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Richard S. Lindzen

Abstract

The models developed in Part I for radiative transfer and ozone photochemistry in the mesosphere are incorporated into a two-level model for baroclinic flow, and the effect of radiative and photochemical processes on the stability of the flow is separately investigated for radiative and photochemical conditions obtaining at 30 km and 52.5 km. In each case it is found that the flow is unstable for all non-zero values of shear, in contrast to the adiabatic case where instability required that the shear exceed some critical shear. At 30 km the instabilities at low shears differ considerably from the instabilities for higher shears near the critical shear of the adiabatic theory. The latter have a dominant wavelength of the order of 10,000 km and a phase speed relative to the mean zonal wind of about −20 m sec−1. The former have a dominant wavelength of about 5000 km and a relative phase speed of about −2 m sec−1. The effect of the advection of ozone on the heating appears to be responsible for the low shear mode. This effect is negligible at 52.5 km where there are no significant differences (apart from growth rate) between low and high shear instabilities. The instabilities at this level have a dominant wavelength of about 7900 km and a relative phase speed of about −20 m sec−1.

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Richard S. Lindzen

Abstract

The stability of a baroclinic, axially symmetric vortex on an f-plane to axially symmetric disturbances is investigated. It is found that with photochemistry and radiative transfer acting, such disturbances are unstable regardless of the value of the Richardson number. The growth rates under conditions relevant to the mesosphere are, however, very small.

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Richard S. Lindzen

Abstract

No abstract available.

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Richard S. Lindzen

Abstract

This paper considers the vertical propagation of a long-period, small-amplitude perturbation in a medium in which radiative transfer and photochemistry play important roles. The perturbation and the basic field are assumed to be axially symmetric and symmetric about the equator; the basic wind field is geostrophic and the basic temperature field is in radiative equilibrium.

It is found that long-period perturbations can only propagate by virtue of the physical effects of radiative transfer and photochemistry. The computed wave propagates downwards and, for a period of 2.2 years, the phase speed is close to the observed speed of 1.5 km month−1 for the “26-month” equatorial oscillation. The observed relative phases of velocity and temperature fields, and the sharp attenuation of the oscillation below 20 to 25 km are also found in the model wave.

There are discrepancies between the model and the observed “26-month” oscillation, which are to be expected in view of the nonlinearity of the observed phenomenon. However, it appears that, for complex reasons, the observed wave may satisfy equations similar to those occurring in the linear theory.

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Richard S. Lindzen

Abstract

The response of the mesosphere to slow fluctuations in ultraviolet and visible radiation intensity is investigated using the simplified models for photochemistry and radiative transfer developed by Lindzen and Goody (1965). Fluctuations in oxygen's ultraviolet bands, ozone's ultraviolet bands and ozone's visible band are separately considered. It is found that the mesosphere is most sensitive to fluctuations in ozone's ultraviolet bands above 35 km and to fluctuations in oxygen's ultraviolet bands above 30 km. At levels of peak sensitivity, fluctuations of about 12 per cent in either of these bands will give rise to temperature fluctuations of 2 deg K. This appears to rule out minute changes in solar ultraviolet emission as a cause for the ‘26-month’ oscillation in the equatorial mesosphere. It is also found that the mesosphere is quite sensitive to fluctuations in visible light in the region between 20 and 35 km where fluctuations of 3–6 per cent in visible radiation can give rise to fluctuations of 2 deg K in temperature. On the average, about 26 per cent of the visible radiation in the mesosphere is received via reflection from below. Much of the reflection is from clouds and hence, variation in cloud cover forms an effective way of varying visible light in the mesosphere. In this connection it is found that the winter distribution of cloud cover in the subarctic is such as to introduce into the mesosphere a temperature disturbance whose amplitude and spatial distribution are such as to be able to trigger a sudden warming of the Northern Hemisphere winter variety.

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Richard S. Lindzen

Abstract

No Abstract available.

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Richard S. Lindzen

Abstract

It is noted that gravity waves for which |u¯−c| (u¯=mean flow speed, c=wave phase speed) has a sharp minimum in the upper troposphere or lower stratosphere will have decaying amplitudes above this level despite exponentially decreasing mean density. Eventually this decay ceases and growth resumes. Thus, if a gravity breaks below the level of |u¯−c|min, it will cease breaking above this level. Breaking will, however, resume at some higher level. This second breaking level is a lower bound for the level of breaking in the mesosphere since waves too weak to break where |u¯−c|=|u¯−c|min will break at still higher levels in the mesosphere. Explicit calculations show the “second” breaking levels to be close to observed levels of mesospheric gravity wave breaking. Evidence is also cited for wave breaking in the lower atmosphere, and for the importance of this breaking in the momentum budget of the lower atmosphere.

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Richard S. Lindzen

Recollections of the discovery of the quasi-biennial oscillation (QBO) of the equatorial stratosphere, and of the development of our present theoretical understanding of this phenomenon are presented.

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Richard S. Lindzen

Abstract

A simple model is presented where response to forcing with components at 20K, 40K and 100K yr (where forcing, however, is strongly dominated by 20K yr) is primarily at 100K yr. The main features of the model are a very sensitive response of the snow and sea ice line to solar input, a threshold to transitions between large snow and sea ice coverage (to 53° latitude) and very little snow and sea ice coverage, and a glaciation cycle forced by the snow and sea-ice fine position.

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Richard S. Lindzen

Abstract

The usual assumption that vertically propagating internal gravity waves will cease growing with height once their amplitudes are such as to permit convective instability anywhere within the wave is reexamined. Two factors lead to amplitude limitation:

(i) wave clipping associated with convective mixing, and

(ii) energetic constraints associated with the rate at which the wave can supply energy to the convection.

It is found that these two factors limit supersaturation to about 50% for waves with short horizontal wavelengths and high relative phase speeds. Usually the degree of supersaturation will be much less. These factors also lead to a gradual, rather than sudden, cessation of wave growth with height.

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