The occurrence of internal gravity waves and volumetric acoustic oscillations in the atmosphere. Kochin A lexander V.

2025-05-06 3 0 725.36KB 13 页 10玖币

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The occurrence of internal gravity waves and volumetric acoustic oscillations

in the atmosphere.

Kochin Alexander V.

Highlights

- The causes of internal gravity waves require additional justification.

- Oscillatory movements with an amplitude of tens of meters are present in the atmosphere

in the absence of powerful clouds and fronts.

- Passing the solar terminator causes antiphase pressure and illumination fluctuations.

- The possible process of generating volumetric acoustic oscillations due to the formation

of an acoustic resonator in the atmosphere is considered as an alternative to IGW.

Correspondence: Alexander Kochin (amarl@mail.ru)

Abstract

Mesoscale wave processes are a transport mechanism for the energy exchange between the

troposphere and stratosphere, since the tropopause blocks such an exchange. It is believed that

internal gravity waves (IGW) are the main wave process in the atmosphere. However, the

explanation of the process of IGW occurrence cannot be considered sufficiently substantiated,

because the reasons for the appearance of the selected air parcel, which is affected by the initial

disturbance, are not yet clear. The data of the performed experiments on the detection of wave

processes shows the presence of noticeable oscillatory movements not only during the passage of

fronts or other disturbances, but also in good weather. As an alternative to IGW, I consider a

possible process of generating volumetric acoustic oscillations due to the formation of an acoustic

resonator in the atmosphere. The probability of formation of such oscillations is high, because they

do not require the presence of sharp changes in the parameters of the atmosphere to appear.

1. Introduction

Wave motions are important and integral components of dynamic processes in the atmosphere.

They form the links between different layers of the atmosphere. They are one of the types of

mesoscale turbulence and determine the transport coefficients values on scales of tens and

hundreds of kilometers. The tropopause blocks the exchange between the troposphere and the

stratosphere due to a sharp decrease in the turbulent transport coefficient. Therefore, the exchange

of mass and energy near the tropopause is largely determined by wave processes. Also, wave

processes affect the rate of air displacement in the general circulation of the atmosphere, which

affects the redistribution of air mass above the Earth's surface. To adequately take into account the

contribution of wave processes to the energy and momentum fluxes, it is necessary to know the

spatial and temporal characteristics of the perturbation region. However, due to a little number of

wave variations studies in the atmosphere and ionosphere, the physical mechanisms that determine

the processes of generation and propagation of waves in the atmosphere and ionosphere remain

not fully understood. Currently, internal gravity waves (IGW) are considered to be the main wave

process that has a noticeable effect on the atmosphere (Alexander, 2004; Gossard, 1975, Holton

2004). However, the possibility of IGW generation raises some doubts, so there is no reason to

assume that IGW is the only possible and main type of oscillations. Therefore, the search for

alternative types of wave motions seems to be an actual problem.

2. Generation of internal gravity waves

Oscillations of any type can be generated either in a hard mode (Fig. 1a) or in a soft mode (Fig.

1b).

Fig.1. Oscillation generation modes.

In the hard mode, generation starts immediately from the maximum amplitude (Fig. 1a). Then the

oscillation amplitude either decreases due to energy dissipation, or remains constant if there is an

external energy source that compensates for the losses. Generation in the soft mode starts from

zero amplitude and is possible only in the presence of continuous pumping, which provides an

increase and maintenance of the oscillation energy due to an external source (Fig. 1b).

The generation of internal gravity waves occurs in a hard mode, as follows from the description of

their occurrence (Gossard, 1975; Holton 2004, Tong). It is believed that stable atmospheric

stratification contributes to their generation. Therefore, the stratosphere with stable stratification

is considered as the main area of IGW formation.

Fig.2. Formation of internal gravity waves. Cb - powerful cumulonimbus cloud, H - the vertical

extent of the volume, dh - the initial displacement of the volume.

An external source (a front or a powerful cumulonimbus cloud) creates an impulse that causes an

initial displacement(dh) of some volume as shown in Fig. 2. The size of the air parcel displaced

by the initial impulse is not unambiguously determined. Most researchers believe that the

dimensions of the air parcel are in the order of a few kilometers in thickness and tens of kilometers

long (Alexander, 2004; Gossard, 1975; Kshevetsky, 2015; Kulichkov 2017). We made a paired

launch of radiosondes to estimate these sizes (Fig. 3). The results will be discussed in detail below.

It turned out that the velocity pulsations remain in-phase on a vertical scale of at least 1.5 km and

at a horizontal distance of up to 5 km. The total size of the oscillation region should be several

times larger than the region with in-phase velocity oscillations. Thus, the minimum dimensions

are 5 km vertically and 20 km horizontally. The mass of such the air parcel in the stratosphere at

a height of 15 km with an air density of 0.1 kg/m3 is more than 100 million tons. The amplitude of

the initial displacement is tens or hundreds of meters (Fig. 2). The reasons for the sudden

displacement of only one specific massive air parcel are difficult to explain. Prior to the occurrence

of oscillations, the stratosphere is homogeneous and motionless. The impulse from a powerful

cumulonimbus cloud or an atmospheric front affects the entire atmosphere, including the

troposphere and stratosphere (Fig. 2). The process that causes the appearance of heterogeneities in

the stratosphere with dimensions of tens of kilometers horizontally and a few kilometers vertically

has not been described by anyone. If there are no inhomogeneities, then there is no selected object

that acquires an additional momentum. Therefore, the existence of oscillations of an isolated air

parcel in the stratosphere seems unlikely. There are inhomogeneities in the troposphere, but the

oscillations require a vertical component of the added momentum. However, the height difference

between irregularities and momentum sources in the troposphere is small, so the vertical

component of the added momentum is small. As already noted, the generation of internal gravity

waves occurs in a hard mode. Due to damping, the IGW lifetime cannot be long. However,

undamped oscillations are recorded for several hours (Borchevkina, 2016), which is also poorly

consistent with the mechanism of IGW formation.

The derivation of the the Brunt-Väisälä frequency ratio is done in the hydrostatic approximation,

which is valid only for an infinitely small displacement amplitude of an air parcel. It is assumed

that the pressure equalization time in the surrounding space is much less than the process time.

Equalization of pressures in the medium occurs at the sound speed (Fig. 3).

Fig.3. Displacements of a sound wave and an attached air mass during particle oscillation. The

added mass of air is highlighted in gray, which is displaced by particle oscillations.

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Theoccurrenceofinternalgravitywavesandvolumetricacousticoscillationsintheatmosphere.KochinAlexanderV.Highlights-Thecausesofinternalgravitywavesrequireadditionaljustification.-Oscillatorymovementswithanamplitudeoftensofmetersarepresentintheatmosphereintheabsenceofpowerfulcloudsandfronts.-Passingtheso...

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The occurrence of internal gravity waves and volumetric acoustic oscillations in the atmosphere. Kochin A lexander V.

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