Showing papers on "Rossby radius of deformation published in 1989"

A Composite Model of Mesoscale Convective Complexes

Abstract: A composite analysis technique is used to investigate the evolution of mesoscale features of mesoscale convective complexes (MCCs). The early stage of the MCC lifecycle is characterized by convergence, vertical motion and heating being centered in the lower troposphere. As the MCC matures the level of peak upward motion and heating shifts to the upper troposphere. The system achieves and maintains its maximum divergence, upward motion, and anticyclonic vorticity in the upper troposphere during the latter half of the life cycle. This is in contrast to GATE tropical clusters where the maximum divergence, upward motion, and anticyclonic vorticity occurred at the mature stage of the cluster and then weakened. This difference might be explained by an MCC being an inertially stable form of mesoscale convective system whose radius exceeds the Rossby radius of deformation. The MCC is shown to be an efficient rain producer, exhibiting a precipitation efficiency exceeding 100% at the mature stage due to th...

Rossby-Kelvin instability: a new type of ageostrophic instability caused by a resonance between Rossby waves and gravity waves

Abstract: An ageostrophic version of Phillips’ model is studied. All instabilities found are systematically interpreted in terms of resonance of wave components. The instability occurs if there is a pair of wave components which propagate in the opposite direction to the basic flow and these wave components have almost the same Doppler-shifted frequency. A new instability, identified as a resonance between the Kelvin wave and the Rossby waves, is found at Froude number F ≈ 0.7. The Rossby waves are almost completely in geostrophic balance while the ageostrophic Kelvin wave is the same as in a one-layer system. Doppler shifting matches frequencies which would otherwise be very different. This instability is presumably the mechanism of the frontal instability observed by Griffiths & Linden (1982) in a laboratory experiment. Ageostrophic, baroclinic instability with non-zero phase speed is also observed in the numerical calculation. This instability is caused by resonance between different geostrophic modes.

On the Role of Off-Equatorial Oceanic Rossby Waves during ENSO

Abstract: Recent theoretical and numerical modeling studies of the coupled tropical atmosphere-ocean system suggest that equatorial ocean wave dynamics may play an important role in the evolution of ENSO (El Nino/Southern Oscillation). These studies emphasize that the oceanic wave signal is confined to within a narrow equatorial band (within 6° of the equator). In this study we use a coupled atmosphere–ocean model to investigate the role of off-equatorial Rossby waves observed in the western North Pacific Ocean during the ENSO cycle. We find that these off-equatorial Rossby waves (found poleward of 6° from the equator) are formed through both eastern boundary reflection of the equatorial Kelvin wave signal generated in a warm event (El Nino), and changes in the off-equatorial wind stress curl. Our results indicate that, independent of the generation mechanism, off-equatorial Rossby waves should be thought of as the product and not the triggering mechanism for an ENSO event.

Internal hydraulic control in rotating fluids—applications to oceans

Abstract: Laboratory experiments and analysis of shallow water equations in a rotating fluid show that channel flow is governed by the ratio of the width of the channel to the Rossby radius of deformation R= √[gΔρH/ρf 2]. Flows through narrow ocean openings exhibit blocking and clear evidence of hydraulic control. These imply that formulae can be derived for width, volume flux, and velocity scales of the currents. A new version of the constant potential vorticity problem is solved, and it is shown to predict volume flux within 22% of the zero potential vorticity results. Next a systematic method of predicting volume flux through ocean passages is described. Some examples are given from the Denmark Straits overflow and the flow of Antarctic Bottom Water into the western Atlantic Ocean. Two-layer flows and counter-flows with rotation in a narrow passage, the so-called lock exchange flow problem, duplicate flows at a number of important straits and openings to bays. A potential vorticity formulation is ...

Rossby wave packets in baroclinic mean currents

Abstract: A WKB description of the propagation of Rossby wave packets in a shallow water model of the tropical oceans indicates that the presence of the baroclinic mean currents can modify the characteristics of wave propagation significantly. For currents with weak latitudinal shear the effect of the current itself is less important than the effect of the associated variations in the depth of the thermocline, except near critical layers where waves are absorbed. For example, a westward current, and the associated shoaling of the thermocline towards the equator, can cause the speed of the long Rossby waves to decrease with decreasing latitude. (The speed increases towards the equator in the absence of mean currents.) Westward currents inhibit meridional propagation, but eastward currents enhance it. The amplification and decay of a wave packet as it propagates through a mean current are described in terms of these conservation of wave action. Implications of these results for the propagation of Rossby waves in the real ocean are discussed.

Further evidence of normal mode Rossby waves

Abstract: Further observational evidence of normal mode Rossby waves with higher meridional mode numbers is presented with the aid of global data from the troposphere to the stratosphere over the period November 1979 through April 1986.

Rossby Wave Frequencies and Group Velocities for Finite Element and Finite Difference Approximations to the Vorticity-Divergence and the Primitive Forms of the Shallow Water Equations

Abstract: In this paper Rossby wave frequencies and group velocities are analyzed for various finite element and finite difference approximations to the vorticity-divergence form of the shallow water equations. Also included are finite difference solutions for the primitive equations for the staggered grids B and C from Wajsowicz and for the unstaggered grid A. The results are presented for three ratios between the grid size and the Rossby radius of deformation. The Yortcity-divergence equation schemes give superior solutions to those based on the primitive equations. The best results come from the finite element schemes that use linear basis functions on isosceles triangles and bilinear functions on rectangles. All of the primitive equation finite difference schemes have problems for at least one Rossby deformation-grid size ratio.

The behavior of double‐diffusive intrusion in a rotating system

Abstract: The effects of Earth's rotation on the stability of a thermohaline front of finite width are studied by means of a linear theory. It is found that when the rotation is present, two different types of unstable modes are possible. When the front is narrow and a Rossby radius of deformation based on Ruddick and Turner's (1979) vertical scale is large in comparison with the width of the front, the fastest growing intrusion is nearly two dimensional (nonrotational mode), and its vertical scale is given by Ruddick and Turner's scale. When the Rossby radius becomes small, in addition to the nonrotational mode there appears another unstable mode (the rotational mode) which has a smaller vertical wave number than the nonrotational mode. With the introduction of rotation, the fastest growing mode has nonzero along-frontal wave number; that is, the intrusion becomes tilted in the along-frontal direction. When the Rossby radius of deformation is sufficiently small in comparison with the width of the front, transition from the nonrotational mode to the rotational one occurs. The transition from nonrotational to rotational mode becomes less pronounced when the width of the front is increased for fixed horizontal density-compensating gradients of temperature and salinity. For a wide front the growth rate and vertical wave number for both modes becomes similar, which agrees with the results of previous studies for infinite fronts that rotation does not modify the behavior of the intrusion except for the occurrence of along-frontal tilt.

Transmission and reflection of the Rossby waves in a stratified ocean with bottom topography

Abstract: Transmission and reflection coefficients are calculated for Rossby waves incident on a bottom topography with constant slope in a continuously stratified ocean. The characteristics of the coefficients are interpreted in terms of the quasigeostrophic waves on the slope. In the parameter range where only the barotropic Rossby waves can propagate in the region outside the slope, the bottom trapped wave plays the same role as the topographic Rossby wave in a homogeneous ocean, and hence the transmission is weak unless phase matching takes place. When both of the barotropic and baroclinic Rossby waves can propagate outside the slope, the total transmission can be strong. The bottom trapped wave affects the transmission and reflection, and it leads to the possibility that the Rossby wave is transmitted as a mode different from the incident mode. When the number of the wavy modes on the slope is smaller than that of the Rossby wave modes outside the slope, strong reflection occurs.

Propagation of Rossby waves in stratified shear flows

Abstract: A study is made of the propagation of Rossby waves in a stably stratified shear flows. The wave equation for the Rossby waves is derived in an isothermal atmosphere on a beta plane in the presence of a latitudinally sheared zonal flow. It is shown that the wave equation is singular at five critical levels, but the wave absorption takes place only at the two levels where the local relative frequency equals in magnitude to the Brunt Vaisala frequency. This analysis also reveals that these two levels exhibit valve effect by allowing the waves to penetrate them from one side only. The absorption coefficient exp(2) is determined at these levels. Both the group velocity approach and single wave treatment are employed for the investigation of the problem.

On the generation of baroclinic rossby waves in the presence of a semi-circular boundary

Abstract: An analytical model is constructed for the generation of baroclinic Rossby waves by a vorticity source in the presence of a semi-circular boundary. The vorticity source is used to represent the effect of the Agulhas retroflection to the south of Southern Africa. The displacement of the interface between the two layers of the model ocean consists of quantized waves near the coast and a train of Rossby waves drifting westward further offshore.