Wave Of Zonal Wavenumber Research Articles

On the Momentum Budget of the Quasi-Biennial Oscillation in the Whole Atmosphere Community Climate Model

Abstract This study documents the contribution of equatorial waves and mesoscale gravity waves to the momentum budget of the quasi-biennial oscillation (QBO) in a 110-level version of the Whole Atmosphere Community Climate Model. The model has high vertical resolution, 500 m, above the boundary layer and through the lower and middle stratosphere, decreasing gradually to about 1.5 km near the stratopause. Parameterized mesoscale gravity waves and resolved equatorial waves contribute comparable easterly and westerly accelerations near the equator. Westerly acceleration by resolved waves is due mainly to Kelvin waves of zonal wavenumber in the range k = 1–15 and is broadly distributed about the equator. Easterly acceleration near the equator is due mainly to Rossby–gravity (RG) waves with zonal wavenumbers in the range k = 4–12. These RG waves appear to be generated in situ during both the easterly and westerly phases of the QBO, wherever the meridional curvature of the equatorial westerly jet is large enough to produce reversals of the zonal-mean barotropic vorticity gradient, suggesting that they are excited by the instability of the jet. The RG waves produce a characteristic pattern of Eliassen–Palm flux divergence that includes strong easterly acceleration close to the equator and westerly acceleration farther from the equator, suggesting that the role of the RG waves is to redistribute zonal-mean vorticity such as to neutralize the instability of the westerly jet. Insofar as unstable RG waves might be present in the real atmosphere, mixing due to these waves could have important implications for transport in the tropical stratosphere.

Local Dynamics of Baroclinic Waves in the Martian Atmosphere

Abstract The paper investigates the processes that drive the spatiotemporal evolution of baroclinic transient waves in the Martian atmosphere by a simulation experiment with the Geophysical Fluid Dynamics Laboratory (GFDL) Mars general circulation model (GCM). The main diagnostic tool of the study is the (local) eddy kinetic energy equation. Results are shown for a prewinter season of the Northern Hemisphere, in which a deep baroclinic wave of zonal wavenumber 2 circles the planet at an eastward phase speed of about 70° Sol−1 (Sol is a Martian day). The regular structure of the wave gives the impression that the classical models of baroclinic instability, which describe the underlying process by a temporally unstable global wave (e.g., Eady model and Charney model), may have a direct relevance for the description of the Martian baroclinic waves. The results of the diagnostic calculations show, however, that while the Martian waves remain zonally global features at all times, there are large spatiotemporal changes in their amplitude. The most intense episodes of baroclinic energy conversion, which take place in the two great plain regions (Acidalia Planitia and Utopia Planitia), are strongly localized in both space and time. In addition, similar to the situation for terrestrial baroclinic waves, geopotential flux convergence plays an important role in the dynamics of the downstream-propagating unstable waves.

Quantification of the contribution of equatorial Kelvin waves to the QBO wind reversal in the stratosphere

Both global scale waves (e.g., Kelvin, equatorial Rossby, or Rossby‐gravity waves) and mesoscale gravity waves contribute to the wind reversals of the quasi biennial oscillation (QBO). The relative contributions of the different wave types are highly uncertain. In our work we quantify the contribution of equatorial Kelvin waves to the reversal from stratospheric easterlies to westerlies averaged over two QBO cycles in the period 2002–2006. Our analysis is based on longitude‐time spectra of temperatures measured by the SABER satellite instrument, as well as temperatures from ECMWF operational analyses. Kelvin waves of zonal wavenumber 1–6 and periods longer than 2.5 days are covered. It is found that the contribution of Kelvin waves is about 30–50% of the observed wind reversal and only 20–35% of the expected total wave forcing. The larger part of the wave forcing therefore has to be contributed by other waves, likely mesoscale gravity waves.

Rayleigh lidar observations of enhanced stratopause temperature over Gadanki (13.5° N, 79.2° E) during major stratospheric warming in 2006

Abstract. Rayleigh lidar observations of temperature structure and gravity wave activity were carried out at Gadanki (13.5° N, 79.2° E) during January–February 2006. A major stratospheric warming event occurred at high latitude during the end of January and early February. There was a sudden enhancement in the stratopause temperature over Gadanki coinciding with the date of onset of the major stratospheric warming event which occurred at high latitudes. The temperature enhancement persisted even after the end of the high latitude major warming event. During the same time, the UKMO (United Kingdom Meteorological Office) zonal mean temperature showed a similar warming episode at 10° N and cooling episode at 60° N around the region of stratopause. This could be due to ascending (descending) motions at high (low) latitudes above the critical level of planetary waves, where there was no planetary wave flux. The time variation of the gravity wave potential energy computed from the temperature perturbations over Gadanki shows variabilities at planetary wave periods, suggesting a non-linear interaction between gravity waves and planetary waves. The space-time analysis of UKMO temperature data at high and low latitudes shows the presence of similar periodicities of planetary wave of zonal wavenumber 1.

A Parameter Sweep Experiment on Quasiperiodic Variations of a Polar Vortex due to Wave–Wave Interaction in a Spherical Barotropic Model

Abstract Weakly nonlinear aspects of a barotropically unstable polar vortex in a forced–dissipative system with a zonally asymmetric surface topography are investigated in order to obtain a deeper understanding of rather periodic variations of the winter circumpolar vortex in the Southern Hemisphere stratosphere that are characterized by the wave–wave interaction between the stationary planetary wave of zonal wavenumber 1 (denoted as Wave 1) and the eastward traveling Wave 2 as studied by Hio and Yoden in 2004. The authors use a spherical barotropic model with a forcing of zonally symmetric jet, dissipation, and sinusoidal surface topography. A parameter sweep experiment is performed by changing the amplitude of the surface topography, which forces the stationary Wave 1, and the width of the prescribed zonally symmetric jet, which controls the barotropic instability, to generate the traveling Wave 2. Several types of solutions from a time-independent solution to a nonperiodic irregular solution are obtained for the combination of these external parameters, but the predominant solution obtained in a wide parameter space is periodic. Details of the wave–wave interactions are described for the transition from a quasiperiodic vacillation to a periodic solution as the increase of the amplitude of topography. Phase relationships are locked at the transition, and variations of zonal-mean zonal flow and topographically forced Wave 1 synchronize with periodic progression of Wave 2 in the periodic solution. A diagnosis with a low-order “empirical mode expansion” of the vorticity equation gives a limited number of dominant nonlinear triad interactions among the zonal-mean, Wave-1, and Wave-2 components around the transition point.

Evidence for short vertical wavelength Kelvin waves in the Department of Energy‐Atmospheric Radiation Measurement Nauru99 radiosonde data

Stratospheric zonal wind and temperature oscillations observed in a 30 day time series of four times per day radiosonde data taken as part of Nauru99 show a strong negative quadrature relationship at periods of 9.5 days and 5 days. The 9.5‐day waves have vertical wavelength of about 4.5 km, and are deduced to be Kelvin waves of zonal wave number 2; the 5‐day waves have vertical wavelengths in the range of 3–4.5 km, and are deduced to be Kelvin waves of zonal wavenumber 4. Vertical momentum fluxes associated with the 9.5‐ and 5‐day modes are estimated to be 1×10−4 m2s−2 and 2×10−4 m2s−2, respectively, in the 20–25 km altitude range where the mean zonal flow is characteristic of the westerly phase of the quasi‐biennial oscillation (QBO). These are an order of magnitude smaller than momentum fluxes associated with traditional wavenumber 1 Kelvin waves analyzed in the easterly phase of the QBO.

Observation of low frequency Kelvin waves in the mesosphere

This paper presents evidence for a quasi-stationary Kelvin wave of zonal wavenumber 1 that is forced in situ in the upper mesosphere and propagates upward from there. Although large scale quasi-stationary longitudinal variations are common in the mesopause equatorial winds, these variations often do not have a structure associated with a vertically propagating mode. However, during the periods of easterly winds that occur near the equinoxes in association with the mesopause semi-annual oscillation (MSAO), a Kelvin wave propagates away from the presumed altitude of forcing. The momentum transport by the Kelvin wave is small and does not make a significant contribution to the SAO wind evolution.

The horizontal structure of traveling planetary‐scale waves in the upper stratosphere

The temperature amplitude and phase structures of zonally propagating planetary waves are determined between 79S and 79N latitudes at the 10 and 1 mbar levels through the use of spectral analysis and complex‐valued empirical orthogonal function analysis. The data are temperatures regressed from Nimbus 4 and 5 satellite infrared radiances during the solsticial seasons of 1970–1974. The structures of three westward moving normal modes and two eastward moving modes linked to instabilities near the winter stratopause are described. Waves of zonal wavenumber 4, 5, and 6 which are eastward moving in the summer Southern Hemisphere are also observed; a wave 1, 16‐day period, westward moving mode commonly observed in geopotential fields is not seen here. The results concur with recent studies of traveling waves and provide estimates of average wave temperature amplitudes in the upper stratosphere.

Upper Tropospheric Waves in the Tropics. Part I: Dynamical Analysis in the Wavenumber-Frequency Domain

Abstract Space and time spectra of large-scale wave disturbances at the 200 mb level in the latitude belt 20°S to 40°N are studied based on wind data for the period June–August 1967. The kinetic energy of the transient waves shows a minimum near 10°N where convective activity attains a maximum. The transient eddies in the zonal wind component have dominant scales of zonal wavenumbers s = 3–6 in most latitudes, while those in the meridional wind component have dominant scales of s = 6–8. By decomposing the wind data into symmetric and antisymmetric components with respect to the equator, three prototype equatorial wave modes are detected: 1) Kelvin waves of zonal wavenumber s = 1 and 2 and a period of 7 days, and s = 1 and periods longer than 20 days; 2) mixed Rossby-gravity waves of s = 4 and a period of 5 days; and 3) Rossby waves (of the lowest meridional nodal number) of s = 2 and a period near 12 days. Westward moving short waves (s = 7–15) gain kinetic energy from the mean easterly flow. Eastward mov...

Stability of the Rossby-Haurwitz wave

A spectral method is used to integrate the primitive equations for the motion on a sphere of a shallow layer of fluid with a free surface. A simple Rossby-Haurwitz wave of zonal wavenumber 4 is found to change its form little over 24 days, whilst one of wavenumber 8 breaks down completely in 5 days. Inspired by the integrations, the barotropic stability of a wave to a perturbation composed of another wave and a zonal flow is considered. The theory suggests that waves of zonal wavenumber greater than 5 may be unstable because the feedback of the perturbation on itself is positive. Those of wavenumber 5 and less are stable, the feedback being negative. The theory and the general barotropic instability of Rossby waves is tested by integrations of the non-divergent barotropic model, in which the wave is perturbed by a zonal flow. Some agreement with theory is found. Waves of zonal wavenumber 8 rapidly break down to produce jets in the same direction as the perturbation zonal flow. The stability of these jets is also examined.

Stability of the Rossby‐Haurwitz wave

AbstractA spectral method is used to integrate the primitive equations for the motion on a sphere of a shallow layer of fluid with a free surface. A simple Rossby‐Haurwitz wave of zonal wavenumber 4 is found to change its form little over 24 days, whilst one of wavenumber 8 breaks down completely in 5 days. Inspired by the integrations, the barotropic stability of a wave to a perturbation composed of another wave and a zonal flow is considered. The theory suggests that waves of zonal wavenumber greater than 5 may be unstable because the feedback of the perturbation on itself is positive. Those of wavenumber 5 and less are stable, the feedback being negative. The theory and the general barotropic instability of Rossby waves is tested by integrations of the non‐divergent barotropic model, in which the wave is perturbed by a zonal flow. Some agreement with theory is found. Waves of zonal wavenumber 8 rapidly break down to produce jets in the same direction as the perturbation zonal flow. The stability of these jets is also examined.