Contribution Of Gravity Waves Research Articles

Gravity and Shear Wave Contributions to Nearshore Infragravity Motions

Abstract Data from a cross-shore array of nine collocated pressure sensors and bidirectional current meters, extending from the shoreline to approximately 4.5-m depth, are used to estimate the relative contributions of gravity waves (e.g., edge and leaky waves) and instabilities of the alongshore current (shear waves) to motions in the infragravity (frequencies nominally 0.004–0.05 Hz) band. The ratio between frequency-integrated velocity and pressure variances is shown to be approximately equal to g/h for a broad spectrum of gravity waves independent of the mode mix of edge and leaky waves. Since shear waves have velocity to pressure variance ratios ≫ g/h, this ratio can be used to estimate the relative contributions of gravity and shear waves to the infragravity band. Outside the surf zone where the shear in the alongshore current is relatively weak, the observed velocity to pressure variance ratios are approximately equal to g/h, consistent with a gravity-dominated wave field. Inside the surf zone wher...

An analysis of the structure and forcing of the equatorial semiannual oscillation in zonal wind

The low‐frequency variability in the equatorial middle atmosphere is dominated by the semiannual oscillation (SAO) and quasi‐biennial (QBO) oscillation. Although the general characteristics and forcing mechanisms of these oscillations are thought to be known, these oscillations are still not well reproduced in modeling experiments of the middle atmosphere. The relatively long period (1992–1996) of Upper Atmosphere Research Satellite (UARS) measurements and U.K. Meteorological Office (UKMO) assimilated model data have been used to examine the zonal wind SAO in the upper stratosphere. The SAO in zonal wind is found to be latitudinally asymmetric about the equator with strongest amplitudes in the southern hemisphere subtropics. The latitudinal asymmetry is diagnosed by using the TEM momentum equation and UKMO data. The asymmetry is mostly due to the different phasing of the peak advective forcing by the mean meridional circulation in each hemisphere. The contribution of gravity waves to the zonal wind SAO are also explored by using the forcing from a linear gravity wave model. Gravity waves are found to be the primary source of westerly momentum in each hemisphere during fall and contribute to the latitudinal asymmetry of the SAO by having more easterly forcing in the southern hemisphere during summer compared to in the northern hemisphere during summer.

Momentum balance of the Venusian midlatitude mesosphere

The downward control principle is applied to estimate the vertically integrated zonal force driving the midlatitude lower mesosphere on Venus. The distinct localization of the forcing in the midlatitude implies two candidates for the momentum carrier: Rossby wave; and internal gravity waves generated by shear instability. Rough estimates of the Eliassen‐Palm (EP) flux suggest that the meridional circulation will be driven principally by the EP flux divergence associated with a Rossby wave, although the contribution of gravity waves cannot be ruled out. It is also shown that advective transport is as important as eddy diffusion for tracer transport.

VHF Doppler Radar Determination of the Momentum Flux in the Upper Troposphere and Lower Stratosphere: Comparison between the Three- and Four-Beam Methods

The vertical flux of horizontal momentum in the upper troposphere and lower stratosphere can be measured by VHF Doppler radars using the mesosphere-stratosphere-troposphere (MST) radar technique. Two methods have been used: one using three beams, one vertical and two oblique, and the other using four beams, two pairs of oblique beams symmetrically offset from the vertical. According to theory the four-beam method should be more accurate, but bemuse some radars do not have the capability of using the four-beam method, it is important to assess the accuracy of the three-beam method. In this study the rapid steerability of the Japanese MU radar was used to make three- and four-beam measurements simultaneously. It is found that the three-beam flux agrees with the four-beam flux only for long-period fluctuations. For shorter periods a systematic error is caused by wind fluctuations with wavelengths that are comparable with the separation between the beams (2–7 km in this study). In this study, performed during summer at 35°N latitude, it is found that the momentum flux due to long-period fluctuations is caused primarily by synoptic-scale or mesoscale disturbances, while short-period flux may primarily be related with intense vertical air motion. Thus, during these observations, the contribution of gravity waves seemed to be unimportant.

Gravity wave fields above Andøya

The meteorological rocket soundings of the MAP/WINE Campaign were used to analyse gravity wave motions statistically in the central part of the middle atmosphere above Andøya (69°N, 16°E). The averaged vertical wave number spectra [PSD(m)] of the horizontal winds in the height range 64-30 km give further evidence that spectra of mesoscale fluctuations, i.e. with vertical scales from about 10 to 1 km, are dominated by gravity wave contributions. The saturated wave field, i.e. with scales of 2-1 km, results in a spectrum ~ m − s , with s of the order of 3. Averaged vertical profiles of wave amplitudes indicate damping of wave energy below about 50 km according to exp ( z/2 h 0), h 0 ranging from 15 to 21 km. The breaking height for waves with vertical scales less than about 10 km was around 50 km during the MAP/WINE period above Andøya.

Contributions of gravity waves to the momentum, heat and turbulent energy budget of the upper mesosphere and lower thermosphere

The role of gravity waves for the momentum and heat budget of the atmosphere between approximately 70 and 110 km height is considered. Parameterization schemes for vertical gravity wave diffusivity, generalized Rayleigh friction, viscous force, heat conduction and kinetic energy dissipation are reviewed. Eddy diffusion parameterization and its relation to the gravity wave approach is also discussed and it is shown that principal similarities exist in both concepts, especially when irregular (stochastic) contributions to the perturbations are modeled. Special attention is paid to the dissipation of perturbation kinetic energy and its contribution to the heat budget of the mesopause region. It is concluded that the amount of energy which can be attributed to the part of the gravity wave spectrum contributing to generalized Rayleigh friction above the mesopause is of the order of 10% of the total perturbation energy.