T. Duncan Fairlie

Chaotic advection in the stratosphere : implications for the dispersal of chemically perturbed air from the polar vortex

The Lagrangian evolution of material lines within the northern hemisphere winter stratospheric vortex is determined using isentropic winds and diabatic heating rates obtained from the NASA Langley Research Center (LaRC) atmospheric circulation model. Transient, subtropical anticyclones lead to deformation of the material lines near the edge of the polar vortex which then rapidly evolve into elongated filaments as material is drawn around the anticyclones. The rate of stretching of the material lines is shown to be exponential, with typical e-folding times of the order of 4 to 8 days. These results provide evidence for “chaotic advection” near the edge of the stratospheric polar vortex which leads to rapid mixing of vortex air with tropical and midlatitude air. The characteristic timescales of these mixing processes and the extent to which the mixing penetrates the polar vortex have important implications for the dispersal of chemically perturbed air from the polar vortex throughout the northern hemisphere and attendant ozone depletion.

A climatology of stratospheric polar vortices and anticyclones

[1] United Kingdom Meteorological Office global analyses from 1991 to 2001 are used to create a global climatology of stratospheric polar vortices and anticyclones. New methodologies are developed that identify vortices in terms of evolving three-dimensional (3-D) air masses. A case study illustrates the performance of the identification schemes during February and March of 1999 when a merger of anticyclones led to a stratospheric warming that split the Arctic polar vortex. The 3-D structure and temporal evolution of the Arctic vortex and identified anticyclones demonstrates the algorithm’s ability to capture complicated phenomena. The mean geographical distribution of polar vortex and anticyclone frequency is shown for each season. The frequency distributions illustrate the climatological location and persistence of polar vortices and anticyclones. A counterpart to the Aleutian High is documented in the Southern Hemisphere: the ‘‘Australian High.’’ The temporal evolution of the area occupied by polar vortices and anticyclones in each hemisphere is shown as a function of potential temperature. Large polar vortex area leads to an increase in anticyclone area, which in turn results in a decrease in the size of the polar vortex. During Northern winter and Southern spring, 9 years of daily anticyclone movement are shown on the 1200 K (36 km, 4 hPa) isentropic surface. Preferred locations of anticyclogenesis are related to cross-equatorial flow and weak inertial stability. Regimes of traveling and stationary anticyclones are discussed. INDEX TERMS: 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3319 Meteorology and Atmospheric Dynamics: General circulation; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); KEYWORDS: polar vortex, stratospheric anticyclones

Mixing Processes within the Polar Night Jet

Abstract Lagrangian material line simulations are performed using U.K. Meteorological Office assimilated winds and temperatures to examine mixing processes in the middle- and lower-stratospheric polar night jet during the 1992 Southern Hemisphere spring and Northern Hemisphere winter. The Lagrangian simulations are undertaken to provide insight into the effects of mixing within the polar night jet on observations of the polar vortex made by instruments onboard the Upper Atmosphere Research Satellite during these periods. A moderate to strong kinematic barrier to large-scale isentropic exchange, similar to the barrier identified in GCM simulations, is identified during both of these periods. Characteristic timescales for mixing by large-scale isentropic motions within the polar night jet range from 20 days in the Southern Hemisphere lower stratosphere to years in the Northern Hemisphere middle stratosphere. The long mixing timescales found in the Northern Hemisphere polar night jet do not persist. Instead,...

Stratospheric versus pollution influences on ozone at Bermuda: Reconciling past analyses

[1] Conflicting interpretations of the spring ozone maximum observed at Bermuda (32°N, 65°W) have fueled the debate on stratospheric influence versus tropospheric production as sources of tropospheric ozone. We use a global three-dimensional (3-D) model of tropospheric ozone-NO x -hydrocarbon chemistry driven by assimilated meteorological observations to reconcile these past interpretations. The model reproduces the observed seasonal cycle of surface ozone at Bermuda and captures the springtime day-to-day variability (r = 0.82, n = 122, p < 0.001) driven by high-ozone events. We find that transport of North American pollution behind cold fronts is the principal contributor to springtime surface ozone at Bermuda and is responsible for all the high-ozone events. The model reproduces the observed positive correlations of surface ozone with 7 Be and 210 Pb at Bermuda; the correlation with 7 Be reflects the strong subsidence behind cold fronts, resulting in the mixing of middle-tropospheric air with continental outflow in the air arriving at Bermuda, as indicated by the positive 7 Be- 210 Pb correlation. This mixing appears to have been an obfuscating factor in past interpretations of subsiding back-trajectories at Bermuda as evidence for a stratospheric or upper tropospheric origin for ozone. Isentropic back-trajectories computed in our model reproduce the previously reported subsidence associated with high-ozone events. Even in the free troposphere, we find that the stratosphere contributes less than 5 ppbv (<10%) to spring ozone over Bermuda. Positive O 3 - 7 Be and negative O 3 - 210 Pb correlations observed at Tenerife (28°N, 16°W, 2.4 km) in summer are reproduced by the model and are consistent with a middle-tropospheric source of ozone, not an upper tropospheric or stratospheric source as previously suggested. A regional budget for the North Atlantic in spring indicates that the stratosphere contributes less than 10 ppbv ozone (<5%) below 500 hPa, while the lower troposphere contributes 20-40 ppbv ozone throughout the troposphere.

The contribution of mixing in Lagrangian photochemical predictions of polar ozone loss over the Arctic in summer 1997

Measurements from the Halogen Occultation Experiment, together with assimilated winds, temperatures, and diabatic heating rates from the NASA Goddard data assimilation office, are used in the NASA Langley Research Center trajectory-photochemical model to compute photochemistry along three-dimensional air parcel trajectories for the Northern Hemisphere for the period March through September 1997. These calculations provide a global perspective for the interpretation of constituent measurements made from the ER-2 platform during the Photochemistry of Ozone Loss in the Arctic Region in Summer aircraft campaign. An important component of the model is a parameterization of sub-grid-scale diffusive mixing. The parameterization uses an n-member mixing approach which includes an efficiency factor that enhances the mixing in regions where strain dominates the large-scale flow. Model predictions of O 3 and CH 4 are compared with in situ measurements made from the ER-2. Comparison of the in situ data with model predictions, conducted with and without diffusive mixing, illustrates the contribution that irreversible mixing makes in establishing observed tracer-tracer correlations. Comparisons made for an ER-2 flight in late April 1997 show that irreversible mixing was important in establishing observed tracer-tracer correlations during spring 1997. Comparisons made in late June 1997, when filaments of very low N 2 O and CH 4 were observed, indicate that remnants of air from the polar vortex survived unmixed in the low stratosphere 6 weeks after the breakup of the polar vortex in May. The results demonstrate that the sub-grid-scale mixing parameterization used in the model is effective not only for strong mixing conditions in late winter and early spring, but also for relatively weak mixing conditions that prevail in summer.

Lagrangian forecasting during ASHOE/MAESA: Analysis of predictive skill for analyzed and reverse‐domain‐filled potential vorticity

A statistical analysis is conducted to determine to what extent analyzed and 5-day reverse-domain-filled (RDF) potential vorticity (PV) obtained from meteorological analyses can predict ATLAS nitrous oxide (N2O) tracer structure encountered along the ER-2 flight track during the Airborne Southern Hemisphere Ozone Experiment / Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) campaign. The results indicate that RDF PV shows no statistically significant improvement in forecast skill over analyzed PV in predicting tracer structure along the ER-2 flight track. In fact, RDF generally shows a degradation in predictive skill. RDF does show some success in refining large-scale gradients and small-scale structures, present in the analyzed PV fields. In at least one case, RDF PV captured a filament encountered by the ER-2, but in general, such structure is marked by low confidence in the RDF PV analyses.

Response of middle atmosphere chemistry and dynamics to volcanically elevated sulfate aerosol: Three‐dimensional coupled model simulations

The NASA Langley Research Center Interactive Modeling Project for Atmospheric Chemistry and Transport (IMPACT) model has been used to examine the response of the middle atmosphere to a large tropical stratospheric injection of sulfate aerosol, such as that following the June 1991 eruption of Mount Pinatubo. The influence of elevated aerosol on heterogeneous chemical processing was simulated using a three-dimensional climatology of surface area density (SAD)developed using observations made from the Halogen Occultation Experiment, Stratospheric Aerosol and Gas Experiment II, and Stratospheric Aerosol Measurement satellite instruments beginning in June 1991. Radiative effects of the elevated aerosol were represented by monthly mean zonally averaged heating perturbations obtained from a study conducted with the European Center/Hamburg (ECHAM4) general circulation model combined with an observationally derived set of aerosol parameters. Two elevated-aerosol simulations were integrated for 31/2 years following the volcanic injection. One simulation included only the aerosol radiative perturbation, and one simulation included both the radiative perturbation and the elevated SAD. These perturbation simulations are compared with multiple-year control simulations to isolate relative contributions of transport and heterogeneous chemical processing. Significance of modeled responses is assessed through comparison with interannual variability. Dynamical and photochemical contributions to ozone decreases are of comparable magnitude. Important stratospheric chemical/dynamical feedback effects are shown, as ozone reductions modulate aerosol-induced heating by up to 10% in the lower stratosphere and 25% in the middle stratosphere. Dynamically induced changes in chemical constituents which propagate into the upper stratosphere are still pronounced at the end of the simulations.

Large‐scale stratospheric ozone photochemistry and transport during the POLARIS Campaign

Measurements from the Halogen Occultation Experiment (HALOE) on board the UARS satellite and assimilated winds, temperatures, and diabatic heating rates from the NASA Goddard data assimilation office (DAO) are used with the NASA Langley Research Center (LaRC) Lagrangian photochemical model to compute 3-D air parcel trajectories with photochemistry for all Northern Hemisphere HALOE observations during the period March-September 1997. Results from ensemble means of the photochemical trajectory calculations provide a global perspective for the interpretation of constituent measurements made from the ER-2 and balloon platforms during the POLARIS aircraft campaign. Lagrangian photochemical predictions are shown to compare favorably with ER-2, balloon, Total Ozone Mapping Spectometer (TOMS), and subsequent coincident HALOE observations. Model predictions show large-scale photochemical ozone loss in high latitudes at ER-2 flight altitudes of over 10% per month in June and July, in good agreement with steady state photochemical calculations constrained with ER-2 observations of radical and long-lived species. Largest summertime photochemical ozone losses (over 1.4 ppmv/month) are found to occur poleward of 60°N above 30 mbar, in good agreement with steady state photochemical calculations constrained with observations from the balloon-borne Fourier transform infrared solar absorption spectrometer (MkIV) instrument. Summertime polar photochemical ozone losses are driven largely by NO x chemistry and are largest for air parcels with high NO x /NO y ratios that have experienced continuous sunlight for several days. Differences between predicted net changes in ozone and changes due to photochemistry are used to estimate residual changes due to transport processes. Photochemical and residual transport tendencies tend to be of similar magnitude but opposite sign. Photochemical loss of ozone tends to outweigh positive transport tendencies in high latitudes, while upwelling of low ozone below the tropical ozone maximum moderates photochemical production there. The estimated transport tendencies are generally consistent with expectations based on transformed Eulerian circulation derived from the DAO assimilated data and the mean ozone distribution. A net (photochemical plus transport) ozone decrease of over 0.2 ppmv/ month is predicted throughout the middle and lower stratosphere poleward of 70°N during the summer months.

Tropical aerosol in the Aleutian High

Stratospheric aerosol profiles at high northern latitudes from the Stratospheric Aerosol Measurement (SAM) II experiment are used to document the aerosol maxima that occur in the major wintertime anticyclones. Fourteen years (1978 -1991) of 1 mm extinction are used to calculate median values for each season in bins of 58 latitude by 308 longitude by 1 km altitude. Longitude-altitude sections of estimated surface area density show that tropical, aerosol rich air tends to accumulate in the Aleutian High from 15 to above 30 km, and in the North Atlantic High in the 15-25 km layer. A trajectory case study with winds from the European Center for Medium-Range Weather Forecasting is used to investigate the hypothesis that the observed aerosol maxima are maintained by episodic poleward surges of high aerosol air from the tropical stratospheric reservoir. Lagrangian trajectories are initialized and run backward in time, from both a high- resolution grid and SAM II occultations, for selected days when high aerosol is found in the Aleutian High. Results show that during the case study provided, a deep sheet of aerosol rich air originating over Africa is advected poleward and eastward around the polar vortex and entrained into the Aleutian High.

Photochemical calculations along air mass trajectories during ASHOE/MAESA

The practicality of conducting photochemical calculations along trajectories of air masses is investigated. An isentropic trajectory package is used in conjunction with a detailed photochemical model to compare predictions of the mean chemical content of air masses initialized with the Halogen Occultation Experiment (HALOE) data with coincident in situ observations from instruments onboard the ER-2 aircraft. Comparisons are made for 10 ER-2 flights originating from Christchurch, New Zealand, during the May to June and October 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) deployments. Between 54 and 84 coincidences are found, depending on the species measured. Correlations between the ER-2 and HALOE air mass/box model calculations are high (0.56–0.90) for most species considered except for H2O (0.14) and HCl (0.24). Statistically significant low biases in the prediction of HCl, H2O, and OH are found. Kolmogorov-Smirnov (KS) significance tests are used to quantify the agreement between the distribution of species observed by the ER-2 and predicted by the HALOE trajectory/ photochemical model. The model predictions agree with the observed variance within the distributions at significance levels greater than 0.80 (greater than 80% confidence that the predicted and observed variance are identical) for H2O, ClO, O3, and NOy. The impact of computational errors in the trajectory calculations and measurement uncertainty in the computed confidence levels are investigated using Monte Carlo techniques. Computational trajectory errors are found to play a small role in reducing confidence levels. The error analysis shows that the HALOE trajectory/photochemical model calculations reproduce the large-scale variability found in the in situ ER-2 constituent measurements to within the expected uncertainties in the HALOE observations for all species considered. It is concluded that the combined trajectory/photochemical model is an effective tool for interpreting in situ aircraft observations within the global perspective provided by remote satellite measurements.