Anne R. Douglass

Stratosphere-troposphere exchange

In the past, studies of stratosphere-troposphere exchange of mass and chemical species have mainly emphasized the synoptic- and small-scale mechanisms of exchange. This review, however, includes also the global-scale aspects of exchange, such as the transport across an isentropic surface (potential temperature about 380 K) that in the tropics lies just above the tropopause, near the 100-hPa pressure level. Such a surface divides the stratosphere into an “overworld” and an extratropical “lowermost stratosphere” that for transport purposes need to be sharply distinguished. This approach places stratosphere-troposphere exchange in the framework of the general circulation and helps to clarify the roles of the different mechanisms involved and the interplay between large and small scales. The role of waves and eddies in the extratropical overworld is emphasized. There, wave-induced forces drive a kind of global-scale extratropical “fluid-dynamical suction pump,” which withdraws air upward and poleward from the tropical lower stratosphere and pushes it poleward and downward into the extratropical troposphere. The resulting global-scale circulation drives the stratosphere away from radiative equilibrium conditions. Wave-induced forces may be considered to exert a nonlocal control, mainly downward in the extratropics but reaching laterally into the tropics, over the transport of mass across lower stratospheric isentropic surfaces. This mass transport is for many purposes a useful measure of global-scale stratosphere-troposphere exchange, especially on seasonal or longer timescales. Because the strongest wave-induced forces occur in the northern hemisphere winter season, the exchange rate is also a maximum at that season. The global exchange rate is not determined by details of near-tropopause phenomena such as penetrative cumulus convection or small-scale mixing associated with upper level fronts and cyclones. These smaller-scale processes must be considered, however, in order to understand the finer details of exchange. Moist convection appears to play an important role in the tropics in accounting for the extreme dehydration of air entering the stratosphere. Stratospheric air finds its way back into the troposphere through a vast variety of irreversible eddy exchange phenomena, including tropopause folding and the formation of so-called tropical upper tropospheric troughs and consequent irreversible exchange. General circulation models are able to simulate the mean global-scale mass exchange and its seasonal cycle but are not able to properly resolve the tropical dehydration process. Two-dimensional (height-latitude) models commonly used for assessment of human impact on the ozone layer include representation of stratosphere-troposphere exchange that is adequate to allow reasonable simulation of photochemical processes occurring in the overworld. However, for assessing changes in the lowermost stratosphere, the strong longitudinal asymmetries in stratosphere-troposphere exchange render current two-dimensional models inadequate. Either current transport parameterizations must be improved, or else, more likely, such changes can be adequately assessed only by three-dimensional models.

Overview of the EOS aura mission

Aura, the last of the large Earth Observing System observatories, was launched on July 15, 2004. Aura is designed to make comprehensive stratospheric and tropospheric composition measurements from its four instruments, the High Resolution Dynamics Limb Sounder (HIRDLS), the Microwave Limb Sounder (MLS), the Ozone Monitoring Instrument (OMI), and the Tropospheric Emission Spectrometer (TES). With the exception of HIRDLS, all of the instruments are performing as expected, and HIRDLS will likely be able to deliver most of their planned data products. We summarize the mission, instruments, and synergies in this paper.

A Comparison of the Lower Stratospheric Age-Spectra Derived from a General Circulation Model and Two Data Assimilation Systems

[1] We use kinematic and diabatic back trajectory calculations, driven by winds from a general circulation model (GCM) and two different data assimilation systems (DAS), to compute the age spectrum at three latitudes in the lower stratosphere. The age spectra are compared to chemical transport model (CTM) calculations, and the mean ages from all of these studies are compared to observations. The age spectra computed using the GCM winds show a reasonably isolated tropics, in good agreement with observations; however, the age spectra determined from the DAS differ from the GCM spectra. For the DAS diabatic trajectory calculations there is too much exchange between the tropics and midlatitudes. The age spectrum is thus too broad, and the tropical mean age is too old as a result of mixing older midlatitude air with tropical air. Likewise, the midlatitude mean age is too young because of the in-mixing of tropical air. The DAS kinematic trajectory calculations show excessive vertical dispersion of parcels in addition to excessive exchange between the tropics and midlatitudes. Because air is moved rapidly to the troposphere from the vertical dispersion, the age spectrum is shifted toward the young side. The excessive vertical and meridional dispersion compensate in the kinematic case, giving a reasonable tropical mean age. The CTM calculation of the age spectrum using the DAS winds shows the same vertical and meridional dispersive characteristics of the kinematic trajectory calculation. These results suggest that the current DAS products will not give realistic trace gas distributions for long integrations; they also help explain why the extratropical mean ages determined in a number of previous DAS-driven CTMs are too young compared with observations. Finally, we note that trajectory-generated age spectra show significant age anomalies correlated with the seasonal cycles. These anomalies can be linked to year-to-year variations in the tropical heating rate. The anomalies are suppressed in the CTM spectra, suggesting that the CTM transport scheme is too diffusive.

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.

Interhemispheric differences in springtime production of HCl and ClONO2 in the polar vortices

UARS observations of O3 and ClO (Microwave Limb Sounder), ClONO2 and HNO3 (Cryogenic Array Etalon Spectrometer), NO, NO2, and HCl (Halogen Occultation Experiment), and model calculations are used to produce an exposition of the different processes through which the reservoir gases ClONO2 and HCl are reformed at the end of the polar winter. Comparison of the observations within the polar vortices shows that HCl increases more rapidly in the Antarctic vortex in spring than in the Arctic vortex. Model analysis shows that this occurs because the O3 concentrations in the southern vortex fall well below those in the northern vortex. The Cl/ClO fraction calculated for the southern hemisphere is therefore up to 30 times higher, leading to rapid HCl formation by Cl + CH4. The concentrations of NO observed by HALOE are substantially lower for the northern hemisphere than for the southern hemisphere, even for similar values of the concentration of HNO3 and the production of NOX from HNO3 through photolysis and reaction with OH. This is consistent with the dependence of the NO/NOX ratio on the O3 concentration, i.e., the daytime production rate of NO2 via NO + O3 is reduced, leading to higher NO in the southern hemisphere. This higher concentration of NO also contributes to the rapid HCl increase as Cl production from ClO + NO is enhanced.

Trends in Stratospheric Ozone: Lessons Learned from a 3D Chemical Transport Model

Abstract Stratospheric ozone is affected by external factors such as chlorofluorcarbons (CFCs), volcanoes, and the 11-yr solar cycle variation of ultraviolet radiation. Dynamical variability due to the quasi-biennial oscillation and other factors also contribute to stratospheric ozone variability. A research focus during the past two decades has been to quantify the downward trend in ozone due to the increase in industrially produced CFCs. During the coming decades research will focus on detection and attribution of the expected recovery of ozone as the CFCs are slowly removed from the atmosphere. A chemical transport model (CTM) has been used to simulate stratospheric composition for the past 30 yr and the next 20 yr using 50 yr of winds and temperatures from a general circulation model (GCM). The simulation includes the solar cycle in ultraviolet radiation, a representation of aerosol surface areas based on observations including volcanic perturbations from El Chichon in 1982 and Pinatubo in 1991, and t...

A three‐dimensional simulation of the ozone annual cycle using winds from a data assimilation system

The wind fields from the NASA Goddard stratospheric data assimilation procedure are used in a three-dimensional chemistry and transport model to produce an ozone simulation for the year September 11, 1991 to September 10, 1992. Photochemical production and loss are taken from the Goddard two-dimensional model. The calculated ozone is compared with observations from the total ozone mapping spectrometer (TOMS) onboard Nimbus 7 and the microwave limb sounder on the upper atmospheric research satellite. Although the model total ozone is about 50 Dobson units (DU; =2.69 × 10−16 molecules cm−2) lower than TOMS in the tropics and up to 70 DU higher than TOMS in middle to high latitudes, the simulated ozone fields reproduce many of the features in the observations. Even at the end of this integration, the synoptic features in the modeled total ozone are very similar to TOMS observations, indicating that the model maintains realistic values for the horizontal and vertical gradients, at least in the lower stratosphere. From this good comparison between model and observations on timescales ranging from days to months, we infer that the transport driven by the assimilated wind fields closely approximates the actual atmospheric transport. Therefore the assimilated winds are useful for applications which may be sensitive to the lower stratospheric transport.

Two‐dimensional and three‐dimensional model simulations, measurements, and interpretation of the influence of the October 1989 solar proton events on the middle atmosphere

The very large solar proton events (SPEs) which occurred from October 19 to 27, 1989, earned substantial middle-atmospheric HOx and NOx constituent increases. Although no measurements of HOx increases were made during these SPEs, increases in NO were observed by rocket instruments which are in good agreement with calculated NO increases from our proton energy degradation code. Both the HOx and the NOx increases can cause ozone decreases; however, the HOx-induced ozone changes are relatively short-lived because HOx species have lifetimes of only hours in the middle atmosphere. Our two-dimensional model, when used to simulate effects of the longer-lived NOx, predicted lower-stratospheric polar ozone decreases of greater than 2% persisting for one and a half years past these SPEs. Previous three-dimensional model simulations of these SPEs (Jackman et al., 1993) indicated the importance of properly representing the polar vortices and warming events when accounting for the ozone decreases observed by the solar backscattered ultraviolet 2 instrument two months past these atmospheric perturbations. In an expansion of that study, we found that it was necessary to simulate the November 1, 1989, to April 2, 1990, time period and the November 1, 1986, to April 2, 1987, time period with our three-dimensional model in order to more directly compare to the stratospheric aerosol and gas experiment (SAGE) II observations of lower stratospheric NO2 and ozone changes between the end of March 1987 and 1990 at 70°N. Both the NOx increases from the October 1989 SPEs and the larger downward transport in the 1989–1990 northern winter compared to the 1986–1987 northern winter contributed to the large enhancements in NO2 in the lower stratosphere observed in the SAGE II measurements at the end of March 1990. Our three-dimensional model simulations predict smaller ozone decreases than those observed by SAGE II in the lower stratosphere near the end of March 1990, indicating that other factors, such as heterogeneous chemistry, might also be influencing the constituents of this region.

Using transport diagnostics to understand chemistry climate model ozone simulations

We use observations of N2O and mean age to identify realistic transport in models in order to explain their ozone predictions. The results are applied to 15 chemistry climate models (CCMs) participating in the 2010 World Meteorological Organization ozone assessment. Comparison of the observed and simulated N2O, mean age and their compact correlation identifies models with fast or slow circulations and reveals details of model ascent and tropical isolation. This process-oriented diagnostic is more useful than mean age alone because it identifies models with compensating transport deficiencies that produce fortuitous agreement with mean age. The diagnosed model transport behavior is related to a models ability to produce realistic lower stratosphere (LS) O3 profiles. Models with the greatest tropical transport problems compare poorly with O3 observations. Models with the most realistic LS transport agree more closely with LS observations and each other. We incorporate the results of the chemistry evaluations in the Stratospheric Processes and their Role in Climate (SPARC) CCMVal Report to explain the range of CCM predictions for the return-to-1980 dates for global (60°S-60°N) and Antarctic column ozone. Antarctic O3 return dates are generally correlated with vortex Cly levels, and vortex Cly is generally correlated with the models circulation, although model Cl chemistry and conservation problems also have a significant effect on return date. In both regions, models with good LS transport and chemistry produce a smaller range of predictions for the return-to-1980 ozone values. This study suggests that the current range of predicted return dates is unnecessarily broad due to identifiable model deficiencies.

Application of a monotonic upstream-biased transport scheme to three-dimensional constituent transport calculations

Abstract The application of van Leers scheme, a monotonic, upstream-biased differencing scheme, to three-dimensional constituent transport calculations is shown. The major disadvantage of the scheme is shown to be a self-limiting diffusion. A major advantage of the scheme is shown to be its ability to maintain constituent correlations. The scheme is adapted for a spherical coordinate system with a hybrid sigma-pressure coordinate in the vertical. Special consideration is given to cross-polar flow. The vertical wind calculation is shown to be extremely sensitive to the method of calculating the divergence. This sensitivity implies that a vertical wind formulation consistent with the transport scheme is essential for accurate transport calculations. The computational savings of the time-splitting method used to solve this equation are shown. Finally, the capabilities of this scheme are illustrated by an ozone transport and chemistry model simulation.