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Kaoru Sato

Abstract

This paper describes small-scale wind disturbances associated with Typhoon Kelly (October 1987) that were observed by the MU radar, one of the MST (mesosphere, stratosphere, and troposphere) radars, continuously for about 60 hours with fine time and height resolution. First, in order to elucidate the background of small-scale disturbances, synoptic-scale variation in atmospheric stability related to the typhoon structure during the observation is examined. When the typhoon passed near the MU radar site, the structure was no longer axisymmetric. There is deep convection only in the front (north-northeast) side of the typhoon while convection behind it is suppressed by a synoptic-scale cold air mass moving eastward to the west of the typhoon. A drastic change in atmospheric stability over the radar site as indicated by echo power profiles is likely due to the passage of the sharp transition zone of convection.

Strong small-scale wind disturbances were observed around the typhoon passage. It is shown that the statistical characteristics are significantly different before (BT) and after (AT) the typhoon passage, especially in frequency spectra of vertical wind fluctuations. The spectra for BT are unique compared with earlier studies of vertical winds observed by VHF radars. Another difference is dominance of a horizontal wind component with a vertical wavelength of about 3 km, which is observed only in AT.

Further analyses are made of detailed characteristics and vertical momentum fluxes for dominant disturbances. It is found that some of the disturbances are generated so as to remove the momentum of cyclonic wind rotation of the typhoon. Deep convection, topographic effects in strong winds, and strong vertical shear of horizontal winds around an inversion layer are possible sources of the dominant disturbances. Moreover, two monochromatic disturbances lasting for more than 10 h in the lower stratosphere observed in BT and AT, respectively, are identified as inertio-gravity waves, by obtaining wave parameters consistent with all observed quantities. Both of the inertio-gravity waves propagate energy away from the typhoon.

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Kaoru Sato

Abstract

An analysis is made of vertical wind disturbances (VWDs) observed by the MU radar (Shigaraki, Shiga, Japan) in terms of the wave structures, sources, and vertical momentum flux. First, it is shown through spectral and lag-correlation analyses that there are several common features in three observation periods when the VWDs appeared: large power in the vertical wind fluctuations exists below a height of ∼20 km and is distributed largely at lower frequencies. The decrease in power near 20 km is sharp and the correlation between vertical winds above and below the level is very small, suggesting that the height of 20 km is a special level for the VWDs such as a critical level. The disturbances below 20 km consist of several modes in a height-frequency space, some of which oscillate almost in phase in a wide height range spanning ∼8 km at the maximum.

Second, using eight sets of the MU radar data, the relationship between vertical wind activity and horizontal wind near the surface is examined in order to identify the source of the VWDS. The activity of the vertical wind is closely related to both direction and strength of the horizontal wind near the surface at Yonago (about 250 km west of Shigaraki). This indicates that the VWDs are due mainly to gravity waves generated by the effect of topography with heights of ∼1000 m located between Yonago and Shigaraki. The features obtained through the wave structure analysis can be interpreted well as characteristics of topographically forced waves.

Finally, vertical momentum flux associated with the VWDs is examined. Although it is generally difficult to examine the vertical momentum flux associated with quasi-stationary mountain waves from observations at one location, the estimation is possible to some extent when the spatial phase fluctuates largely according to temporal changes of the background wind. As a result, some of the VWDs' characteristics on the momentum flux consistent with the view of mountain waves are obtained.

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Masashi Kohma and Kaoru Sato

Abstract

The tropopause is the boundary between the troposphere and stratosphere and is normally defined by the temperature lapse rate. Previous studies have noted that synoptic-scale and planetary-scale disturbances bring about lapse-rate-tropopause (LRT) height fluctuations on time scales from several days to several years. In the present study, a diagnostic expression for the tendency of LRT height is derived by assuming that the LRT can be characterized as a discontinuity in the vertical gradient of the potential temperature. In addition, the contribution from each term in the thermodynamic equation to the LRT height is quantified. The derived equation is validated by examining the time variation of the LRT height associated with baroclinic waves in an idealized numerical calculation, that of the zonal-mean LRT height in GPS radio occultation data, and that of the LRT height in reanalysis data.

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Kaoru Sato and Motoyoshi Yoshiki

Abstract

Intensive radiosonde observations were performed at Syowa Station (69.0°S, 39.6°E) over about 10 days in each of March, June, October, and December 2002 to examine inertia–gravity wave characteristics in the Antarctic lower stratosphere. Based on the 3-hourly observation data, two-dimensional (i.e., vertical wavenumber versus frequency) spectra of wind fluctuations were examined, utilizing a double Fourier transform method. Clear signals of gravity waves whose phases propagate upward, suggesting downward energy propagation, are detected in June and October when the polar night jet (PNJ) was present. On the other hand, downward phase propagation (i.e., upward energy propagation) components are dominant in all months. There is a spectral peak around the inertial frequency in a wide range of vertical wavenumbers in December when the background wind was weak, whereas large spectral densities are distributed over lower-frequency regions in June and October. These spectral characteristics are consistent with the results obtained using a gravity wave–resolving global circulation model (GCM) by Sato et al. Dynamical characteristics are examined separately for upward- and downward-propagating gravity waves in June, using a hodograph analysis method. As a result, it is found that upward- and downward-propagating wave packets observed simultaneously in the same height regions have similar horizontal wavelengths and phase velocities. This fact suggests that these gravity waves are generated from the same source with a similar mechanism. When the wave packets were observed, both the local Rossby number and the residual in the nonlinear balance equation estimated using NCEP–NCAR reanalysis data are large around the PNJ situated slightly to the lower latitudes of Syowa Station. Therefore, it is likely that the observed inertia–gravity waves are generated by a spontaneous adjustment around the geostrophically unbalanced PNJ and propagate toward Syowa Station. The possibility of spontaneous gravity wave generation around the PNJ is confirmed by comparison with the GCM simulation by Sato et al.

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Takenari Kinoshita and Kaoru Sato

Abstract

A companion paper formulates the three-dimensional wave activity flux (3D-flux-M) whose divergence corresponds to the wave forcing on the primitive equations. However, unlike the two-dimensional wave activity flux, 3D-flux-M does not accurately describe the magnitude and direction of wave propagation. In this study, the authors formulate a modification of 3D-flux-M (3D-flux-W) to describe this propagation using small-amplitude theory for a slowly varying time-mean flow. A unified dispersion relation for inertia–gravity waves and Rossby waves is also derived and used to relate 3D-flux-W to the group velocity. It is shown that 3D-flux-W and the modified wave activity density agree with those for inertia–gravity waves under the constant Coriolis parameter assumption and those for Rossby waves under the small Rossby number assumption.

To compare 3D-flux-M with 3D-flux-W, an analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) data is performed focusing on wave disturbances in the storm tracks during April. While the divergence of 3D-flux-M is in good agreement with the meridional component of the 3D residual mean flow associated with disturbances, the 3D-flux-W divergence shows slight differences in the upstream and downstream regions of the storm tracks. Further, the 3D-flux-W magnitude and direction are in good agreement with those derived by R. A. Plumb, who describes Rossby wave propagation. However, 3D-flux-M is different from Plumb’s flux in the vicinity of the storm tracks. These results suggest that different fluxes (both 3D-flux-W and 3D-flux-M) are needed to describe wave propagation and wave–mean flow interaction in the 3D formulation.

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Takenari Kinoshita and Kaoru Sato

Abstract

The large-scale waves that are known to be trapped around the equator are called equatorial waves. The equatorial waves cause mean zonal wind acceleration related to quasi-biennial and semiannual oscillations. The interaction between equatorial waves and the mean wind has been studied by using the transformed Eulerian mean (TEM) equations in the meridional cross section. However, to examine the three-dimensional (3D) structure of the interaction, the 3D residual mean flow and wave activity flux for the equatorial waves are needed. The 3D residual mean flow is expressed as the sum of the Eulerian mean flow and Stokes drift. The present study derives a formula that is approximately equal to the 3D Stokes drift for equatorial waves on the equatorial beta plane (EQSD). The 3D wave activity flux for equatorial waves whose divergence corresponds to the wave forcing is also derived using the EQSD. It is shown that the meridionally integrated 3D wave activity flux for equatorial waves is proportional to the group velocity of equatorial waves.

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Yuki Hayashi and Kaoru Sato

Abstract

In previous studies, a steady-state assumption has been frequently used for the analysis of wave-induced meridional circulation. In general, however, the wave forcing is not constant and thus induced circulation can vary in time. Thus, to understand such transient behaviors, time evolutions of a slow variable describing balanced flows and two fast variables describing gravity waves and flows that are slaved to balanced flows are investigated. A Boussinesq equation system is used to examine zonal-mean flow responses to unsteady zonally uniform forcing. Green’s function is used to analytically obtain the evolution of meridional circulation. Responses to zonal wave forcing are mainly examined although responses to a diabatic heating and to wave forcing are discussed in brief. For forcing with a step function shape in time, gravity waves are radiated as a transient response. The time needed to form the circulation depends on the aspect ratio (i.e., latitudinal to vertical lengths) of wave forcing, which determines the group velocity of gravity waves. When the forcing time scale is longer than the inertial period, the response does not include gravity wave radiation and mainly involves a meridional circulation, which is similar to the solution for steady forcing. The two-celled meridional circulation describes the early stage response to the forcing and can be used to examine how the wave forcing is distributed to zonal wind acceleration and Coriolis torque. It is shown that the distribution depends on the aspect ratio of the forcing.

Open access
Miho Yamamori and Kaoru Sato

Abstract

The mechanism for the amplification of medium-scale tropopausal waves (with horizontal wavelengths of 2000–3000 km and wave periods of 1–2 days) that are basically neutral is investigated using a three-dimensional wave-activity flux. Distinctive upward wave-activity fluxes are found in the midtroposphere (400 hPa) upstream of the region where the medium-scale waves are dominant, that is, where the horizontal components of the wave-activity flux are large. These upward fluxes converge in the upper troposphere (300 hPa). This convergence is a major source of medium-scale waves, producing large values of the horizontal components of the wave-activity flux in the active regions.

Examination of the spatial structure of the heat flux shows that the upward fluxes are due to occasional baroclinic interactions between upper-level medium-scale waves and lower-tropospheric disturbances with the same temporal scales. The seasonal and geophysical variations of the waves depend mostly on the amount of baroclinic coupling with the lower disturbances.

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Arata Amemiya and Kaoru Sato

Abstract

The Asian monsoon anticyclone, which develops in the upper troposphere and lower stratosphere during boreal summer, exhibits significant subseasonal variability with a characteristic spatial structure. The dynamics of this variability is investigated using a nonlinear β-plane shallow-water model. The equivalent depth is estimated using reanalysis data to relate the three-dimensional dynamics in isentropic coordinates to the shallow-water model. Composite analysis reveals the resemblance of the horizontal structures between the Montgomery streamfunction and thickness on the 360-K level. However, the coefficients of the linear regressions between those two variables are strongly dependent on latitude. The estimated equivalent depths of the northern region are more than 2 times greater than those of the southern region. This is attributable to the background thermal structure around the tropopause. Based on this, a latitude-dependent mean depth is incorporated into the shallow-water model to numerically investigate responses to a steady localized forcing in the subtropics. With the inclusion of the latitudinal dependence of the mean depth, the vortex shedding state is able to have a longitudinally confined structure, which differs from the conventional case of constant mean depth. The spatial structure of this numerical solution corresponds to the observed structure, in which low-PV air is largely confined to finite longitudes within the Asian monsoon anticyclone. This suggests the possible role of dynamical instability and the interaction with the subtropical jet in determining the characteristic structure of the Asian monsoon anticyclone.

Open access
Takehiko Satomura and Kaoru Sato

Abstract

The generation of small-scale gravity waves associated with the breaking of mountain waves in the stratosphere has been simulated within a fully compressible, nonhydrostatic, two-dimensional numerical model. The model includes the mesosphere and offers sufficiently high spatial resolution to characterize the breaking and generation of gravity waves in the stratosphere. The mean flow is initialized by CIRA86 at 40°N in February. A bell-shaped mountain with 30-km half-width and 1.5-km height is located in the bottom of the modeled domain.

The primary wave forced by the mountain propagates into the stratosphere with amplitude increasing with height and subsequently breaks in the lower stratosphere. After the primary wave breaking, significant wave activity is simulated in the stratosphere. These secondary waves are generated at both the upstream and the downstream edges of the breaking zone. Analysis is mostly focused on downstream small-scale waves. The horizontal and vertical wavelengths of the secondary gravity waves are 3–8 km and 3–20 km, respectively, and phase velocities are −1.5 to +4 m s−1. The amplitudes of the vertical velocity component of the secondary waves are 0.1–0.2 m s−1 at altitudes of about 20 km. Theoretical consideration and model simulation suggest that the winter stratosphere can be a wave duct for small-scale gravity waves once generated there as in the present simulation, because zonal winds are minimized in the winter stratosphere between the height regions of the subtropical jet and the mesospheric jet. It is also suggested that both the convective instability and an instability related with normal-modes act separately in different areas to generate the secondary gravity waves.

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