Models of the Middle Atmosphere

9 Models of the Middle Atmosphere 3.9.1 General Circulation Models 

Three-dimensional models which provide solutions to some form of the primitive equations outlined in Section 3.3 are called general circulation models (GCMs). These models can provide insight on the coupling between dynamical and radiative processes in the atmosphere. They resolve large-scale waves and synoptic eddies, and include state-of-the- 

Ideally, the solution of the primitive equations for specified external constraints (e.g., the solar irradiance at the top of the atmosphere) and appropriate boundary conditions (e.g., observed sea-surface temperature) should provide a comprehensive representation in space and time of the atmospheric dynamical system. In practice, however, limitations in computer capabilities impose limits on the spatial resolution of these models, so that small-scale processes, rather than being explicitly reproduced, must be parameterized. The uncertainties associated with these physical parameterizations (e.g., boundary layer exchanges, convection, clouds, gravity wave breaking, etc.) often limit the overall accuracy in the model results. 

Chemical-transport models (CTMs) simulate the formation, transport, and destruction of chemical compounds in the atmosphere. These models can be directly coupled to the general circulation models 

The first attempts to simulate the three-dimensional global distribution of chemically active species in the stratosphere have been based on the on-line approach. Hunt (1969) introduced ozone in the early GCM developed at GFDL, and studied the transport of this compound, using a simple parameterization for its photochemistry. Somewhat more sophisticated coupled models for ozone and related compounds have been developed by Cunnold et al. (1975) and Schlesinger and Mintz (1979). 

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The aim of this study is to describe the dynamical consequences (e.g. thermal response) of the middle atmosphere, which are directly related to the ozone distribution by using a mechanistic three-dimensional model of the middle atmosphere and the 3-D GEOS ozone data. 

In this study the numerical simulations were performed with a 3-D mechanistic global Cologne Model of the Middle Atmosphere (COMMA) based on the primitive equations expressed in spherical coordinates for the horizontal and log-pressure coordinates in the vertical direction. The model equations are solved on the basis of an explicit numerical scheme (leapfrog) with a fixed time step of 450 sec. To avoid separate evolution at even and odd time steps, a Robert time filter is used. 

To adjust the GEOS ozone data for simulation with the 3-D model of the middle atmosphere the following steps were performed. After three lower (850, 700 and 600 hPa) ozone data levels were removed from consideration due to a number of indefinite means, the GEOS ozone data were interpolated to vertical levels and horizontal grid of the 3-D mechanistic model. 

Dameris M., U.Berger, G.Guenter and A.Ebel, (1991) The ozone hole dynamical consequences as simulated with a three-dimensional model of the middle atmosphere Ann. Geophysicae 9, p. 661 -668,... 

The sections of this chapter deal with the following elements of atmospheric dynamics vertical structure of the atmosphere (Section 3.2), fundamental equations of atmospheric motions (Section 3.3), transport of chemical constituents and the relative importance of dynamical and chemical effects on photochemical species (Section 3.4), atmospheric waves (Section 3.5), the mean meridional circulation and the use of the transformed Eulerian formalism to illustrate the roles of mean meridional and eddy transports (Section 3.6), the important role of wave transience and dissipation (Section 3.7), vertical transport by molecular diffusion in the thermosphere (Section 3.8), and finally, models of the middle atmosphere (Section 3.9). 

Figure 3.38. Comparison of the zonal wind calculated for January by several general circulation models of the middle atmosphere with climatological values. From Pawson et al. (2000).

Brasseur, G.P., M.H. Hitchman, S. Walters, M. Dymek, E. Falise, and M. Pirre, An interactive chemical dynamical radiative two-dimensional model of the middle atmosphere. J Geophys Res 95, 5639, 1990. 

Huang, T.Y.W., and G.P. Brasseur, The effect of long-term solar variability in a two-dimensional interactive model of the middle atmosphere. J Geophys Res 98, 20,410, 1993. 

Important alterations to the global distribution of stratospheric ozone are currently predicted by the best available models which synthesize the chemistry, radiation and dynamics of the middle atmosphere. While these predictions have fluctuated significantly since the first crude estimates were offered in the mid-1970s [31, progress in many fields has brought a growing realization that the stratosphere may well be the first natural system to submit to the scientific method. 

Fig. 1. Box model of the middle and lower atmosphere indicating the major removal processes for source molecules carried upward from the surface. The small complement of molecules which actually reach the lower stratosphere then become the precursors for free radical chain carrying reactions which are terminated by radical-radical recombination reactions to form reservoir molecules. Downward transport to the tropopause of these reservoir molecules maintains mass continuity.

Garcia, R.R., and S. Solomon, A numerical model of the zonally averaged dynamical and chemical structure of the middle atmosphere. J Geophys Res 88, 1379, 1983. 

Mahlman, J.D., and L.J. Umscheid, Dynamics of the middle atmosphere Successes and problems of the GFDL SKIHI general circulation model, in Proc of the U.S.-Japan Seminar on Middle Atmosphere Dynamics, Terra Scientific Pub., Tokyo, 1983. 

Manzini, E., N.A. McFarlane, and C. McLandress, Impact of the Doppler-spread parameterization on the simulation of the middle atmosphere circulation using the MA/ECHAM4 general circulation model. J Geophys Res 102, 25,751, 1997. 

Brasseur, G., The response of the middle atmosphere to long-term and short-term solar variability A two-dimensional model. J Geophys Res 98, 23,079, 1993. 

Steil, B. Briihl, C. Manzini, E. Cmtzen, P.J. Lelieveld, J. Rasch, P.J. Roeckner, E. Kruger, K., 2003 A New Interactive Chemistry-Climate Model 1. Present-Day Climatology and Interannual Variability of the Middle Atmosphere Using the Model and 9 years of HALOE/UARS Data , in Journal of Geophysical Research, 108, doi 10.1029/2002JD002971. 

The Oxford results have recently been used by Solomon and Garcia to examine the distribution of long-lived tracers and chlorine species in the middle atmosphere. This important paper has crystallized many of the issues relating to the hydroxyl and chlorine species, particularly the relationship between the variability in methane concentration and the variability in CIO. Figure 10 summarizes the correlation between the two-dimensional model of Solomon and Garcia [18] and the Oxford CH4 maps. These results are then used to define the expected variability in local CIO concentrations reported by in situ observations. 

Mills F. P. (1998) 1. Observations and photochemical modeling of the Venus middle atmosphere. 11. Thermal infrared spectroscopy of Europa and Callisto. Doctoral Thesis, California Institute of Technology, Pasadena, CA. 

Numerical models have been used to predict the potential ozone depletion in response to the emission of halocarbons, based on different plausible scenarios. All of these models indicate that the time required for the middle atmosphere to respond to surface emissions of these halocarbons is very long (several decades). In particular, even with the measures taken to reduce or phase-out the emissions of the CFCs and other halocarbons, it is expected that the Antarctic ozone hole will be observed each spring (September-October) at least until the year 2040. It should also be noted that the halocarbons are active in the infrared, and contribute to the greenhouse effect. 

Grose, W.L., J.E. Nealy, R.E. Turner, and W.T. Blackshear, Modeling the transport of chemically active constituents in the stratosphere, in Transport Processes in the Middle Atmosphere. G. Visconti, and R. Garcia, eds., 229, D. Reidel, Massachusetts, 1987. 

Rose, K., and G. P. Brasseur, A three-dimensional model of chemically active trace species in the middle atmosphere during disturbed winter conditions. J Geophys Res 94, 16,387, 1989. 

Band models are useful for practical applications but have some basic limitations for example, they are only capable of yielding low spectral resolution and they do not always account for the effects of band wings. Kiehl and Ramanathan (1983) have shown, for example, that in the case of the CO2 band at 15 /tm the Goody and Malkmus formulation can only yield accurate results if the spectral interval 6n is less than 10 cm-1, and that the wide band models are probably more suitable for radiative transfer formulations in the middle atmosphere. With the development of fast computer systems, however, band models are becoming obsolete for many applications (e.g., retrieval of temperature and chemical constituents from satellite measurements) and are progressively being replaced by more accurate line-by-line integrations. 

The determination of the O2 photodissociation frequency in the Schumann-Runge bands is obtained from a computation including the complex rotational structure of the band system. A major difficulty in the calculation of Schumann-Runge band photolysis in the middle atmosphere, in addition to requirements for high spectral resolution, arises from the temperature dependence of absorption cross sections. Polynomial expressions have been derived to reproduce the temperature variation provided by the line-by-line calculations (Minschwaner et al., 1992) this kind of approach, however, is generally not feasible to be implemented in detailed middle atmosphere models, but can be used to develop more efficient broad-band parameterizations (see Section 4.7.3). 

Apruzese, J.P., D.F. Strobel, and M R. Schoeberl, Parameterization of IR cooling in the middle atmosphere dynamics model, 2. Non LTE radiative transfer and the globally averaged temperature of the mesosphere and lower thermosphere. J Geophys Res 89, 4917, 1984. 

Siskind, D.E., K. Minschwaner, and R.S. tick man, Photodissociation of oxygen and water vapor in the middle atmosphere Comparison of numerical methods and impact on modeled ozone and hydroxyl. Geophys Res Lett 21, 863, 1994. 

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