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Taroh Matsuno

This volume consists of some papers presented at the AMS Symposium held to honor the memory of the late Professor Michio Yanai as well as additional works inspired by his research. By the nature of this volume, many of the contributed papers describe the development of tropical meteorology over the past half-century or so in connection with Professor Yanai’s influence on it. While most of the chapters address specific areas and discuss timely issues, in this prologue I will describe some of Professor Yanai’s contributions during the early period of his career from my own point of view. As this is a personal reminiscence, I would like to emphasize how Professor Yanai influenced me.

Both Professor Yanai and I became graduate students at the University of Tokyo to begin our career as meteorologists in 1956 and 1957, respectively. Since we studied and worked together so closely for a long time, in this article I will call him Yanai-san as I have done in our personal interactions.

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Taroh Matsuno

Abstract

The dynamics of the stratosphere sudden warming phenomenon is discussed in terms of the interaction of vertically propagating planetary waves with zonal winds. If global-scale disturbances are generated in the troposphere, they propagate upward into the stratosphere, where the waves act to decelerate the polar night jet through the induction of a meridional circulation. Thus, the distortion and the break-down of the polar vortex occur. If the disturbance is intense and persists, the westerly jet may eventually disappear and an easterly wind may replace it. Then “critical layer interaction” takes place. Further intensification of the easterly wind and rapid warming of the polar air are expected to occur as well as weakening of the disturbance. The model is verified by numerical integrations of the adiabatic-geostrophic potential vorticity equation. Computed results possess features similar to those observed in sudden warming phenomena.

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Taroh Matsuno

Abstract

Planetary-scale, stationary disturbances in the winter stratosphere are considered to be upward propagating internal Rossby waves forced from below. Numerical solutions to the linearized equation for wave propagation are obtained by assuming a realistic profile of zonal winds as the basic state and imposing observed monthly mean heights of the 500-mb surface as the lower boundary condition. The computed wave structures in the meridional section show good agreement with the observed state for the component of zonal wavenumber 1. For wavenumber 2, the computed amplitude is too small to compare with the observed. Wave energy density attains a maximum in the lower and middle stratosphere at high altitudes, where strong upward transfer of wave energy appears. A region of small latitudinal gradient of potential vorticity of the basic state is found above the tropospheric jet, which acts as a barrier for wave propagation and confines wave energy to the polar region. Above 40 km the wave tends to spread to lower latitudes. A line of zero zonal winds in the tropics is a major sink of up-flowing planetary wave energy.

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Taroh Matsuno
and
Kenji Nakamura

Abstract

The Eulerian-mean (usual zonal mean) and the Lagrangian-mean (an average in the zonal direction along a curved “material line,” consisting of definite individual air particles) meridional circulations are investigated, given an upward propagating planetary wave incident on a critical level. The Eulerian-mean circulation is such that an upward motion appears at higher latitudes, a downward motion at lower latitudes, and an equatorward meridional flow near the critical level. The Lagrangian circulation is quite different from the Eulerian one. A strong poleward meridional flow appears concentrated at the critical level. In connection with this meridional flow, a Lagrangian-mean vertical motion occurs which diverges from this level at higher latitudes, i.e., it consists of a downward motion below the critical level and an upward motion above the level. At lower latitudes, vertical motions converge toward the critical level. The vertical motion field of this four-sector structure in the meridional section causes corresponding zonal mean temperature changes as observed in real sudden warnings.

From a set of equations derived by Andrews and McIntyre (1978b), which govern the Lagrangian-mean quantities, we can obtain the above results directly. By interpreting the physical meaning of this system of equations, we interpret the strong meridional flow at the critical level as an Ekman-transport-like motion caused by a strong westward force at the critical level, which arises from a sharp gradient of the zonal component of the radiation stress associated with the wave. The mechanism of a sudden warming as viewed in terms of the Lagrangian-mean motion is discussed.

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