meridional transport, resulting in a phase difference of mean tropopause ozone mixing ratio in the higher Southern Hemisphere latitudes with respect to the corresponding Northern Hemisphere season and location. Finally, this study suggests that individual pathways of cross-tropopause transport are unlikely to be the result of simultaneous adiabatic and diabatic mechanisms.

Response to Comment on "Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes"

Abstract: Pielke and Chase ([ 1 ][1]) contend that the troposphere has not warmed since 1979, and that tropospheric warming cannot contribute to observed increases in tropopause height ([ 2 ][2]). They base this contention on the absence of tropospheric warming in thickness data and geopotential heights

On the life cycle of a stratospheric intrusion and its dispersion into polluted warm conveyor belts

Abstract: [1] The aircraft-based 2002 Intercontinental Transport and Chemical Transformation experiment intercepted and chemically analyzed pollution plumes transported from Asia to the western United States. The research flight on 10–11 May 2002 detected mixing between polluted and stratospheric air at midtropospheric levels above the California coast. This study uses a Lagrangian domain-filling trajectory technique to illustrate that this event was the result of mixing between two warm conveyor belts (WCB) containing Asian pollution and the remnants of a deep tropopause fold from a downstream midlatitude cyclone (referred to as the stratospheric component of a dry airstream or SCDA). Advection of the trajectory particles shows how the SCDA decayed over 7.5 days. One component dispersed into a downstream WCB, while another component descended into the lower troposphere and became entrained by an upwind WCB. After 7.5 days of transport 22% of the SCDA mass was transported into the troposphere. The portions of the SCDA that penetrated to the lowest altitudes had the greatest likelihood of being transported into the troposphere. For example, over 90% of the SCDA at altitudes below the 600 hPa level was transported to the troposphere, but none of the mass at the 200 hPa level was exchanged. More than half of the exchange occurred during the first 48 hours as the deepest portions of the tropopause fold decayed over the Pacific. The rest of the exchange occurred over the following 5.5 days as the remnants of the SCDA sheared apart along the edge of the stratospheric polar vortex and became entrained into subsequent tropopause folds and vortex breakaway features. Stratosphere to troposphere exchange resulted in the transport of 0.5 Tg of stratospheric ozone to the troposphere during the 7.5 day study period. Roughly half of the SCDA particles that entered the troposphere dispersed into the upwind and downwind WCBs.

Tracing troposphere-to-stratosphere transport above a mid-latitude deep convective system

Abstract: . Within the project SPURT (trace gas measurements in the tropopause region) a variety of trace gases have been measured in situ in order to investigate the role of dynamical and chemical processes in the extra-tropical tropopause region. In this paper we report on a flight on 10 November 2001 leading from Hohn, Germany (52oN) to Faro, Portugal (37oN) through a strongly developed deep stratospheric intrusion. This streamer was associated with a large convective system over the western Mediterranean with potentially significant troposphere-to-stratosphere transport. Along major parts of the flight we measured unexpectedly high NOy mixing ratios. Also H2O mixing ratios were significantly higher than stratospheric background levels confirming the extraordinary chemical signature of the probed air masses in the interior of the streamer. Backward trajectories encompassing the streamer enable to analyze the origin and physical characteristics of the air masses and to trace troposphere-to-stratosphere transport. Near the western flank of the intrusion features caused by long range transport, such as tropospheric filaments characterized by sudden drops in the O3 and NOy mixing ratios and enhanced CO and H2O can be reconstructed in great detail using the reverse domain filling technique. These filaments indicate a high potential for subsequent mixing with the stratospheric air. At the south-western edge of the streamer a strong gradient in the NOy and the O3 mixing ratios coincides very well with a sharp gradient in potential vorticity in the ECMWF fields. In contrast, in the interior of the streamer the observed highly elevated NOy and H2O mixing ratios up to a potential temperature level of 365 K and potential vorticity values of maximum 10 PVU cannot be explained in terms of resolved troposphere-to-stratosphere transport along the backward trajectories. Also mesoscale simulations with a High Resolution Model reveal no direct evidence for convective H2O injection up to this level. Elevated H2O mixing ratios in the ECMWF and HRM model are seen only up to about tropopause height at 340 hPa and 270hPa, respectively, well below flight altitude of about 200 hPa. However, forward tracing of the convective influence as identified by satellite brightness temperature measurements and counts of lightning strokes shows that during this part of the flight the aircraft was closely following the border of an air mass which was heavily impacted by convective activity over Spain and Algeria. This is evidence that deep convection at mid-latitudes may have a large impact on the tracer distribution of the lowermost stratosphere reaching well above the thunderstorms anvils as claimed by recent studies using cloud-resolving models.

Titan's stratospheric composition driven by condensation and dynamics

Abstract: [1] Atmospheric transport of chemical compounds and organic haze in the stratosphere of Titan is investigated with an axisymmetric general circulation model. It has been shown previously that the meridional circulation, dominated by global Hadley cells, is responsible both for the creation of an intense stratospheric zonal flow and for the accumulation of chemical compounds and haze in high latitudes. The modified composition in turn intensifies the meridional circulation and equator-to-pole thermal contrasts. This paper analyzes in detail the transport processes responsible for the observed vertical and latitudinal variations of atmospheric composition. It is shown that the competition between rapid sinking of air from the upper stratosphere in the winter polar vortex and latitudinal mixing by barotropic planetary waves (parameterized in the model) controls the vertical gradient of chemical compounds. The magnitude of polar enrichment (of a factor 1.4 to 20 depending on the particular species) with respect to low latitudes is mostly controlled by the way the meridional advection increases the concentrations of chemical compounds in the clean air which is rising from the troposphere, where most of the chemical compounds are removed by condensation (the temperature at the tropopause being close to 70 K). The agreement between the observed and simulated contrasts provides an indirect but strong validation of the simulated dynamics, thus confirming the explanation put forward for atmospheric superrotation. It is shown also that by measuring the atmospheric composition, the Cassini-Huygens mission will provide a strong constraint about Titan's atmospheric circulation.

Deep convective cloud-top heights and their thermodynamic control during CRYSTAL-FACE

Abstract: Infrared (11 micron) radiances from GOES-8 and local radiosonde profiles, collected during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) in July 2002, are used to assess the vertical distribution of Florida-area deep convective cloud top height and test predictions as to its variation based on parcel theory. The highest infrared tops (Z(sub 11)) reached approximately to the cold point, though there is at least a 1-km uncertainty due to unknown cloud-environment temperature differences. Since lidar shows that visible 'tops' are 1 km or more above Z(sub 11), visible cloud tops frequently penetrated the lapse-rate tropopause (approx. 15 km). Further, since lofted ice content may be present up to approx. 1 km above the visible tops, lofting of moisture through the mean cold point (15.4 km) was probably common. Morning clouds, and those near Key West, rarely penetrated the tropopause. Non-entraining parcel theory (i.e., CAPE) does not successfully explain either of these results, but can explain some of the day-to-day variations in cloud top height over the peninsula. Further, moisture variations above the boundary layer account for most of the day-today variability not explained by CAPE, especially over the oceans. In all locations, a 20% increase in mean mixing ratio between 750 and 500 hPa was associated with about 1 km deeper maximum cloud penetration relative to the neutral level. These results suggest that parcel theory may be useful for predicting changes in cumulus cloud height over time, but that parcel entrainment must be taken into account even for the tallest clouds. Accordingly, relative humidity above the boundary layer may exert some control on the height of the tropical troposphere.