How Well Do Current Models Represent Chemical and Physical Processes in the Upper Troposphere and Lower Stratosphere?

3.1 HOW WELL DO CURRENT MODELS REPRESENT CHEMICAL AND PHYSICAL PROCESSES IN THE UPPER TROPOSPHERE AND LOWER STRATOSPHERE? Donald J Wuebbles 1,*, Jennifer C. Wei1, Douglass Kinnison2 1University of Illinois at Urbana-Champaign, Urbana, IL 2National Center For Atmospheric Research, CO 1. INTRODUCTION disturbances in the subtropical region (Rood et al., 1997). There remain significant uncertainties in Given the complexity of the transport and understanding the changes in atmospheric chemical processes in the upper troposphere and chemical and physical processes affecting ozone lower stratosphere, it is not surprising that the and other constituents in the upper troposphere UT/LS is of great interest towards understanding and lower stratosphere (UT/LS), the first few the capabilities to model the processes acting in kilometers above and below the tropopause. In this region. We are using newest version of the recent years, through new insights in atmospheric three-dimensional chemical-transport of the global dynamics coupled with an increase in constituent atmosphere, Model for OZone And Related measurements, the understanding of the UT/LS chemical Tracers, called MOZART-3, which has a has evolved greatly. It is no longer adequate to complete stratosphere and mesosphere as well as think of the tropopause as being a sharp boundary troposphere, to try to improve our understanding and transport barrier between the troposphere and of the processes that control the chemical stratosphere (Rosenolf, 1995). Wave-induced composition and structure of the upper zonal forces in the extratropical stratosphere troposphere and lower stratosphere, while at the induce a global-scale meridional circulation in same time trying to understand the capabilities of which mass is pulled upward and poleward in the the model. Comparisons are being made, tropics and pushed downward in the extratropics wherever possible, with available observations, to (e.g., Hoor et al., 2002), transporting air from the examine the current theoretical understanding of troposphere to the stratosphere, and then from the the UT/LS region. As a result of these studies, we stratospheric “overworld” to the lowermost hope to better understand the capability of a state- stratosphere, the “middleworld”, where more rapid of-the-art model like MOZART-3 to evaluate the transport is possible between the stratosphere and representation of the model in terms of chemical troposphere along isentropic surfaces. Logan and physical processes in the upper troposphere (1999) suggested that ozone in the lowermost and lower stratosphere (UT/LS) stratosphere builds up during winter in the extratropics as a result of downward transport in the large-scale meridional circulation, and reaches 2. MODEL DESCRIPTION a minimum during autumn as a result of exchange with lower latitudes and the troposphere in spring The MOZART-3 model is developed from the and summer. During all seasons, there is a MOZART-2 model, which was primarily designed transition zone near the tropopause that contains for studies of tropospheric processes. Although air characteristic of both the troposphere and the MOZART-2 extends to 3 hPa (about 35 km), the stratosphere. The extraction of the net transport representation of chemical processes focuses on still remains uncertain, but may occur either by the troposphere, whereas MOZART-3 is extended isentropic transport caused by wave breaking to include the entire troposphere, stratosphere and (Holton et al., 1995) or by diabatic cross-isentropic mesosphere. A short description of MOZART-2 transport associated with synoptic baroclinic and the major differences found in MOZART-3 are —————————————————— given below. A more complete description of MOZART-2 and its capabilities is given in Horowitz * Corresponding author address: Donald J. et al. (2002). The earlier initial version of MOZART Wuebbles, University of Illinois, Department of is described in Brasseur et al. (1998) and Atmospheric Sciences, Urbana, IL 61801; email: Hauglustaine et al. (1998). [email protected]. Using meteorological fields (global wind, The meridional circulation of the stratosphere temperature, humidity, cloud fields, convective is also known as the “Brewer-Dobson circulation” mass fluxes and diffusion parameters) from the based originally on observations of stratospheric NCAR Whole Atmospheric Community Climate water vapor (Brewer, 1949) and stratospheric Model (WACCM, up to approximately 140 ozone (Dobson, 1956). This circulation comprises kilometers), MOZART-3 has a horizontal of upwelling in the tropics and subsidence in the resolution of about 2.8° latitude by 2.8° longitude, middle and high latitudes in the summer time, with 64 levels in the vertical. MOZART-3 can also whereas the circulation is from the tropics into high use assimilated meteorological fields, but have not latitudes in the winter time (Rosenolf, 1995). Since been fully tested. However, we have used National the stratosphere is characterized by strong Center for Environmental Prediction (NCEP) stability to vertical displacement, most tracers reanalyses in studies of PEM-TROPICS data with show horizontal gradients between regions of MOZART-2. The MOZART-2 model includes a mean upward motion and downward motion. N2O mass conserving advection scheme based on the serves as a useful tracer of stratospheric work of Lin and Rood (1996). The chemical transport. Its only sources are in the troposphere, scheme includes 140 chemical and photochemical and it is long lived in the stratosphere. It is also the reactions, as well as washout for 10 soluble primary stratospheric source of reactive nitrogen species. The distributions of more than 50 species NOy, and the observed correlation between NOy are calculated, including O3, HOx, NOx, methane, and N2O in the lower stratosphere is well defined and a number of non-methane hydrocarbons and linear. (NMHC). In addition, the most recent version of Figure 1 is one example of the characteristic MOZART-2 includes a detailed representation of shape for monthly zonal average distribution of the mass mixing ratio of sulfate, black carbon, and N O, as measured by the Cryogenic Limb Array ammonium (Tie et al., 2001) and, more recently, 2 Etalon Spectrometer (CLAES) on the Upper mineral dust aerosol abundance. These aerosols Atmosphere Research Satellite (UARS) (Douglass are primarily derived from precursor emissions at et al., 1999). The characteristic shape for the long- the surface. Heterogeneous chemical processes lived tracers, such as N O, is its isopleths bulging are also included on these aerosol types. Although 2 upward in the tropics and poleward/downward in concerns about mass consistency in the model the extratropics. Especially in the winter or spring have been fully resolved in the latest version, the hemisphere, there are strong gradients in the existing MOZART-2 model still tends to subtropics, relatively flat isopleths in middle overestimate ozone in the upper troposphere at latitudes, and strong gradients near 60 degrees in high northern latitudes, probably as a result of too latitude in the winter hemisphere. The model much cross-tropopause transport of ozone results (Figure 2) show similar features to the although too little vertical resolution in the observations (Douglass et al., 1999; Strahan et al., tropopause region may also contribute (Horowitz 1999). Mean N O on pressure levels provide a et al., 2002). Nonetheless, in general, the model 2 gross measure of the balance between transport does an excellent job of capturing the by the mean winds and horizontal mixing. Such characteristics of tropospheric chemistry as, the maximum N O at any pressure level is processes. 2 found in the tropics, and the N O decreases with With the extension into the upper atmosphere, 2 altitude at all latitudes. There is outflow from the the MOZART-3 model currently includes 90 tropics at all levels associated with upward chemical species and approximately 250 transport, and the tracer distribution at middle photochemical reactions. Essentially all of the latitudes depends upon a balance between the chemical constituents and associated chemical mean horizontal and vertical transport as well as reactions and photochemical reactions important mixing across the subtropics. In the Northern to the stratosphere and mesosphere have been Hemisphere (NH), the N O contours at middle included in MOZART-3. Heterogeneous processes 2 latitudes are flatter during winter season (e.g., on sulfate aerosols and polar stratospheric clouds January) than Fall (e.g., September) and the high- (Type 1a, 1b, and 2) are included. Denitrification latitude contours show evidence of winter decent. and dehydration processes are also represented. The sharpness of the horizontal gradient in the NH winter is produced by the combined influence of the residual circulation and the horizontal mixing 3. RESIDUAL CIRCULATION AND MIXING IN driven by planetary wave transport (Douglass et THE LOWERMOST STRATOSPHERE al., 1999). This gradient contrasts with the more constant horizontal gradient seen in the NH the winter than in the fall (one example flight is autumn. shown in Figure 3). (a) (a) (b) (b) Figure 2. Zonal average N2O distribution calculated from MOZART 3 for (a) January and (b) September. Figure 1. Averaged N2O distribution based on CLAES observations from (a) September 18 to 24, 1992 and (b) from January 6 to 13, 1993 (right) (Douglass et al., 1999) A sense of the importance of the transport processes can be gained by considering

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