A31E-01 08:00h
Detecting the Recovery of the Antarctic Ozone Hole
The Antarctic ozone hole develops each year and culminates by early Spring. Antarctic ozone values have been monitored since 1979 using satellite observations from the TOMS instrument. The severity of the hole has been assessed from TOMS using the minimum total ozone value from the October monthly mean (depth of the hole) and by calculating the average size during the September-October period. Ozone is mainly destroyed by halogen catalytic cycles, and these losses are modulated by temperature variations in the collar of the polar lower stratospheric vortex. In this presentation, we show the relationships of halogens and temperature to both the size and depth of the hole. Because atmospheric halogen levels are responding to international agreements that limit or phase out production, the amount of halogens in the stratosphere should decrease over the next few decades. Using projections of halogen levels combined with age-of-air estimates, we find that the ozone hole is recovering at an extremely slow rate and that large ozone holes will regularly recur over the next 2 decades. We will show estimates of both when the ozone hole will begin to show first signs of recovery, and when the hole will fully recover to pre-1980 levels.
A31E-02 08:30h
Are Emissions of Restricted Halocarbons in the USA and Canada Still Globally Significant?
The global manufacture of halocarbons regulated by the Montreal Protocol has dropped substantially in response to the January 1, 1996, production phase-out deadline (1994 for halons) for developed (Article 5) countries like the United States and Canada. Contemporary emissions of these ozone-depleting substances (ODS) emanate from ongoing production in developing countries and releases of banked halocarbons world-wide. ODS emissions in developing nations can be appraised from reported production figures, but not so for developed nations where recent manufacture is negligible. Emissions in the United States and Canada are increasingly difficult to estimate because of limited information about bank sizes and release rates in the post-production era. In addition, regional- or national-scale emission estimates should no longer be derived wholly from localized measurements because of the potentially patchy spatial distributions of modern emissions. We estimate ODS emissions in the USA and Canada from $>$1000 simultaneous, in situ measurements each of CO and six restricted halocarbons (CFC-11, CFC-12, CFC-113, methyl chloroform, carbon tetrachloride, and halon-1211) in and above the planetary boundary layer during the 2003 CO$_{2}$ Budget and Regional Airborne - North America (COBRA-NA 2003) study. The data obtained during 87 flight hours are geographically extensive ($>$30,000 km) including two 11,000 km flight circuits across both countries. More than 50 pollution "events" with statistically significant ODS:CO emission ratios were sampled, and for each event we have determined a flux footprint using the Stochastic Time-Inverted Lagrangian Transport (STILT) model. The model also calculates footprint-weighted average population densities and CO fluxes which we convert to footprint-weighted average ODS fluxes using the measured ODS:CO emission ratios. Statistically robust relationships between footprint-averaged ODS fluxes and population densities for several ODS indicate that population-based extrapolations of these relationships to national levels are warranted. Emission estimates for the USA and Canada in 2003 will be presented and compared to the magnitudes of global emissions.
A31E-03 08:45h
On tracer correlations in the troposphere: The case of ethane and propane
To investigate the reasons for and the utility of tropospheric tracer correlations, we examine the interrelationship between ethane and propane on the basis of the observations and global 3-D chemical transport model (GEOS-CHEM) simulations. Ethane and propane have reasonably well defined sources and sinks. We chose to examine the correlation between propane and ethane/propane ratio because it is more sensitive to mixing and is less dependent on temperature than that between ethane and propane. We find that the linear correlation in the log space of propane and ethane/propane ratios are better characterized away from the source regions. Observations at northern mid and high latitudes (TOPSE) and over the tropical Pacific (PEM-Tropics B) are therefore investigated. We show that the correlation generally follows the one determined by chemical (loss) kinetics and that the deviation from the kinetics slope reflects the difference in the strength of mixing relative to chemical loss between ethane and propane. The comparison of observed and simulated correlation slopes can therefore be used to evaluate the coupling of chemistry and transport simulated in the model. At northern mid and high latitudes, the model is generally in agreement with the observations in February and March but simulates a wrong seasonal change of the correlation from March to May. The model appears to overestimate the transport from lower to mid latitudes and the horizontal mixing at high latitudes in May. Over the tropical Pacific, the model reproduces well the observed two-branch structure of the correlation. The minor branch results from different transport and chemistry coupling characteristics in the southern from northern hemisphere. The discrepancy between observed and simulated correlation slope values appears to reflect an underestimate of continental convective transport at northern mid latitudes and an overestimate of latitudinal transport into the tropics. In addition, we show that the correlation can be used to define the subset of observations, for which the coupling between chemistry and transport is simulated reasonably well in the model. Using the subsets of observed and simulated data for inverse modeling would reduce (systematic) biases introduced by systematic model transport errors. We show that in the data we examined, only those for March at mid latitudes and February-April at high latitudes conform to the criterion. On the basis of those data, we find that the model underestimates the emissions of ethane and propane by 14°+-5%.
A31E-04 09:00h
Errors in Tracer-Transport Modeling: Can CTMs Find the Right Answer?
In working with the NASA Global Modeling Initiative's framework for chemistry-transport models (CTMs), it became clear that different algorithms for tracer transport produce surprisingly different results. This difference can only be interpreted as tracer-transport error; it contributes significantly to the overall CTM error in simulating atmospheric chemistry; and it is comparable to the likely errors in meteorological fields, emissions, photochemistry, and sub-grid processes. In simulating the atmospheric distribution of fossil-fuel CO2, it was found that the UCI CTM (Prather second-order moments) and the GMI core CTM (Lin and Rood algorithm) showed differences in (i) stratospheric age-of-air of more than one year and (ii) tropospheric gradients that would alter the inferred CO2 sources/sinks. A new tactic in establishing the correct answer for the tracer-transport problem, and thus in defining CTM error, is taken here: doubling-to-convergence. The premise of this work is that the numerical error in tracer transport (as opposed to the error due to incorrect wind fields, for example) is proportional in some manner to the grid size, i.e., there are no systematic errors in the UCI and GMI CTMs in either coding or basic algorithms. By successive halving of the grid size (1, 1/2, 1/4, 1/8), we show that the CTMs converge. Note that the computational burden for the 1/8-grid case increases by a factor of 4096. In this simple case, we can quantify the transport error in both CTMs, and further we can derive an Aitken acceleration factor that allows a single doubling (1 and 1/2) to be used to produce a best answer with error estimates.
A31E-05 09:15h
A DARE approach for 3D cloud resolving simulations of large scale atmospheric circulation
We present a Diabatic Acceleration and REscaling (DARE) approach that enables large scale atmospheric circulations to be modeled by 3D cloud resolving simulations at a much reduced computational cost. In the DARE approach, the three-dimensional, natural-scale interactions between large scale circulation and deep convection are resolved. As a result, the DARE approach offers important advantages over existent parameterization approaches, including the Cloud Resolving Convective Parameterization, and is expected to be useful in a wide range of problems that involve interactions between large scale circulation, deep convection and associated cloud processes, whose investigation have been long plagued by deficiencies in cumulus/cloud parameterizations. Initial results from equatorial $\beta$-plane simulations using the DARE approach will be presented.
A31E-06 09:30h
Empirically Evaluating Short-Term Atmospheric Predictability
Defining predictability as divergence rate of trajectories initially emanating from neighboring points, in this paper I first estimate actual (i.e., observed) seasonal northern hemisphere mid-tropospheric IPV predictability. I use a simple, model-independent but inherently linear method. Using essentially the same method, I also estimate lower-atmospheric persistence, the rate of trajectory departure from an initial state. The time scale for complete loss of information (i.e., for knowledge of the state along the other trajectory in the divergence problem, or the initial state in the persistence problem, to provide no useful information about the current state of the primary trajectory) is about 4-6 days, between the cyclogenesis and blocking time scales. I next explore the operators that govern the dynamics and that yield the predictability and persistence properties established in the first part. The winter operators display non-normal growth potential; the summer ones do not, and are very nearly neutrally stable.
A31E-07 09:45h
A dynamic similarity model for the subgrid-scale mixing of reactants in LES of atmospheric turbulent reacting flows
The chemical lifetime of reactants in the atmosphere can vary within a wide range of time scales. Some highly reactive compounds such as OH and HO2 radicals have typical time scales smaller than a second. For such species, chemistry can be so active that the chemical compounds react in situ and are hardly transported by the flow. In large eddy simulations (LES) of atmospheric reacting flows, homogeneous and instantaneous mixing of reactants within a grid-cell is normally assumed. However, this assumption can result in large errors in the estimation of the reaction rates due to the fact that highly reactive species can be segregated or pre-mixed at small scales. Since this process occurs at scales smaller than the grid length (sub-grid process), it requires a subgrid parameterization. In this paper, we propose a parameterization for the subgrid chemical transformations. Its formulation relies on the description of the subgrid covariance, i.e. the quantity that accounts for the mixing within a grid-cell, by using similarity arguments. The model is tested in large eddy simulations of a convective atmospheric boundary layer with reactive chemical species at different resolutions. The new model is able to capture the expected changes of magnitude of the subgrid covariance associated with changes in resolution. As a result, the simulations yield resolution-independent overall reaction rates (resolved plus subgrid reaction rates) and concentrations of reactants.