David B. Considine

Past, present, and future modeled ozone trends with comparisons to observed trends

The NASA Goddard Space Flight Center (GSFC) two-dimensional (2-D) model of stratospheric transport and photochemistry has been used to predict ozone changes that have occurred in the past 20 years from anthropogenic chlorine and bromine emissions, solar cycle ultraviolet flux variations, the changing sulfate aerosol abundance due to several volcanic eruptions including the major eruptions of El Chichon and Mount Pinatubo, solar proton events (SPEs), and galactic cosmic rays (GCRs). The same linear regression technique has been used to derive profile and total ozone trends from both measurements and the GSFC model. Derived 2-D model ozone profile trends are similar in shape to the Solar Backscattered Ultraviolet (SBUV) and SBUV/2 trends with highest percentage decreases in the upper stratosphere at the highest latitudes. The general magnitude of the derived 2-D model upper stratospheric negative ozone trend is larger than the trends derived from the observations, especially in the northern hemisphere. The derived 2-D model negative trend in the lower stratosphere at middle northern latitudes is less than the measured trend. The derived 2-D model total ozone trends are small in the tropics and larger at middle and high latitudes, a pattern that is very similar to the Total Ozone Mapping Spectrometer (TOMS) derived trends. The differences between the derived 2-D model and TOMS trends are generally within 1–2% in the northern hemisphere and the tropics. The derived 2-D model trends are generally more in southern middle and high latitudes by 2–4%. Our 2-D model predictions are also compared with the temporal variations in total ozone averaged between 65°S and 65°N over the TOMS observing period (1979–1993). Inclusion of anthropogenic chlorine and bromine increases, solar cycle ultraviolet flux variations, and the changing sulfate aerosol area abundance into our model captures much of the observed TOMS global total ozone changes. The model simulations predict a decrease in ozone of about 4% from 1979 to 1995 due to the chlorine and bromine increases. The changing sulfate aerosol abundances were computed to significantly affect ozone and result in a maximum decrease of about 2.8% in 1992 in the annually averaged almost global total ozone (AAGTO) computed between 65°S and 65°N. Solar ultraviolet flux variations are calculated to provide a moderate perturbation to the AAGTO over the solar cycle by a maximum of ±0.6% (about 1.2% from solar maximum to minimum). Effects from SPEs are relatively small, with a predicted maximum AAGTO decrease of 0.22% in 1990 after the extremely large events of October 1989. GCRs are computed to cause relatively minuscule variations of a maximum of + 0.02% in AAGTO over a solar cycle.

The middle atmospheric response to short and long term solar UV variations: analysis of observations and 2D model results

Abstract We have investigated the middle atmospheric response to the 27-day and 11-yr solar UV flux variations at low to middle latitudes using a two-dimensional photochemical model. The model reproduced most features of the observed 27-day sensitivity and phase lag of the profile ozone response in the upper stratosphere and lower mesosphere, with a maximum sensitivity of +0.51% per 1% change in 205 nm flux. The model also reproduced the observed transition to a negative phase lag above 2 mb, reflecting the increasing importance with height of the solar modulated HO x chemistry on the ozone response above 45 km. The rnodel revealed the general anti-correlation of ozone and solar UV at 65–75 km, and simulated strong UV responses of water vapor and HO x species in the mesosphere. Consistent with previous 1D model studies, the observed upper mesospheric positive ozone response averaged over ±40° was simulated only when the model water vapor concentrations above 75 km were significantly reduced relative to current observations. Including the observed temperature-UV response in the model to account for temperature-chemistry feedback improved the model agreement with observations in the middle mesosphere, but did not improve the overall agreement above 75 km or in the stratosphere for all time periods considered. Consistent with the short photochemical time scales in the upper stratosphere, the model computed ozone-UV sensitivity was similar for the 27-day and 11-yr variations in this region. However, unlike the 27-day variation, the model simulation of the 11-yr solar cycle revealed a positive ozone-UV response throughout the mesosphere due to the large depletion of water vapor and reduced HO x -UV sensitivity. A small negative ozone response at 65–75 km was obtained in the 11-yr simulation when temperature-chemistry feedback was included, In agreement with observations, the model computed a low to middle latitude total ozone phase lag of +3 days and a sensitivity of +0.077% per 1% change in 205 nm flux for the 27-day solar variation, and a total ozone sensitivity of +0.27% for the 11-yr solar cycle. This factor of 3 sensitivity difference is indicative of the photochemical time constant for ozone in the lower stratosphere which is comparable to the 27-day solar rotation period but is much shorter than the 11-yr solar cycle.

Stratospheric effects of Mount Pinatubo aerosol studied with a coupled two‐dimensional model

A new interactive radiative-dynamical-chemical zonally averaged two-dimensional model has been developed at Goddard Space Flight Center. The model includes a linear planetary wave parameterization featuring wave-mean flow interaction and the direct calculation of eddy mixing from planetary wave dissipation. It utilizes family gas phase chemistry approximations and includes heterogeneous chemistry on the surfaces of both stratospheric sulfate aerosols and polar stratospheric clouds. This model has been used to study the effects of the sulfate aerosol cloud formed by the eruption of Mount Pinatubo in June 1991 on stratospheric temperatures, dynamics, and chemistry. Aerosol extinctions and surface area densities were constrained by satellite observations and were used to compute the aerosol effects on radiative heating rates, photolysis rates, and heterogeneous chemistry. The net predicted perturbations to the column ozone amount were low-latitude depletions of 2-3% and northern and southern high-latitude depletions of 10-12%, in good agreement with observations. In the low latitudes a depletion of roughly 1-2% was due to the altered circulation (increased upwelling) resulting from the perturbation of the heating rates, with the heterogeneous chemistry and photolysis rate perturbations contributing roughly 0.5% each. In the high latitudes the computed ozone column depletions were mainly a result of heterogeneous chemistry occurring on the surfaces of the volcanic aerosol. Temperature anomalies predicted were a low-latitude warming peaking at 2.5 K in mid-1992 and high-latitude coolings of 1-2 K which were associated with the high-latitude ozone reductions. The sensitivity of the predicted perturbations to changes in the specification of the planetary wave forcings was examined. The maximum globally averaged column ozone depletions ranged from 2 to 4% for the cases studied.

Simulation of stratospheric tracers using an improved empirically based two-dimensional model transport formulation

We have developed a new empirically based transport formulation for use in our Goddard Space Flight Center (GSFC) two-dimensional chemistry and transport model. In this formulation, we consider much of the information about atmospheric transport processes available from existing data sets. This includes zonal mean temperature, zonal wind, net heating rates, and Eliassen-Palm flux diagnostics for planetary and synoptic-scale waves. We also account for the effects of gravity waves and equatorial Kelvin waves by utilizing previously developed parameterizations in which the zonal mean flow is constrained to observations. This scheme utilizes significantly more information compared to our previous formulation and results in simulations that are in substantially better agreement with observations. The new model transport captures much of the qualitative structure and seasonal variability observed in stratospheric long lived tracers, such as isolation of the tropics and the southern hemisphere winter polar vortex, the well-mixed surf-zone region of the winter subtropics and midlatitudes, and the latitudinal and seasonal variations of total ozone. Model simulations of carbon 14 and strontium 90 are in good agreement with observations, capturing the peak in mixing ratio at 20–25 km and the decrease with altitude in mixing ratio above 25 km. We also find mostly good agreement between modeled and observed age of air determined from SF6 outside of the northern hemisphere polar vortex. However, inside the vortex, the model simulates significantly younger air compared to observations. This is consistent with the model deficiencies in simulating CH4 in this region and illustrates the limitations of the current climatological zonal mean model formulation. The model correctly propagates the phase of the lower stratospheric seasonal cycles in 2CH4+H2O and CO2. The model also qualitatively captures the observed decrease in the amplitude of the stratospheric CO2 seasonal cycle between the tropics and midlatitudes. However, the simulated seasonal amplitudes were attenuated too rapidly with altitude in the tropics. The generally good model-measurement agreement of these tracer simulations demonstrate that a successful formulation of zonal mean transport processes can be constructed from currently available atmospheric data sets.

Is upper stratospheric chlorine decreasing as expected

The monthly-mean total chlorine abundance (Cl T ) at 55 km inferred from HALOE HCl observations increases from 1992 to 1997 and then subsequently decreases. The pre-1997 increase is consistent with surface measurements of Cl T time-lagged by around 6 years. However, a decrease after 1997 is inconsistent with such a time lag, which would predict a peak in late 1999. Accounting for stratospheric mixing processes produces an expected stratosphere Cl T which is in agreement with the HALOE Cl T time series considering the uncertainty in the HALOE data. However, the peak in Cl T is still predicted to occur in later 1999 rather than 1997. We find that reasonable low frequency changes in transport, chlorine partitioning, anomalous buildup of organic chlorine at 55 km, and tropospheric rainout of inorganic chlorine do not reconcile the expected and HALOE Cl T time series. At present, we are unable to explain how upper stratospheric Cl T could decrease as early as 1997.

Chlorine catalyzed destruction of ozone: Implications for ozone variability in the upper stratosphere

The annual mean and the annual amplitude of ozone have been derived from ozone measurements from the SBUV and SBUV/2 spectrometers on board the Nimbus-7 and NOAA-11 satellites. These values differ significantly from values calculated using a two-dimensional model of stratospheric photochemistry and dynamics with standard chemistry. We have found that the differences between the calculated and data-derived values are considerably improved by changing the partitioning in the Cly family to create a larger reservoir of HCl and reducing ClO. This is accomplished by including a channel for the products HCl+O2 from the reaction ClO+OH in addition to the products Cl+HO2. This partitioning also improves the agreement between the calculated and measured values of ClO/HCl ratio.

Two‐dimensional model simulations of the QBO in ozone and tracers in the tropical stratosphere

[1] Meteorological data from the United Kingdom Meteorological Office (UKMO) and constituent data from the Upper Atmospheric Research Satellite (UARS) are used to construct yearly zonal mean dynamical fields for the 1990s for use in the NASA/Goddard Space Flight Center (GSFC) two-dimensional (2-D) chemistry and transport model. This allows for interannual dynamical variability to be included in the model constituent simulations. In this study, we focus on the tropical stratosphere. We find that the phase of quasi-biennial oscillation (QBO) signals in equatorial CH4 and profile and total column O3 data are resolved quite well using this empirically based 2-D model transport framework. However, the QBO amplitudes in the model constituents are systematically underestimated relative to the observations at most levels. This deficiency is probably due in part to the limited vertical resolutions of the 2-D model and the UKMO and UARS input data sets. We find that using different heating rate calculations in the model affects the interannual and QBO amplitudes in the constituent fields, but has little impact on the phase. Sensitivity tests reveal that the QBO in transport dominates the ozone interannual variability in the lower stratosphere, with the effect of the temperature QBO being dominant in the upper stratosphere via the strong temperature dependence of the ozone loss reaction rates. We also find that the QBO in odd nitrogen radicals, which is caused by the QBO modulated transport of NOy, plays a significant but not dominant role in determining the ozone QBO variability in the middle stratosphere. The model mean age of air is in good overall agreement with that determined from tropical lower-middle stratospheric OMS balloon observations of SF6 and CO2. The interannual variability of the equatorial mean age in the model increases with altitude and maximizes near 40 km, with a range of 4–5 years over the 1993–2000 time period. INDEX TERMS: 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334); 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; 3319 Meteorology and Atmospheric Dynamics: General circulation; KEYWORDS: interannual variability, stratospheric circulation, ozone

Space shuttle's impact on the stratosphere: An update

To assess their impact on the stratosphere, a launch scenario of nine shuttles and three Titans per year is simulated in a two-dimensional photochemistry and transport model that includes heterogeneous reactions on a stratospheric sulfate aerosol (SSA) layer and polar stratospheric clouds (PSCs). These rocket launches are predicted to cause small constituent changes in the stratosphere. Maximum total inorganic chlorine enhancements are computed to be about 12 parts per trillion by volume (∼0.4% on a 3 parts per billion by volume background) in the middle to upper stratosphere at northern middle to high latitudes. Maximum ozone decreases associated with these chlorine increases are calculated to be about 0.14% in the middle to upper stratosphere at northern middle to high latitudes. Column ozone decreases are predicted to be a maximum of about 0.05% at northern polar latitudes in the early spring. Model results using (1) gas phase only reactions, (2) gas phase reactions and heterogeneous reactions on the SSA layer, and (3) gas phase reactions and heterogeneous reactions on the SSA layer and PSCs have also been compared with one another. The simulations from these three versions of our model gave annually averaged global total ozone decreases of (1) 0.0056%, (2) 0.010%, and (3) 0.014%. Stratospheric effects from heterogeneous reactions promoted by the alumina emitted from these rockets could be larger than those predicted from the chlorine emissions and need to be investigated further.

Chemical reaction rate sensitivity and uncertainty in a two‐dimensional middle atmospheric ozone model

The NASA Goddard Space Flight Center two-dimensional (2-D) model has been used to study the sensitivity of model ozone concentrations to input chemical reaction rates, and the uncertainty of the model-calculated concentrations. Ozone sensitivity coefficients to changes in chemical reaction rates are defined as logarithmic partial derivatives of the ozone concentration with respect to the chemical reaction rates. These logarithmic derivatives are estimated using a finite difference technique. The ozone sensitivity coefficients to 96 gas phase chemical reactions in the 2-D model show that the ozone concentration is sensitive to the rates of about 25 reactions. The magnitude of the ozone sensitivity coefficients varies from 0.05 to 0.9. The latitude-altitude distributions of the ozone sensitivity coefficients to several reactions are presented. The uncertainty of the model-calculated ozone concentration is evaluated using a guided Monte Carlo (GMC) method from a probability distribution function. The GMC method judiciously combines uncertainty estimates derived from the sensitivity information with Monte Carlo runs of the model. The uncertainty of the model ozone concentration due to uncertainties in gas phase reaction rates is calculated from published chemical rate uncertainties and varies from 10-20% in the lower stratosphere to 30-40% in the mesosphere. Details concerning the GMC method are discussed, and the latitude-altitude distribution of the uncertainty of the model-calculated ozone is presented.

Sensitivity of tracers and a stratospheric aircraft perturbation to two‐dimensional model transport variations

We examine the sensitivity of two-dimensional model simulations of stratospheric tracers to uncertainties in the model transport and explore how such uncertainties impact the simulation of a lower stratospheric perturbation due to high-speed civil transport (HSCT) aircraft emissions. To define the transport uncertainty, we vary the model transport fields so that the resulting tracer simulations roughly bracket the observations. This provides an estimate of the upper and lower limits on realistic transport rates in our two-dimensional (2-D) model. Increasing the advective residual circulation strength or the lower stratospheric vertical diffusion (Kzz) decreases the mean age and residence time of the HSCT emissions and diminishes the negative response in total column ozone globally. Increasing the stratospheric horizontal diffusion (Kyy) either globally or in the tropics only has the opposite effect of increasing the age and emission residence time and enhancing the negative total ozone response. Uncertainties in the mechanical eddy forcing derivation affect both Kyy and the residual circulation simultaneously, resulting in some cancellation of effects. This produces a smaller range of uncertainty in the tracer and perturbation simulations than given by uncertainties in the circulation or Kyy components separately. The model simulations in the lower and middle stratosphere are relatively insensitive to the strength of the mesospheric gravity wave effects and the magnitude of the horizontal diffusive transport across the tropopause. The base model transport compares most favorably with tracer data and gives a global and annual mean steady state HSCT perturbation response in total ozone of −0.62%, assuming a NOx emission index of 5 g/kg, 500 airplanes, and a 10% gas-to-particle conversion of the SO2 emission. For the range of transport uncertainty examined in this study, the global total ozone perturbation response ranges from −0.34% to −0.74%, with a mainly strong correlation between the total ozone response and mean age.