A21B-0723 0800h
Sensitivity of Age-of-air and Ozone to Vertical Resolution in a Chemical Transport Model Coupled With a GCM
We evaluated stratospheric transport characteristics of an off-line chemical transport model coupled with a general circulation model which were developed at the Meteorological Research Institute/Japan Meteorological Agency. Two versions used here had vertically 45- and 68-level which had a vertical spacing of about 2 and 0.5 km respectively in the stratosphere; and the same T42 spectral truncation horizontally. The transport module was a hybrid semi-Lagrangian scheme, which was formulated in an ordinal semi-Lagrangian scheme horizontally and a mass-conserving flux-form vertically. As a diagnostic of transport characteristics in the stratosphere, we calculated the age-of-air with using assimilated atmospheric fields to ERA40 re-analysis for the period 1981-2000. The 68-level model was better able to capture the observed mean ages than the 45-level model. And we applied the simple leaky tropical pipe model in the tropical lower stratosphere to the age-of-air results; and revealed that the 68-level model reduced the numerical vertical diffusion in this region than the 45-level model. The vertical diffusion of the 68-level model was the same order of the one estimated from observations. And also, we conducted full-chemistry runs for the period 1991-2000. The climatology of ozone and several long-lived trace gases were compared with observations. In the tropical and mid-latitude lower stratosphere, ozone profiles of the 68-level model were remarkably better agreement with observations. This indicated that the transport of long-lived trace gases due to the Brewer-Dobson circulation was closely depend on the vertical resolution.
A21B-0724 0800h
Climatology and origin of small-scale vertical structures of ozone in the lower stratosphere
To understand the mechanism of small-scale advection in the stratosphere which is not resolved by assimilated meteorological data, the climatology and origin of vertical structures (vertical wavelength $<$ 2 km) in stratospheric ozone profiles are investigated by using the global data of routine ozonesonde observations. Vertical structures of ozone in the lower stratosphere can be attributed to either vertical advection or horizontal advection. A useful method to distinguish between them is to evaluate the correlation between the fluctuations of ozone mixing ratio and potential temperature. Positive correlation indicates that vertical advection is responsible for the ozone fluctuation, while no correlation means that horizontal advection is responsible for the ozone fluctuation. It was found that the correlation increases with decreasing latitude in both hemispheres and there exists a distinct seasonal variation in the correlation in the mid-latitude with a maximum in late summer and a minimum in late winter. The cause of the seasonal variation was inferred from the seasonal variations of the correlation coefficient, the ozone fluctuation amplitude and the gravity wave activity. The results show that horizontal advection becomes active and overwhelms vertical advection in late winter, thereby lowering the correlation in this season.
A21B-0725 0800h
Ten Years of MOZAIC Measurements in the UTLS - Past and Future
MOZAIC was initiated in 1993 as a cooperation between european Scientific Institutions, Avionic Industry and five european Airlines in order to generate representative and high quality information on the chemical composition of the UTLS region. Starting with measurements of O3 and H2O in 1994, instruments for CO and NOy were added in 2001. After three phases of funding by the European Commission, MOZAIC has provided data from more than 150000 flight hours in the UTLS and 40000 vertical profiles in the Troposphere. The poster discusses the climatologies of O3, H2O, CO and NOy in the UT and LS and gives an overview of the vertical profiles available over many cities of the world. Important examples are the extreme CO concentrations observed in the UT over large areas in East Asia, the interannual variability of ozone and its correlation with NAO. The data also demonstrate the importance of large scale lofting of boundary layer pollution over the North American East Coast in spring and summer for the odd-nitrogen budget in the UT. The correlation between O3, CO and NOy in the LS gives insight into the exchange between stratosphere and troposphere. The poster presents the outline of a new design study IAGOS (Integration of routine Aircraft measurements into a Global Observing System) just accepted by the European Commission that will allow establish regular observations from in-service aircraft as a long-term component of a global observing system.
http://www.aero.obs-mip.fr/mozaic/
A21B-0726 0800h
Rapid and Simple quantifying system for $^{17}$O anomaly in CO$_{2}$ by using CF/IRMS
We developed a rapid and simple measurement system for $\delta$$^{17}$O, $\delta$$^{18}$O and $\delta$$^{13}$C values in nmol quantities of CO$_{2}$ by using continuous-flow isotope ratio mass spectrometry. The analytical system consisted sequentially of an automatic sample injection system, a helium-purged automatic CO$_{2}$ purification line, a GC capillary, combustion oven, and CF/IRMS. One of the features of our system is that we use CO$_{2}$ molecule to determine the stable isotopic compositions including $\delta$$^{17}$O. The $\delta$$^{17}$O, $\delta$$^{18}$O and $\delta$$^{13}$C values of CO$_{2}$ in a sample are calculated from the mass ratios of both 45/44 and 46/44 of two different kinds of CO$_{2}$, that have been purified quantitatively from different aliquot of a sample. While one (rCO$_{2}$) flows directly into IRMS after GC separation, the other one (eCO$_{2}$) flows through 900degC CuO unit prior to the introduction into IRMS in order to exchange oxygen atoms in the sample CO$_{2}$ molecules with mass-dependent oxygen atoms in CuO. To improve analytical precision statistically, both rCO$_{2}$ and eCO$_{2}$ are introduced alternately to IRMS repeatedly. The standard deviation of 0.57$\permil$ for $\delta$$^{17}$O can be realized by 9 repeated cycles of the pair analysis, using as little as 7nmol CO$_{2}$ within 3 hour. Analytical blanks associated with the single CO$_{2}$ introduction are as small as 1pmol. Based on this system, we determined vertical distribution of $\delta$$^{17}$O values in CO$_{2}$ in the stratospheric air over Japan.
A21B-0727 0800h
A Modeling Study of the Interaction Between Meteorological Background Conditions and the Dynamical Development of a Boreal Pyrocumulonimbus Cloud
Large boreal forest fires emerge as an additional, previously under-appreciated direct source of smoke gases and aerosol particles in the upper troposphere and lower stratosphere (UT/LS). A well documented case of a fire induced convective blow-up with direct injection of smoke into the lower stratosphere is given presented by the Chisholm fire, that burned in Alberta, Canada and peaked on 28 May 28, 2001. There is observational evidence that convective blow-ups of boreal pyro-clouds with significant injection of smoke into the UT/LS region are triggered by the passage of synoptic scale cold fronts. This suggests that the advent of a cold front not only results in an intensification of the fire itself, but also in an intensification of the fire induced atmospheric convection. There are several possible mechanisms through which the convection dynamics can be influenced by the cold front. First, the fire- induced convection is very sensitive to the wind shear and the orientation of the fire front relative to the wind direction, both of which change as the cold front passes. Second, whereas the surface temperature has a sharp negative gradient, there is a maximum of specific humidity at the cold front. This gives rise to a maximum in convective available potential energy (CAPE). Third, large-scale convergence enhances the onset of convection. We have performed numerical simulations of the pyro-cloud convection of over the Chisholm fire using the non-hydrostatic cloud resolving plume model ATHAM (Active Tracer High Resolution Atmospheric Model). Our results show that the fluxes of sensible heat and water vapor are indeed strong enough to explain the penetration of the fire plume into the tropopause region. Different meteorological background conditions are adopted from ECMWF reanalysis data and used to investigate the sensitivity of the pyro-cloud convection to the variation of the wind field as well as the temperature and humidity profiles as during the cold front passage. Using a two-moment scheme, we also analyzed the sensitivity to interactions between the fire-smoke aerosols and cloud processes. Whereas we find a strong sensitivity to the meteorological background conditions, our results suggest that the sensitivity of the plume dynamics to the effects of microphysical interactions between fire aerosols and cloud processes is rather weak, at least with the currently implemented parameterization.
A21B-0728 0800h
Examination of Organic Reactions in UT/LS Aerosols: Temperature Dependence in Sulfuric Acid Solution
Sulfuric acid has been used for decades as an industrial catalyst for organic reactions, but its parallel role in atmospheric aerosols is relatively unexplored, despite identification of a wide array of organic compounds in particles. Several recent studies have demonstrated possible reactions in acidic particles, generally involving carbonyl groups (C=O) and leading to the formation of larger molecules. Reactions of oxygenated organic compounds in acidic solution are most often studied near room temperature, while the sulfate particles of the upper troposphere and lower stratosphere are significantly colder. Our studies of ethanal (acetaldehyde) suggest that reactivity in ~50 wt% H2SO4 solutions may be enhanced at lower temperatures, contrary to expectations. We will present temperature-dependent results of acid catalyzed condensation reactions, leading to formation of higher molecular weight products. Implications for aerosol composition and reactivity will be discussed.
A21B-0729 0800h
Observations of Chloroform (CHCl$_3$)-Depleted Air Masses at the Jungfraujoch High-Altitude Observatory
Episodes of chloroform (trichloromethane, CHCl$_3$) depletion in air masses have been observed over the past 4 years at the high-altitude (3580 m.a.s.l) research station at Jungfraujoch, Switzerland, using in situ gas chromatography-mass spectrometry (GC-MS) measurement techniques. These $\sim$10 episodes occurred primarily during winter and early spring time and lasted typically 1 to 3 days. They are characterized by chloroform concentrations below 2 pmol mol$^{-1}$ compared to a mean background concentrations of $\sim$10 pmol mol$^{-1}$ over the observational period. Relative humidity during these episodes is very low, suggesting a recent stratospheric origin of the air masses. However, positive ozone anomalies are absent for these episodes. In addition, backtrajectory analysis suggests that the air masses resided in the upper troposphere in northern and north-eastern Europe for at least 5 days prior to the arrival at Jungfraujoch. Also, other chlorinated trace gases with similar known atmospheric sinks, such as CH$_2$Cl$_2$ and CH$_3$CCl$_3$, are not depleted during these episodes. Our findings are therefore suggestive of an unknown selective removal mechanism for chloroform in the upper troposphere or lower stratosphere.
A21B-0730 0800h
Resonance Fluorescence Instrument for the {\it in Situ} Detection of BrO in the Atmosphere: Instrument Description, Calibration, and Flight Data Analysis
Losses of ozone over midlatitudes of the northern hemisphere are well documented in both the scientific literature and the public policy arena. Accurately defining secular trends in the distribution of ozone and establishing the mechanisms responsible for the observed losses are two dominant and enduring issues. The latest WMO report, {\it Scientific Assessment of Ozone Depletion: 2002}, states: ``The vertical, latitudinal, and seasonal characteristics of changes in midlatitude ozone are broadly consistent with the understanding that halogens are the primary cause of these changes, in line with similar conclusions from the 1998 Assessment.'' Other studies, however, have indicated alternative explanations to midlatitude ozone loss. Therefore, given the continued uncertainty, it is prudent to examine the full complement of halogen radical families in the lower stratosphere from the tropics to midlatitudes. Accurate BrO measurements, in particular, are critical due to the high efficiency of bromine compounds at ozone destruction. However, efforts to test the abundance and speciation of bromine in the stratosphere have been particularly hampered by low atmospheric concentrations. Here we present details of our redesigned resonance fluorescence axis for {\it in situ} BrO detection, which is incorporated into the Harvard Halogen flight instrument. These modifications provide the capability to detect BrO with greater sensitivity, precision, and spatial resolution than in any previous {\it in situ} aircraft measurement. Instrument calibrations have been ongoing, and details of this work, along with analysis of the BrO data from twelve flights of the SOLVE mission to Kiruna, Sweden, are presented. We estimate an accuracy of BrO measurements from the Harvard Halogen flight instrument of \pm20% (1$\sigma$) with a detection limit of 3 pptv in 5 minutes.
A21B-0731 0800h
Anomalous Vortex Intensification and Associated Stratosphere-Troposphere Evolution
Composite stratosphere-troposphere evolution is observed during the Northern Hemisphere extended winters in association with stratospheric polar vortex intensification events. These events are initially marked by the descent of zonal wind and temperature anomalies from the upper stratosphere to the tropopause, in connection with strong anomalous heat fluxes in the upper stratosphere. As the events mature, enhanced anomalous momentum fluxes appearing in the upper troposphere accompany an increase in the amplitude of the Northern Hemisphere Annular Mode and related wind and temperature anomalies near the surface. Overall, the observed dynamics exhibit a largely linear relationship (similar, but opposite in sign) to those found during stratospheric polar vortex weakening events (as characterized by Sudden Stratospheric Warming). However, notable differences occur in the tropical region. In particular, anomalous vortex intensification tends to coincide with La Nina events.
A21B-0732 0800h
Inconsistencies in Tropical Tropopause Temperatures Between Radiosonde and GPS Radio Occultation Measurements
Accurate temperature measurements at the tropical tropopause are critical to diagnosing the relationship between water vapor saturation mixing ratio and stratospheric water vapor and, accordingly, the mechanisms for stratosphere-troposphere exchange. The radiosonde network has provided the most accurate temperature record in the tropics to date, but a self-consistent temperature mapping of the tropical tropopause layer (TTL) with radiosonde data is impaired by (i) very limited spatial sampling, especially over the predominantly marine tropics, (ii) differences in radiosonde instrument packages, and (iii) solar radiation effects on reported temperatures. Global positioning system (GPS) radio occultation measurements offer a powerful approach to examining the temperature structure of the TTL that provides homogeneous spatial coverage of the tropics while still maintaining high vertical resolution. We use a GPS occultation data set obtained from the CHAMP satellite for 2001-2003 with retrievals performed at the Jet Propulsion Laboratory (JPL) and subjected to objective quality control. These occultations are compared with radiosonde measurements from the WMO global network that have been processed through the complex quality control of NCEP. GPS occultations and radiosondes show significant differences in (i) the mean cold-point tropopause temperature, (ii) the distribution of cold-point temperatures, and (iii) the height of the cold-point tropopause. We investigate differences between radiosonde and occultation climatologies of the TTL, paying special attention to the merits and deficiencies of each measurement approach. We also compare the GPS occultation retrievals of JPL to other retrieval algorithms to investigate potential biases. The temperature differences between GPS occultations and radiosondes at the cold-point tropopause could have profound implications for the water vapor budget of the stratosphere.
A21B-0733 0800h
Isotopic Measurements of Water Vapour and Methane from the MIPAS Satellite Instrument
The isotopic composition of stratospheric water vapour depends on the sources of water vapour and on the temperature and precipitation history of the stratospheric air. Isotopic measurements of water vapour therefore have the potential to aid investigation of dehydration in the polar vortex, of stratospheric/tropospheric exchange, of upward transport in the tropics and of subsidence at polar latitudes. The isotopic composition of methane affects that of water vapour, since methane oxidation is an important source of water vapour in the stratosphere. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a high resolution limb sounder flying on the European Space Agency's Envisat satellite. MIPAS measures infrared atmospheric limb emission spectra from 685-2410cm$^{-1}$ over an altitude range of 6-68km. These spectra are acquired continuously during both day and night, providing a three--dimensional, almost global map of the atmospheric emission every 24 hours. After suitable ground processing, these spectra allow the retrieval of concentration profiles of numerous atmospheric trace gases. The MIPAS spectra contain enough information to distinguish minor isotopes of many of these gases, including water vapour (H$_2^{18}$O, H$_2^{17}$O and HDO). It is also possible to obtain measurements of CH$_3$D, giving the potential to investigate the deuterium budget of the stratosphere. Here we present global isotopic measurements of water vapour and methane from the MIPAS instrument, spanning the upper troposphere and lower stratosphere.
A21B-0734 0800h
CASE STUDY OF STRATOSPHERE-TROPOSPHERE EXCHANGE ASSOCIATED WITH A SUMMER EXTRATOPICAL LOW: MOZAIC AIRCRAFT MEASUREMENTS ANALYSIS AND MODELLING.
There is lack of knowledge of summer season case studies of stratosphere Troposphere Exchange, especially for intense and irreversible cases. Here we present a summer case of deep tropopause fold associated with dynamics of extratropical lows. The case study has been revealed by analysis of ozone, water vapor and carbon monoxide measurements from two MOZAIC aircrafts.The first aircraft captures a tropopause fold in the low troposphere. The second one flies in the upper level divergent outflow of the low and captures air masses processed by the Warm Conveyor Belt (WCB) and the Dry Airstream (DA). We perform backtrajectories (LAGRANTO tool) using 3-hourly ECMWF analysis to retrieve the origin of ozone and CO features in the flight time series, and to re-construct the 3 dimensional fields of Potential Vorticity with Reverse Domain Filling technique. Contribution of mixing processes between the WCB and the DA (convection, clear air turbulence, ...) involved during the event are analysed with a mesoscale model.
http://www.aero.obs-mip.fr/mozaic/
A21B-0735 0800h
MIXING LAYER: THE MID-LATITUDE TROPOPAUSE AS SEEN BY MOZAIC AIRCRAFT IN-SITU MEASUREMENTS OF OZONE AND CARBON MONOXIDE.
MOZAIC in-situ measurements of ozone, water vapour, carbon monoxide and nitrogen oxide in the Upper-Troposphere and Lower-Stratosphere (UTLS) are daily collected since January 2002 by 5 commercial aircraft along routes between Europe and western America and eastern Asia. Mixing lines of O3 and CO in the UTLS region are analysed. A mixing layer across the tropopause is identified. Observations of the mixing layer are sorted by regions (Europe, Atlantic, Northeast America, Asia) and seasons, and are analysed with respect to zonal differences in upper level dynamics and in surface CO emissions. Interpretations are further conducted with the analysis of 5-days backward trajectories of air parcels initialized along aircraft paths and driven by ECMWF-analysed winds. Case studies of MOZAIC observations of long-range tropospheric transport and of stratospheric injection of high CO values due to the conjunction of boreal biomass fires and deep convection over Northwest Canada in June-July 2004 during the ICARTT campaign will be presented.
http://www.aero.obs-mip.fr/mozaic/
A21B-0736 0800h
Control of the Tropical Tropopause and Vertical Transport and Vertical Transport Across it
A 3D primitive equation model is used to investigate how the tropical tropopause is influenced by cumulus convection in the troposphere and mean upwelling in the stratosphere. The model simulates the residual mean circulation explicitly, whereas it represents the influence of convection on large-scale structure through statistical properties of cloud. Within this global framework, a change of extratropical planetary waves induces a large change of downwelling over the winter hemisphere, compensated at lower latitudes by a change of upwelling. This exerts a major influence on thermal structure in the extratropics, but only a minor one in the tropics. On seasonal time scales, however, the influence in the tropics is significant. During northern winter, extratropical planetary waves are sharply amplified. The accompanying intensification of tropical upwelling, while small compared to the intensification of extratropical downwelling, accounts for about half of the observed seasonal change of the tropical tropopause. Remarkably, anomalous thermal structure extends even into the summer hemisphere, where the tropopause is anomalously high and cold. Just the reverse is found in the winter hemisphere. Contrasting with this is the dependence on convection, which is large in the tropics. An intensification or deepening of convection "elevates and cools" the tropical tropopause. Accompanying those changes overhead is anomalous downwelling in the lowermost stratosphere. It is forced by convective cooling, at and above the Level of Neutral Buoyancy (LNB), where overshooting cumulus are colder than their environment. Anomalous temperature is out of phase above and below the LNB, consistent with cumulus detrainment and observed changes that accompany the outbreak of cold cloud. Conversely, an elevation of the LNB, as would accompany an increase of moist static energy (e.g., SST), "elevates but warms" the tropical tropopause. This dependence may explain geographical variations of the tropopause. A change of tropical convection also influences the extratropical circulation, secondarily through the absorption of planetary waves, which then modulates downwelling and temperature over the winter hemisphere. Vertical transport into the stratosphere depends on both mechanisms, which interact. Above the LNB, convective cooling drives environmental downwelling that transports stratospheric air into the troposphere at sites of deep convection. There, air of high theta mixes with air of low theta that has been convected above the LNB inside overshooting cumulus. The mixture, having been cooled mechanically, then experiences enhanced radiative warming that carries it upward at sites removed from convection.
A21B-0737 0800h
Water and Methane in the Upper Troposphere and Stratosphere Based on ACE-FTS Measurements
The SCISAT-1 mission, also known as the Atmospheric Chemistry Experiment (ACE), is a Canadian satellite mission to investigate the chemical and dynamical processes that control the distribution of ozone in the stratosphere and upper troposphere. The primary instrument on SCISAT-1 is the ACE-FTS, a high resolution Fourier transform spectrometer operating in the 750-4500 cm$^{-1}$ range. ACE-FTS occultation spectra dating back to January 2004 have now been processed to produce a suite of data products including temperature and pressure profiles from the mid-troposphere to the mesopause, as well as VMR profiles of H$_{2}$O, N$_{2}$O, CO, NO, HCl, HF, HNO$_{3}$, CH$_{4}$, O$_{3}$, NO$_{2}$, HDO, SF$_{6}$, COF$_{2}$, N$_{2}$O$_{5}$, ClONO$_{2}$, CCl$_{3}$F, CCl$_{2}$F$_{2}$ and CHF$_{2}$Cl with other VMR profiles to come in the near future. Recently, there has been some debate within the atmospheric community regarding increases in upper tropospheric and stratospheric water vapor that have been observed over the last 50 years. Understanding trends in atmospheric water vapor are necessary for our understanding of both stratospheric ozone depletion and global climate change. This paper will report on the ACE-FTS H$_{2}$O, HDO and CH$_{4}$ measurements and discuss their relationship to trends and measurements by other instruments such as ATMOS, HALOE and POAM III.
A21B-0738 0800h
Cross-tropopause Transport by Deep Convection and Its Implications to Climate and Atmospheric Chemistry
Recent observations by aircraft and meteorological satellites (such as GOES, AVHRR, POAM and MODIS) have indicated unambiguous signature of cross-tropopause transport by deep convective systems. These include cirrus plumes located a few kilometers above the anvils of severe storms as shown by satellite visible and infrared images, measurements of trace gases of tropospheric origin in the lower stratosphere in the outflow region of deep convection, and satellite observations of tropopause penetration of ashes by boreal fire-induced storms. We have performed numerical simulations of the anvil top plumes above severe storms and the fire-induced storms using a 3-D nonhydrostatic cloud model. The simulated plumes and pyrocumulonimbus resemble those observed. Analyses of simulation results show that the irreversible mass transfer through the tropopause is mainly due to the diabatic turbulent diffusion induced by the breaking of gravity waves caused by the deep convection. The same mechanism also explains the controversial "jumping cirrus" phenomenon reported by Ted Fujita in 1980s. Such irreversible mass transfer of water vapor and other chemical species from the troposphere to the lower stratosphere would have significant implications on the global climate and atmospheric chemistry. It is also likely that we need to revise our understanding of the global stratosphere-troposphere exchange process substantially in light of these new observations. A summary of the recent observations and model simulations and discussions on their implications on climate, atmospheric chemistry, and global transport process will be presented in the meeting.
A21B-0739 0800h
Seasonal Variability of H$_{2}$O and O$_{3}$ in the UT/LS and Extent of the Extra-tropical Mixing Layer Based on Airborne SPURT Measurements
The data set presented in this study was obtained during the airborne project SPURT (german: trace gas transport in the tropopause region). The high-resolution \emph{in situ} measurements of a large set of trace gases, thereunder total water (H$_{2}$O) and ozone (O$_{3}$), took place over Europe between November $2001$ and July $2003$. Each annual season was investigated within $2$ campaigns in the region around the polar jet covering a broad latitude range between $20-80$^\circ$N. Based upon this data set, we derived a seasonal climatology and characteristics of trace gas distributions, their gradients and variability in that region. Significantly enhanced H$_{2}$O mixing ratios of several $10$\ ppmv near the tropopause and even higher up in the LS have been detected during all seasons. To get a comprehensive insight into processes, ranging and variability of the measured parameters in the investigated area, correlations, distributions as well as probability density distributions (PD) of the \emph{in situ} measured trace gases H$_{2}$O and O$_{3}$ were determined by using chemical, thermal and dynamical coordinates (e.g.\ potential temperature, potential vorticity, distance to the local tropopause, distance to the jet stream maximum wind speeds, equivalent latitude). The PDs serve as a tool to transform localised aircraft data to a more comprehensive view of the tropopause region. It appears that both trace gases show most compact distributions and are best correlated in the view of potential vorticity and distance to the local tropopause. The seasonal cycles of H$_{2}$O and O$_{3}$ in the UT and LS are also evident in the PDs. An extra-tropical mixing layer following the shape of the tropopause and/or surfaces of potential vorticity was identified. Based on Lagrangian studies this layer is supposed to be influenced by transport processes on time scales of days to weeks. From tracer-tracer-correlations the extent of this layer could be estimated to range from the local tropopause to approximately $8-9$\ PVU, independent from season. By long-term simulations using the J\"{u}lich CLaMS (Chemical Lagrangian Model of the Stratosphere) the tropospheric influence with enhanced H$_{2}$O values even beyond this layer could be reproduced which originates from transport on longer time scales.
A21B-0740 0800h
Stratospheric Ozone Heating and the Downward Reflection of Forced Planetary Waves
Forced planetary waves extend throughout the troposphere and stratosphere and thus provide an important link between these two regions of the atmosphere. Although it is well established that vertically propagating planetary waves generated in the troposphere play a major role in driving the zonal-mean stratospheric circulation, an ever-growing body of evidence shows that the stratosphere, despite its lesser mass, may play a more important role in influencing the tropospheric circulation than previously thought. Here we examine the role of stratospheric ozone heating due to wave-ozone feedbacks in the downward reflection of tropospherically generated planetary waves. Using coupled equations for quasigeostrophic potential vorticity and ozone volume mixing ratio, a WKB analysis yields an analytical expression that shows how coupled wave-ozone interactions in the stratosphere can affect the index of refraction and reflection coefficient of topographically forced planetary waves. The reflection coefficient shows explicitly how key stratosphere features, including wind structure, radiative processes and zonal-mean ozone distribution, can affect the planetary wave structure in the troposphere. The analytical results have been confirmed numerically using climatological distributions of wind, temperature and ozone for each season. The numerical results show that depending on the height of the stratospheric reflecting surface, which is a strong function of season, the wave-ozone feedbacks can alter the planetary wave fluxes in the troposphere by as much as 10-15%. These results are discussed in light of stratospheric ozone perturbations arising from solar variability.
A21B-0741 0800h
Summer Northern Hemisphere annular mode revealed from observation and AGCM simulation
The seasonal variations of the Northern Hemisphere annular mode (NAM) are investigated through empirical orthogonal function analysis of the zonally averaged geopotential height fields for each individual calendar month. Patterns of the winter and summer NAMs differ not only in the geopotential height fields but also in the mean meridional circulation and eddy structure. The summer NAM has a smaller meridional scale, and is displaced poleward as compared to the winter NAM. The antinode on the lower-latitude side in the summer NAM is at the nodal latitude of the winter NAM. The summer NAM is more strongly related to surface air temperatures over Eurasia than the original Arctic Oscillation. The summer NAM is a wave-driven internal atmospheric mode that is maintained by both stationary and transient waves. The summer NAM is associated with the Arctic front, polar jet, and storm track around the Arctic Ocean. The CCSR/NIES/FRCGC AGCM (T106L56) is also able to exhibit the summer NAM. In addition, the summer NAM enables the realization of anomalous summers, such as the hot Europe/cool Japan occurrences in 2003. Anomalous geopotential and temperature polarity of the summer NAM are substantially identical to the summer patterns in 2003.
A21B-0742 0800h
Cloud-Radiative Interactions With Upper Tropospheric Circulations
A mechanism for the interaction of cloud-radiative effects with large-scale upper tropospheric circulations is explored using both idealized numerical models and atmospheric data. In a zonally-symmetric model of the atmosphere containing representations of moist convection, radiation, and partial cloudiness, overturning circulations were induced by the vertical dipoles of radiative heating which exist between the bases and tops of upper tropospheric clouds. Such circulations preferentially enhanced the dynamical variability of the upper troposphere on timescales of several days, as compared to longer seasonal timescales. Vertical velocities from reanalyzed atmospheric data exhibited a similar proportional enhancement with height of such higher frequency variability. Correlations between humidity, vertical velocity, and indicators of cloudiness for this reanalyzed data set are also discussed. All of these results are compared with integrations of a three-dimensional general circulation model in which the radiative heating profiles were vertically smoothed in the upper troposphere, thereby eliminating the vertical heating dipole collocated with high-level clouds.
A21B-0743 0800h
A New 2D-Advection-Diffusion Model Simulating Trace Gas Distributions in the Lowermost Stratosphere
Tracer distributions in the lowermost stratosphere are affected by both, transport (advective and non-advective) and in situ sources and sinks. They influence ozone photochemistry, radiative forcing, and heating budgets. In-situ measurements of long-lived species during eight measurement campaigns revealed relatively simple behavior of the tracers in the lowermost stratosphere when represented in an equivalent-latitude versus potential temperature framework. We here present a new 2D-advection-diffusion model that simulates the main transport pathways influencing the tracer distributions in the lowermost stratosphere. The model includes slow diabatic descent of aged stratospheric air and vertical and/or horizontal diffusion across the tropopause and within the lowermost stratosphere. The diffusion coefficients used in the model represent the combined effects of different processes with the potential of mixing tropospheric air into the lowermost stratosphere such as breaking Rossby and gravity waves, deep convection penetrating the tropopause, turbulent diffusion, radiatively driven upwelling etc. They were specified by matching model simulations to observed distributions of long-lived trace gases such as CO and N2O obtained during the project SPURT. The seasonally conducted campaigns allow us to study the seasonal dependency of the diffusion coefficients. Despite its simplicity the model yields a surprisingly good description of the small scale features of the measurements and in particular of the observed tracer gradients at the tropopause. The correlation coefficients between modeled and measured trace gas distributions were up to 0.95. Moreover, mixing across isentropes appears to be more important than mixing across surfaces of constant equivalent latitude (or PV). With the aid of the model, the distribution of the fraction of tropospheric air in the lowermost stratosphere can be determined.
A21B-0744 0800h
New Satellite Observations of Upper Tropospheric/Lower Stratospheric Aerosols: Case Studies over the U.S. and Canada
During the past few years, a near-UV aerosol index derived from Total Ozone Mapping Spectrometer (TOMS) data has been used to detect aerosol plumes carried to high altitudes by intense convective activity. Using data from the Terra Multi-angle Imaging SpectroRadiometer (MISR), we can directly measure the heights of certain of these upper tropospheric/lower stratospheric (UTLS) plumes. Stereoscopic height retrievals are performed routinely and globally as part of the MISR data product generation system, using imagery from the instrument's nadir and near-nadir cameras to minimize the computer processing load. On a case-by-case basis, the stereo capability can be enhanced by using very oblique views, thereby increasing aerosol visibility and improving stereo height resolution and plume spatial coverage. In addition, MISR aerosol retrievals provide information on column-integrated particle abundances and microphysical properties such as size distribution and shape. We present case studies of TOMS-MISR observations of UTLS plumes, including an extensive smoke layer originating from the Chisholm fire in Alberta in May 2001, and an aerosol layer identified in TOMS data over the midwestern U.S. in August 2002, and subsequently explored using MISR data. The global prevalence of high-altitude (i.e., above 5 km) smoke- or dust-laden aerosols is unknown. A systematic investigation combining these satellite data sets would lead to new insights regarding plume origins, atmospheric residence times, climatic and environmental significance, and long-range particulate transport.
A21B-0745 0800h
Modelling the impact of convective entrainment on the Tropical Tropopause
In the tropics plumes originating near the surface may have sufficient potential energy to overshoot their level of neutral buoyancy. Because of the relatively long radiative relaxation time in the tropopause, such deep convective events, though rare, might have a significant effect on the dynamics/physics/chemistry in the vicinity of the cold point. Indeed recent model simulations and observational studies indicate that convection in the tropics and summertime mid-latitudes influences the temperature and tracer concentrations (particularly water vapor) to altitudes above 16 km. In this talk we will describe the role and robustness of convective overshoot/entrainment in the Tropical Tropopause layer in a 2- and 3D cloud resolving model. We will also discuss the extent and the amount of overshoot as a function of CAPE, geometry, grid resolution, sub-grid scale turbulence parametrisation, domain size and prescribed microphysics.
A21B-0746 0800h
Observations of Gas- and Condensed-Phase Nitric Acid in the Tropical Upper Troposphere and Lower Stratosphere
Nitric acid (HNO$_{3}$) serves as the primary reservoir species for nitrogen oxides, which are directly involved in the photochemical production of tropospheric ozone. Gas- and condensed-phase HNO$_{3}$ were observed for the first time in the tropical upper troposphere and lower stratosphere (UT/LS) during the NASA Pre-Aura Validation Experiment (Pre-AVE) in January, 2004, onboard the WB-57F high-altitude research aircraft. Vertical profiles of HNO$_{3}$ in the UT/LS reveal low values (150 pptv or less) near the tropical tropopause. Individual nitric acid-containing particles were observed between 3\deg S and 7\deg N during two Pre-AVE flights in the LS. These particles were observed at 18 km altitude and temperatures of 188-192 K, with number densities of 10$^{-4}$-10$^{-6}$ cm$^{-3}$. Understanding the formation mechanism of these particles might provide insight into ice particle formation at the tropical tropopause and the selective nucleation process for large nitric acid trihydrate (NAT) particles previously observed in the Arctic lower stratosphere.
A21B-0747 0800h
Age and Pathway Diagnostics for a Stratospheric General Circulation Model
Using a variety of age diagnostic experiments we examine the stratospheric age spectrum of the Goddard Finite Volume General Circulation Model. Pulse tracer release age-of-air computations are compared to forward and backward trajectory computations. These comparisons show good agreement, and the age-of-air also compares well with observed long lived tracers. Pathway diagnostics show how air arrives in the lowermost stratosphere and the age structure of that region. Using tracers with different lifetimes we can estimate the age spectrum - this technique should be useful in diagnosing transport from various trace gas observations.