Search Results

You are looking at 21 - 30 of 52 items for :

  • Author or Editor: Timothy J. Dunkerton x
  • Journal of the Atmospheric Sciences x
  • All content x
Clear All Modify Search
Timothy J. Dunkerton

Abstract

A semiannual oscillation in monthly mean wind has been observed in the upper mesosphere over Ascension Island (8°S) and Kwajalein (9°N). It is suggested that the selective transmission of gravity and Kelvin waves through the lower-level stratopause semiannual oscillation is responsible for this “mesopause” semiannual oscillation. No in situ semiannual forcing is required at the mesopause.

The theoretical model developed here also illustrates the importance of the time-mean component of the mean zonal flow as it affects wave propagation through the equatorial middle atmosphere.

Full access
Timothy J. Dunkerton

Abstract

The propagation and refraction of stationary inertia–gravity waves in the winter stratosphere is examined with ray tracing. Due to their smaller vertical group velocity these waves experience more lateral ray movement and horizontal refraction that the simple gravity waves recently discussed by Dunkerton and Butchart. Stationary waves are rotated by the transverse horizontal shear and propagate into the polar night jet. Circumstances are found in which the mean flow shear has enhanced unstable wavebreaking by compressing, the wave packet and decreasing the absolute value of wave action density required for breaking. In some other places, reflection from the caustic is more likely.

Full access
Timothy J. Dunkerton

Abstract

It is shown that the analytic transient internal gravity wave solutions derived by Dunkderton (1981a,b) remain qualitatively unchanged when a “saturation hypothesis” is included in the analysis. Furthermore, the wave flux in the saturated region is not constant in height, and experiences considerable falloff as the critical level is approached. Wave transience would appear to lower the level of wavebreaking on the order of a scale height.

It is also shown that these analytic solutions allow shock formation at the trailing edge of the wave packet, for both Boussinesq and atmospheric cases. An “equal-area” rule any be used to determine the position of both internal and trailing shocks. Saturation leads to a net mean flow change slightly different from that of the “nonsaturated” solutions.

Full access
Timothy J. Dunkerton

Abstract

Inertial instabilities on the equatorial beta-plane may take the form of a zonally nonsymmetric disturbance while preserving their centrifugal character. Numerical experiments at finite zonal wavenumber suggest a preferred mode of instability with zonal wavenumber between the symmetric value and a short-wave cutoff in linear cross-equatorial shear. Zonal nonsymmetric results in a slight reduction of the marginally stable shear in the presence of second-order diffusion.

Full access
Timothy J. Dunkerton

Abstract

The dynamical state of the stratosphere influenced by radiative heating, with no internal sources or sinks of angular momentum, is examined. It is shown that there exists a nonlinear Hadley regime driven by antisymmetric (or more generally, asymmetric) thermal equilibria typical of the middle atmosphere at the solstices. This regime consists of a single mean meridional cell, equatorial easterlies and strong winter westerlies. Outside of the circulation region the flow is in thermal equilibrium.

The effect of one-sided friction, acting as a drag on midlatitude westerlies only, is to expand the Hadley cell into the winter hemisphere and increase the magnitude of cross-equatorial flow. This result is possible even in the steady state when the advection of angular momentum in the tropics is made small by reducing the gradient of angular momentum in this region instead of the advecting velocity.

Full access
Timothy J. Dunkerton

Abstract

A simple theoretical model was developed to investigate the inertial instability of zonally nonuniform, non-parallel flow near the equator. The basic state was independent of height and time but included cross-equatorial shear with longitudinal variation, as observed in the tropical mesosphere and elsewhere. Numerical solutions were obtained for the most unstable modes.

It is shown that, in addition to previously known “global” (symmetric and nonsymmetric) modes of inertial instability, there exist “local” modes within regions of anomalous potential vorticity. Local modes may be exactly stationary or display zonal phase propagation, but are distinguished from global modes by their zero group velocity and concentration of amplitude within, or downstream from, the region of most unstable flow. Local stationary instability has the largest growth rate and occurs in strong inhomogeneous shear when the in situ mean flow is near zero, that is, quasi-stationary with respect to the (stationary) basic-state pattern. This situation is expected in an equatorial Rossby wave critical layer.

The local mode has properties similar to those of “absolute” instability of nonparallel flow as discussed elsewhere in fluid dynamics.

Full access
Volkmar Wirth and Timothy J. Dunkerton

Abstract

This paper investigates the occurrence, formation, and maintenance of eyes in idealized axisymmetric balanced vortices with diabatic forcing. Two key elements of the model setup are temperature relaxation toward a specified equilibrium temperature Te and Ekman pumping from a turbulent boundary layer. Furthermore, the flow is assumed to be almost inviscid in the interior. The model does not attempt any closure for moist convection. Previous work by the authors has shown that there is a continuous transition from monsoonlike vortices to hurricane-like vortices. This transition is governed by the ratio F = αT /cD, where αT is the thermal relaxation rate and cD the surface drag coefficient.

An eye is defined in terms of the vertical wind with maximum upwelling occurring at some finite radius rather than at the origin. It is possible to obtain an eye even though Te maximizes at the origin, that is, even though Te does not directly predispose upwelling at some finite radius. The occurrence of an eye is controlled by F, and the transition between vortices without any eye and vortices with a clearly defined eye is rather sudden. These results are robust with respect to the amplitude of the forcing or the specific shape of Te. The key role of F is corroborated through a systematic nondimensionalization. In a steady-state hurricane-like vortex, mechanical forcing from Ekman pumping maximizes at some finite radius and is instrumental for the maintenance of an eyelike secondary circulation. On the other hand, eye formation during spinup is a purely inviscid process. The results imply that eye formation is a robust and general feature in vortices with strong diabatic forcing.

Full access
Kaoru Sato and Timothy J. Dunkerton

Abstract

Horizontal wind and temperature data obtained from operational radiosondes over Japan have recently been available with high vertical resolution. Analyzing these data over 4 yr has indicated horizontal velocity layers with vertical scales of about 5 km lasting for a week or more. The layers appear frequently in winter at several stations simultaneously and are dominant in the height range of 8–16 km. An empirical orthogonal function (EOF) analysis for the time series of layered disturbance amplitude in winter indicates that there are two dominant principal components. The first component (EOF1) describes layered disturbances in the middle of Japan (30°–37°N) and the second one (EOF2) describes disturbances in the south of Japan (23°–30°N). Using global analysis data, the background field of the layered disturbances was examined. An interesting result is that the background potential vorticity (PV) is approximately zero or negative for EOF2 disturbances even though located in a relatively high-latitude region. This fact suggests that the EOF2 disturbances are due to inertial instability. It is also shown that negative PV occurs more than 30% of the time in winter, in a zonally elongated region of 23°–29°N in the western Pacific, on an isentropic surface of 345 K (∼10 km altitude). Such a high frequency of negative PV is not observed at other longitudes in this latitude band. To determine the origin of the anomalous PV, backward trajectories were analyzed. For EOF2 disturbances, air parcels having mostly negative PV are traced back to the equatorial region in the longitude band 20°W–140°E within a few days. This is due to a strong northward branch of the Hadley circulation associated with deep convection over the Maritime Continent and a strong northeastward subtropical jet stream. On the other hand, the background PV is low but scarcely negative for EOF1 disturbances. Air parcels at EOF1 stations are traced back to the far west because they are advected mostly by a strong eastward jet stream. Thus, it is inferred that the EOF1 disturbances may be due to inertia–gravity waves trapped in a duct of the westerly jet core.

Full access
Volkmar Wirth and Timothy J. Dunkerton

Abstract

This paper provides a unified perspective on the dynamics of hurricane- and monsoonlike vortices by identifying them as specific limiting cases of a more general flow system. This more general system is defined as stationary axisymmetric balanced flow of a stably stratified non-Boussinesq atmosphere on the f plane. The model is based on the primitive equations assuming gradient wind balance in the radial momentum equation. The flow is forced by heating in the vortex center, which is implemented as relaxation toward a specified equilibrium temperature Te. The flow is dissipated through surface friction, and it is assumed to be almost inviscid in the interior. The heating is assumed supercritical, which means that Te does not allow a regular thermal equilibrium solution with zero surface wind, and which gives rise to a cross-vortex secondary circulation. Numerical solutions are obtained using time stepping to a steady state, where at each step the Eliassen secondary circulation is diagnosed as part of the solution strategy.

Reality and regularity of the solution is discussed, putting this work in relation to previous work. Scaling analysis suggests that for a given geometry, essential vortex properties are controlled by the ratio F = αT/cD, where αT is the rate of thermal relaxation and cD quantifies the strength of surface friction for a given surface wind. For large F, the temperature is close to Te and the vortex shows properties that can be associated with a hurricane including strong cyclonic surface winds. On the other hand, for small F, the vortex shows properties that can be associated with a monsoon; that is, the surface winds are small and the secondary circulation keeps the temperature significantly away from Te. The scaling analysis is verified by numerical solutions spanning a wide range of the parameter space. It is shown how the two limiting cases correspond with the respective approximate semianalytical theories presented previously. The results imply an important role of αT for hurricane formation.

Full access
Timothy J. Dunkerton and Neal Butchart

Abstract

Longitudinally asymmetric features of gravity wave propagation in a sudden warming are examined theoretically, using observed geostrophic wind fields in the stratosphere for three days of winter 1979. It is shown that the wind patterns accompanying a sudden warming act to reduce, but not eliminate, quasi-stationary gravity wave propagation to the mesosphere. The onset of large-amplitude planetary waves leads to the formation of propagating zones and forbidden zones for gravity waves of intermediate horizontal scale (50–200 km). Lateral ray movement and horizontal refraction are secondary but observable effects for these waves.

To the extent that these waves are excited isotropically in the troposphere, it is possible to evaluate the direction and magnitude of the average wavevector reaching the mesosphere as follows. Stationary waves with wavevector orthogonal to the local mean flow are selectively absorbed in the stratosphere, implying that for these waves the average wavevector transmitted to the mesosphere is antiparallel to the average of the mean flow orientation extrema in the underlying stratosphere.

Full access