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T. N. Palmer
and
C-P. F. Hsu

Abstract

In a, series of idealized numerical experiments, Butchart et al (1982) have recently established that the configuration of the mean zonal wind occurring immediately before the wavenumber-2 major stratospheric warming of February 1979 was crucial in subsequently focusing upward propagating planetary wave-activity into the high latitude stratosphere. In this sense, it was concluded that the stratospheric circumpolar flow should evolve to some preconditioned state before a wavenumber-2 major warming could occur.

In the present paper, the mechanisms responsible for the transition of the circumpolar flow from its normal midwinter state to this preconditioned state are investigated through a combination of observational numerical and theoretical studies. For the 1978–79 winter, this transition occurred during the substantial wavenumber-1 minor warming of January 1979, and the characteristic structure associated with the preconditioned mean zonal flow was established four days after the peak of this warming, during a period of intense high latitude acceleration. This latter phenomenon is referred to as a stratospheric sudden cooling. Observations of Eliassen-Palm flux cross-sections indicate that while wave, zonal mean-flow interaction theory could account for the qualitative evolution of the circumpolar flow during the warming, substantial nonlinear wave interactions were active during the cooling period, and these interactions significantly influenced the evolution of the circumpolar flow.

In a series of numerical experiments using a truncated semi-spectral model, we show that this sudden cooling phenomenon can be realistically reproduced in an idealized integration in which wave-wave interactions are present. By contrast, we were unable to simulate this phenomenon with these interactions removed.

Two different mechanisms are put forward to account for these nonlinearities. One mechanism is that of wave-breaking and associated potential vorticity mixing, as suggested by McIntyre (1982). The second mechanism is based on the notion of wave-activity, forced in the troposphere, propagating relative to isopleths of potential vorticity of some zonally asymmetric basic state.

Results of the observational and numerical study suggest that the first mechanism was dominant, and that potential vorticity mixing in the outer regions of the polar vortex was central to the process of preconditioning. Nevertheless, we believe that the second mechanism plays an important role in the dynamics of the stratosphere.

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S. A. Changnon
,
F. A. Huff
, and
C. F. Hsu

Use of weather modification by farm groups, state agencies, and power companies to perform operational projects continues to expand. Seven percent of the United States experienced cloud seeding during 1977. The major stakeholders—those paying, those performing the seeding, and the scientific community—have all converged on the need to evaluate operational projects. Major assessments of the national situation have recommended that carefully conducted operational projects can be a source of useful scientific information if designed, operated, and evaluated properly. A project has been launched to develop statistical-physical evaluation techniques for operational projects.

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T. Dunkerton
,
C-P. F. Hsu
, and
M. E. McIntyre

Abstract

Some new diagnostics are presented for a wavenumber-2 sudden warming, simulated by a version of Holton's semi-spectral, primitive-equation model. First, Eliassen-Palm cross sections exhibiting the Eliassen-Palm (EP) planetary-wave flux together with contours of the corresponding flux divergence, are presented for selected days of the simulation. Second, a description of zonal-mean-flow evolution in the model, simpler than the conventional Eulerian-mean description and qualitatively like Lagrangian- mean descriptions in some respects, is constructed from the transformed Eulerian-mean equations presented by Andrews and McIntyre (1976). In this description the mean warming is brought about by a thermally direct “residual meridional circulation” arising as an essentially adiabatic response to a wave-induced torque about the earth's axis. The torque itself is equal to the divergence of the EP wave flux and approximately proportional to the northward flux of quasi-geostrophic potential vorticity. Third, some true Lagrangian means and related diagnostics are presented and discussed.

The EP cross sections strikingly display the effect of the mid-stratospheric zero-wind line which invades middle latitudes from the tropics during the first stage of substantial evolution of the mean state. This zero-wind line develops into a partial reflector of planetary waves, splitting the EP wave flux into two branches and deflecting one of them equatorward and the other to high polar altitudes. The consequent focusing of waves into a smaller horizontal area in the polar cap and into altitudes with lower densities helps bring about the reversal of the polar westerlies in the second stage of mean evolution. Focusing of planetary waves into the high-altitude polar cap should be similarly important for real warmings, but there is no evidence that subtropical zero-wind lines play any important role. Possible mechanisms leading to focusing and hence to warnings in the real atmosphere are discussed.

To picture the model warming in Lagrangian terms, we first compare the shape of an isentropic surface near the level of maximum warming with the computed behavior of sets of air parcels. The isentropic surface is approximately a material surface over the short times concerned. As the warming develops the surface dips down over the pole and rises at the equator (and in the real atmosphere this leads to widespread cooling in the summer stratosphere as has often been observed). Thermally direct motion similarly appears in the residual, generalized Lagrangian-mean, and modified Lagrangian-mean meridional circulations near the level of maximum warming, as might have been expected from the theoretical results of Matsuno and Nakamura (1979). The divergence effect, or non-solenoidality of Lagrangian-mean motion, neglected in their study, is strong here because of the large north-south dispersion of air parcels accompanying the highly transient wave activity. Implications for modeling tracer transport are noted.

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