William L. Grose

Evaluation and intercomparison of global atmospheric transport models using 222Rn and other short‐lived tracers

Simulations of 222Rn and other short-lived tracers are used to evaluate and intercompare the representations of convective and synoptic processes in 20 global atmospheric transport models. Results show that most established three-dimensional models simulate vertical mixing in the troposphere to within the constraints offered by the observed mean 222Rn concentrations and that subgrid parameterization of convection is essential for this purpose. However, none of the models captures the observed variability of 222Rn concentrations in the upper troposphere, and none reproduces the high 222Rn concentrations measured at 200 hPa over Hawaii. The established three-dimensional models reproduce the frequency and magnitude of high-222Rn episodes observed at Crozet Island in the Indian Ocean, demonstrating that they can resolve the synoptic-scale transport of continental plumes with no significant numerical diffusion. Large differences between models are found in the rates of meridional transport in the upper troposphere (interhemispheric exchange, exchange between tropics and high latitudes). The four two-dimensional models which participated in the intercomparison tend to underestimate the rate of vertical transport from the lower to the upper troposphere but show concentrations of 222Rn in the lower troposphere that are comparable to the zonal mean values in the three-dimensional models.

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 trace constituents simulated by a three‐dimensional general circulation model: Comparison with UARS data

Constituent distributions are presented from the NASA Langley three-dimensional general circulation model, incorporating a comprehensive chemistry scheme. A 7-year, gas phase model simulation was performed to investigate long-term model stability. In addition, a 1-year simulation was made using parameterized polar heterogeneous processes and reactions occurring on sulfate aerosols. The results of these simulations are compared with species climatologies and with satellite data sets in order to characterize and evaluate model performance and identify aspects of the chemical scheme requiring improvement. The agreement between the modeled seasonal variation of total ozone and the measurement climatologies is satisfactory but with some differences with respect to the depth and persistence of the southern springtime ozone depletion. Comparisons of the model simulation with observations made from UARS were performed. There is good accord between the microwave limb sounder observations of ozone and the model. Areas of agreement and disagreement are revealed between the model and the cryogenic array etalon spectrometer measurements of HNO 3 and ClONO 2 , suggesting the need for a more detailed representation of sulfate aerosol processes in the model. The comparison between the modeled and the measured partitioning of odd chlorine species is improved in the upper stratosphere by the inclusion of an additional pathway to HCl from the reaction of ClO+OH.

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.

Evolution of Southern Hemisphere spring air masses observed by HALOE

The evolution of Southern Hemisphere air masses observed by the Halogen Occultation Experiment (HALOE) during September 21 through October 15, 1992, is investigated using isentropic trajectories computed from United Kingdom Meteorological Office (UKMO) assimilated winds and temperatures. Maps of constituent concentrations are obtained by accumulation of air masses from previous HALOE occultations. Lagged correlations between initial and subsequent HALOE observations of the same air mass are used to validate the air mass trajectories. High correlations are found for lag times as large as 10 days. Frequency distributions of the air mass constituent concentrations are used to examine constituent distributions in and around the Southern Hemisphere polar vortex.

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.

Large‐scale chemical evolution of the Arctic vortex during the 1999/2000 winter: HALOE/POAM III Lagrangian photochemical modeling for the SAGE III—Ozone Loss and Validation Experiment (SOLVE) campaign

Abstract : The LaRC Lagrangian Chemical Transport Model (LaRC LCTM) is used to simulate the kinematic and chemical evolution of an ensemble of trajectories initialized from Halogen Occultation Experiment (HALOE) and Polar Ozone and Aerosol Measurement (POAM) III atmospheric soundings over the SAGE III-Ozone Loss and Validation Experiment (SOLVE) campaign period. Initial mixing ratios of species which are not measured by HALOE or POAM III are specified using sunrise and sunset constituent CH(4) and constituent PV regressions obtained from the LaRC IMPACT model, a global three dimensional general circulation and photochemical model. Ensemble averaging of the trajectory chemical characteristics provides a vortex-average perspective of the photochemical state of the Arctic vortex. The vortex-averaged evolution of ozone, chlorine, nitrogen species, and ozone photochemical loss rates is presented. Enhanced chlorine catalyzed ozone loss begins in mid-January above 500 K, and the altitude of the peak loss gradually descends during the rest of the simulation. Peak vortex averaged loss rates of over 60 ppbv/day occur in early March at 450 K. Vortex averaged loss rates decline after mid- March. The accumulated photochemical ozone loss during the period from 1 December 1999 to 30 March 2000 peaks at 450 K with net losses of near 2.2 ppmv. The predicted distributions of CH4, O(3), denitrification, and chlorine activation are compared to the distributions obtained from in situ measurements to evaluate the accuracy of the simulations. The comparisons show best agreement when diffusive tendencies are included in the model calculations, highlighting the importance of this process in the Arctic vortex. Sensitivity tests examining the large-scale influence of orographically generated gravity wave temperature anomalies are also presented. Results from this sensitivity study show that mountain-wave temperature perturbations contribute an additional 2-8% O(3) loss during the 1999/2000 winter.

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.

Modeling the Transport of Chemically Active Constituents in the Stratosphere

A three-dimensional, spectral, primitive equation, atmospheric model incorporating comprehensive chemistry has been used to study dynamics and transport processes and to simulate the distribution of ozone and other trace constituents in the stratosphere. Preliminary results from a simulation of the seasonally varying evolution of several important constituents are presented. Comparisons of simulated species distributions with data obtained from satellite experiments demonstrate good agreement in many instances. Of particular interest is the occurrence of incursions or tongues of ozone-rich air parcels from lower latitudes into the polar cap region associated with the displaced polar vortex during a mid-winter stratospheric warming. During the period of enhanced dynamical activity, the model successfully simulates many aspects of observed ozone behavior as well as features described as wave-breaking and irreversible mixing observed in isentropic distributions of potential vorticity inferred from satellite temperature data. Examination of the evolving constituent distributions suggests that episodic transport of ozone into the polar region during wave-breaking events culminates with the production of a high-latitude, spring maximum in total column ozone.

Polar ozone depletion: A three-dimensional chemical modeling study of its long-term global impact

The export of ozone-poor air from the polar region following the breakup of the southern hemisphere polar vortex is examined with a three-dimensional chemistry transport model. This volume of depleted ozone, the result of chemical processing during the southern wintertime and springtime, is long-lived in the lower stratosphere and can affect ozone concentrations at southern middle latitudes following its transport out of the polar region. Two 5-year simulations were performed utilizing the NASA Langley Research Center three-dimensional chemistry transport model. One simulation included only gas phase and sulfate aerosol chemistry, while the second simulation also included reactions occurring on polar stratospheric clouds (PSCs). The model-calculated seasonal variation of southern hemispheric O3, HNO3, and active chlorine as a result of PSC chemistry is in reasonable accord with satellite observations. The model reveals that ozone is transported equatorward following the breakup of the polar vortex to approximately 20°S latitude by the first southern summer following the activation of PSC chemistry. A residual column-integrated ozone depletion of 9% remained by the springtime of the second year. In subsequent years, the southern ozone hole itself increased in depth from a column-integrated depletion of 37% in the first year to 43% in the fifth year with respect to the baseline simulation with no PSC chemistry. The isopleths of column-integrated ozone loss showed a slow equatorward movement during the 5-year run. These model results, in general agreement with earlier model studies using parameterized chemistry, show that a potential exists for a long-term accumulation of ozone loss in the southern polar region and a gradual increase in the global impact of polar ozone depletion. Comparison with satellite and ground-based observations of ozone trends at midlatitudes suggests that ozone dilution may be a contributing factor. Experiments were performed to examine the sensitivity of the rate of local ozone recovery following the breakup of the vortex to the depth and spatial extent of the denitrification of polar air. These simulations revealed that deeper denitrification led to a more persistent column-integrated ozone loss and a slight increase in its equatorward progression.