J. David Neelin
In The Global Circulation of the Atmosphere,
T. Schneider and A. Sobel, Eds, in press, 2006.
Preprint (PDF 1.7 MB)
Introduction.
In tropical dynamics, a leading feature is the interaction of the large-scale circulation
with moist processes arising at small scales. This chapter summarizes a recent
avenue of tropical theory for this interaction, in which constraints from the moist
dynamics can actually simplify large-scale theory under certain conditions. Important
tools in this approach include convective quasi-equilibrium (QE) and the
moist static energy (MSE) budget. Convective QE (Arakawa and Schubert 1974;
Emanuel et al. 1994) is the postulate, used in various forms in a number of convective
parameterizations, that moist convection tends to alter the large-scale temperature
and moisture vertical profiles in a manner that reduces the buoyancy available
to small-scale overturning motions. A further summary of QE may be found in
Chapter 7 in this volume. The aspects required here are the tendency of moist convection
to constrain the temperature profile and to establish a relationship between
temperature and moisture.
In outlining the approach to moist dynamics advocated here, three applications are consideredmonsoons, tropical teleconnections and tropical precipitation changes under global warming. Background for these is provided in the remainder of this section, aiming also to convey the challenges of analysing large-scale flow interacting with moist convection. With this motivation, section 10.2 outlines the moist static energy budget and a useful quantity that arises from it under QE approximations, the gross moist stability. A numerical model, the Quasi-equilibrium Tropical Circulation Model (QTCM), whose formulation has been designed to mesh with this theoretical framework is sketched in section 10.3. This model is used in simulating the phenomena considered, but its particular value is the relative simplicity of analysiseither in pointing to solution features to analyse in observations or more complex models, or generating simpler models from its equations. A consequence of viewing the thermodynamics via the moist static energy budget is the importance of the net flux into the atmospheric column and implications for land-sea contrast, as outlined in section 10.4. This ties directly to the first application, mechanisms setting monsoon poleward boundaries, in section 10.5. Theory for how precipitation anomalies arise in these moist dynamical interactions, along with treatment of cloud feedbacks and land versus ocean cases in this framework, is summarized in sections 10.6 and 10.7. How such diagnostics apply to moist teleconnection theory (section 10.9), and tropical precipitation anomalies under global warming (section 10.10) is then addressed.
We know there are limits of validity to the approximations used here, particularly as one moves to the small scales. The boundaries of the regime of validity have not yet been well determined and this is part of the current direction of this research area. Statements of where limitations or modifications are believed by this author (in discussion with several colleagues noted in the acknowledgments) to be likely to arise are included in the form of postulates, denoted Open question: or Postulate as appropriate. The hope is to summarize phenomena and scales where this particular approach shows promise, while tempting the reader with aspects that are as yet unresolved.
Citation. Moist dynamics of tropical convection zones in monsoons, teleconnections and global warming. In The Global Circulation of the Atmosphere, T. Schneider and A. Sobel, Eds, in press, 2006.