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P. L. Read, P. B. Rhines, and A. A. White

Abstract

A scatter diagram may be constructed by choosing an appropriate closed or open horizontal curve in physical space and plotting the value of any scalr quantity q against the geostrophic streamfunction ψ for each data point on the curve. The area enclosed on the scatter diagram is equal to the net geostrophic advective flux of q across the chosen curve in physical space. When q is the (quasi-geostrophic) potential vorticity Q, and suitable normalizations are adopted, this result may he exploited to derive measures of departure from free-mode form Q)= Q(ψ) along the curve in physical space. For a certain class of open space curves, an appropriate measure is the width-to-length ratio of the circuit in (ψ, Q) space. Most scatter diagrams that have appeared in the literature included the (ψ, Q) points corresponding to all the data or grid points within a given horizontal domain. The significance of the area enclosed on these diagrams is less clear, but the spread about some curve Q) = Q(ψ) is evidently a qualitative measure of the extent to which the flow deviates from free-mode form. For steady or time-averaged flows which are approximately of this form, the gradient dQ/dψ of the scatter diagram may be used to infer some properties of the forcing and dissipative processes acting. When dissipation is principally due to Qtransfer by transient eddy motion (or viscosity), the key diagnostic relation iswhere S is the potential vorticity forcing, K the lateral eddy (or viscous) v the horizontal velocity, and the integrals are taken over and around any region enclosed by a mean streamline. Hence dQ/dψis often negative. corresponding to two common properties of quasi-geostrophic circulations: that the eddy motion (or viscosity) transport Q down its mean gradient (K > 0) and that the circulation integral have the same sign as the potential vorticity forcing. Two sets of examples, both involving (Q,ψ) scatter diagrams constructed from numerically simulated data, are presented. One relates to steady baroclinic wave motion in a rotating annulus system, and the other to the time-averaged circulation in an ocean basin.

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P. L. Read, N. P. J. Thomas, and S. H. Risch

Abstract

A series of numerical simulations of steady, thermally stratified flow of a Boussinesq, incompressible fluid in a rotating, cylindrical fluid annulus were carried out over ranges of spatial resolution, grid stretch, and rotation rate. A range of different numerical advection schemes were used for the representation of heat transport, including a conventional conservative second-order Eulerian scheme and three different variants of a semi-Lagrangian scheme used either for temperature advection alone, or for both thermal and momentum advection. The resulting simulations were compared both with each other, and with high precision measurements of velocity, temperature, and total heat transport in the laboratory. The performance of the semi-Lagrangian scheme was found to be quite strongly sensitive to the spatial interpolation algorithm. A basic tensor cubic scheme generally produced good simulations of steady 2D and 3D flows, although the somewhat more accurate tensor quintic scheme (which is, however, also significantly more expensive) appeared to offer some detectable improvements in accuracy and performance in some cases. A split cubic scheme (which is computationally cheaper but formally less accurate) gave generally poor results in practice and is not recommended. In all cases considered, both the fully Eulerian and most forms of the semi-Lagrangian schemes gave good quantitative agreement with the laboratory measurements when extrapolated to very high resolution. Some significant systematic errors in the simulated heat transport and zonal momentum were found with all schemes, however, when run at moderate (though by no means very low) resolution. The semi-Lagrangian schemes had a tendency to overestimate heat transport relative to the laboratory measurements compared with the Eulerian schemes, but the latter tended to overestimate zonal momentum relative to the laboratory flows compared with the fully semi-Lagrangian simulations.

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J. W. Waters, W. G. Read, L. Froidevaux, R. F. Jarnot, R. E. Cofield, D. A. Flower, G. K. Lau, H. M. Pickett, M. L. Santee, D. L. Wu, M. A. Boyles, J. R. Burke, R. R. Lay, M. S. Loo, N. J. Livesey, T. A. Lungu, G. L. Manney, L. L. Nakamura, V. S. Perun, B. P. Ridenoure, Z. Shippony, P. H. Siegel, R. P. Thurstans, R. S. Harwood, H. C. Pumphrey, and M. J. Filipiak

Abstract

The Microwave Limb Sounder (MLS) experiments obtain measurements of atmospheric composition, temperature, and pressure by observations of millimeter- and submillimeter-wavelength thermal emission as the instrument field of view is scanned through the atmospheric limb. Features of the measurement technique include the ability to measure many atmospheric gases as well as temperature and pressure, to obtain measurements even in the presence of dense aerosol and cirrus, and to provide near-global coverage on a daily basis at all times of day and night from an orbiting platform. The composition measurements are relatively insensitive to uncertainties in atmospheric temperature. An accurate spectroscopic database is available, and the instrument calibration is also very accurate and stable. The first MLS experiment in space, launched on the (NASA) Upper Atmosphere Research Satellite (UARS) in September 1991, was designed primarily to measure stratospheric profiles of ClO, O3, H2O, and atmospheric pressure as a vertical reference. Global measurement of ClO, the predominant radical in chlorine destruction of ozone, was an especially important objective of UARS MLS. All objectives of UARS MLS have been accomplished and additional geophysical products beyond those for which the experiment was designed have been obtained, including measurement of upper-tropospheric water vapor, which is important for climate change studies. A follow-on MLS experiment is being developed for NASA’s Earth Observing System (EOS) and is scheduled to be launched on the EOS CHEMISTRY platform in late 2002. EOS MLS is designed for many stratospheric measurements, including HOx radicals, which could not be measured by UARS because adequate technology was not available, and better and more extensive upper-tropospheric and lower-stratospheric measurements.

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