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Isamu Hirota

Abstract

The structure and behavior of Kelvin waves in the equatorial upper stratosphere and mesosphere are investigated by the use of infrared radiation measurements from the Selective Chopper Radiometer (SCR) on the Nimbus 5 satellite during the two years 1973–74.

By making a combination of three upper channels of the SCR, Kelvin waves with vertical wavelengths of ∼20 km are detected in the tropics. The long-term statistics indicate that zonal wavenumber 1 is prominent throughout the two-year period. The predominant period of the wave is determined by power spectrum analysis using the Doppler effect due to the wave migration. For wavenumber 1, the eastward moving Kelvin wave appears to have a period of 4–9 days. It is also found that the intrinsic (Doppler-shifted) phase velocity of the Kelvin wave is almost constant in time, regardless of the seasonal variation of the mean zonal wind.

The dynamical significance of this wave is stressed in connection with the semiannual oscillation of the mean zonal wind in the equatorial middle atmosphere.

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Isamu Hirota

Abstract

An analysis is made of equatorial wave disturbances in the upper stratosphere and mesosphere by the use of meteorological rocket and satellite observations, to clarify their structure and behavior in relation to the semiannual oscillation of the mean zonal wind.

From a power spectral analysis of wind and temperature over Ascension Island during the four years from 1969 to 1972, it is found that in the height region between 25 and 60 km there exists a wave disturbance with a characteristic vertical scale of 15–20 km; the wave is more active in the easterlies than in the westerlies, showing marked semiannual variation.

Compared with the well-known characteristics of equatorial waves in the lower stratosphere, this wave is likely to he identified as a Kelvin wave with a period of about 10 days. It is suggested that this wave plays an essential role in producing the semiannual reversal of the mean zonal wind by supplying the westerly momentum to the equatorial mesospheric levels.

Further discussions are made of the long-term behavior of mesospheric planetary Rossby waves as observed from the Nimbus 6 Pressure Modulator Radiometer, in connection with the semiannual cycle in the upper mesosphere.

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Isamu Hirota and Toshihiko Hirooka

Abstract

A global analysis is made of large-scale traveling planetary waves in the upper stratosphere by the use of stratospheric height and thickness data up to the 1 mb level derived from the Stratospheric Sounding Unit (SSU) on TIROS-N and NOAA-A satellites for the period December 1979 through November 1981.

The results of space-time power and cross-spectral analyses show the global existence of a westward traveling wave of zonal wavenumber 1 with a period of about 5 days and a westward traveling wave of zonal wavenumber 2 with a period of about 4 days.

The structure and behavior of these waves are investigated by using the components of these period bands separated by a numerical band-pass filter. The 5-day wave is very similar to the (1, 1) mode, which is the first symmetric wavenumber 1 Rossby normal mode in an isothermal atmosphere. Similarly, the 4-day wave appears to correspond to the (2, 1) mode. The vertical structure of both waves agrees well with the theoretical expectation of free external waves.

With regard to the seasonal variation, both waves are irregularly predominant throughout the year except for the December–January–February season, and this evidence is significantly related to the temporal variation of the refractive index of the zonal mean field.

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Toshihiko Hirooka and Isamu Hirota

Abstract

As an extension of our recent previous study, the global structure and behavior of higher-mode Rossby waves am investigated with the aid of TIROS-N and NOAA-A satellite observations for the period November 1979 through April 1982.

It is shown that the second antisymmetric and symmetric modes of zonal wavenumbers 1 and 2 exist in the upper stratosphere, although the existence of zonal wavenumber 2 modes somewhat uncertain. These waves are amplified in the winter hemisphere over a period of 1-2 months. The vertical structure of the waves is similar to that of the simple Lamb mode except for a little westward phase tilt.

The period band of the higher-degree modes is variable, as predicted by numerical models. The period band of the second antisymmetric mode of zonal wavenumber 1, i.e.,(1, 2) mode, sometimes falls into that of a 16-day wave which is the manifestation of the (1, 3) mode. However, the two modes don't coexist in the same period band.

The higher-degree modes are often largely amplified simultaneously. In particular, it is remarkable that they are enhanced before the occurrence of stratospheric sudden warmings. Since the amplitude of these waves is of the same order of magnitude as that of stationary waves, the interference between the traveling and stationary waves may play an important role in the sudden warming event.

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Kaoru Sato, Kazutaka Yamada, and Isamu Hirota

Abstract

Global characteristics and seasonal variation of medium-scale (zonal wavelengths of 2000–3000 km) waves observed around the midlatitude tropopause are examined using 6-hourly European Centre for Medium-Range Weather Forecasts operational data over four years (1990–93), covering both hemispheres. Medium-scale waves and synoptic-scale waves are extracted using time filters and their characteristics are compared. Hovmöller diagrams indicate the existence of medium-scale waves in the Southern Hemisphere as well as in the Northern Hemisphere. The zonal wavelengths of medium-scale waves are slightly larger in the Southern Hemisphere than in the Northern Hemisphere. Medium-scale waves are mostly active in three regions: the North Atlantic in winter (Dec–Jan–Feb), the North Pacific in spring (Mar–Apr–May, MAM), and the south Indian Ocean in autumn (MAM). These regions are roughly corresponding to and slightly downstream of storm tracks where synoptic waves are dominant. Significant differences in seasonal variation of the intensity between the two kinds of waves are also found.

The maximum of wave amplitudes is seen around the tropopause at latitudes slightly higher than the jet stream axis, where the meridional gradient of the quasigeostrophic potential vorticity (QPV) is maximized. The positive large QPV gradient is attributed to the atmospheric structure around the midlatitude tropopause that is located in vertical westerly shear of the jet stream. This fact suggests that the medium-scale waves are waves trapped around the midlatitude tropopause.

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