On the low-frequency, planetary-scale motion in the tropical atmosphere and oceans

1987 ◽  
Vol 4 (3) ◽  
pp. 249-263 ◽  
Author(s):  
Maicun Li
Keyword(s):  
Low Frequency ◽  
Tropical Atmosphere ◽  
Planetary Scale ◽  
Scale Motion

scholarly journals Dynamics of Eddy-Driven Low-Frequency Dipole Modes. Part I: A Simple Model of North Atlantic Oscillations

2007 ◽  
Vol 64 (1) ◽  
pp. 3-28 ◽  
Author(s):  
Dehai Luo ◽  
Anthony R. Lupo ◽  
Han Wan
Keyword(s):  
Life Cycle ◽  
Theoretical Model ◽  
North Atlantic ◽  
Low Frequency ◽  
Positive Phase ◽  
Negative Phase ◽  
Synoptic Scale ◽  
Planetary Scale ◽  
The North ◽  
The North Atlantic Oscillation

Abstract A simple theoretical model is proposed to clarify how synoptic-scale waves drive the life cycle of the North Atlantic Oscillation (NAO) with a period of nearly two weeks. This model is able to elucidate what determines the phase of the NAO and an analytical solution is presented to indicate a high similarity between the dynamical processes of the NAO and zonal index, which is not derived analytically in previous theoretical studies. It is suggested theoretically that the NAO is indeed a nonlinear initial-value problem, which is forced by both preexisting planetary-scale and synoptic-scale waves. The eddy forcing arising from the preexisting synoptic-scale waves is shown to be crucial for the growth and decay of the NAO, but the preexisting low-over-high (high-over-low) dipole planetary-scale wave must be required to match the preexisting positive-over-negative (negative-over-positive) dipole eddy forcing so as to excite a positive (negative) phase NAO event. The positive and negative feedbacks of the preexisting dipole eddy forcing depending upon the background westerly wind seem to dominate the life cycle of the NAO and its life period. An important finding in the theoretical model is that negative-phase NAO events could be excited repeatedly after the first event has decayed, but for the positive phase downstream isolated dipole blocks could be produced after the first event has decayed. This is supported by observed cases of the NAO events presented in this paper. In addition, a statistical study of the relationship between the phase of the NAO and blocking activity over Europe in terms of the seasonal mean NAO index shows that blocking events over Europe are more frequent and long-lived for strong positive-phase NAO years, indicating that the positive-phase NAO favors the occurrence of European blocking events.

The interaction between inner and outer regions of turbulent wall-bounded flow

2007 ◽  
Vol 365 (1852) ◽  
pp. 683-698 ◽  
Author(s):  
Jonathan F Morrison
Keyword(s):  
Reynolds Number ◽  
Large Scale ◽  
Low Frequency ◽  
Linear Process ◽  
Asymptotic Condition ◽  
Outer Scale ◽  
Active Motion ◽  
High Reynolds ◽  
Scale Motion ◽  
Turbulent Wall

The nature of the interaction between the inner and outer regions of turbulent wall-bounded flow is examined. Townsend's theory of inactive motion is shown to be a first-order, linear approximation of the effect of the large eddies at the surface that acts as a quasi-inviscid, low-frequency modulation of the shear-stress-bearing motion. This is shown to be a ‘strong’ asymptotic condition that directly expresses the decoupling of the inner-scale active motion from the outer-scale inactive motion. It is further shown that such a decoupling of the inner and outer vorticity fields near the wall is inappropriate, even at high Reynolds numbers, and that a ‘weak’ asymptotic condition is required to represent the increasing effect of outer-scale influences as the Reynolds number increases. High Reynolds number data from a fully developed pipe flow and the atmospheric surface layer are used to show that the large-scale motion penetrates to the wall, the inner–outer interaction is not describable as a linear process and the interaction should more generally be accepted as an intrinsically nonlinear one.

Determination of Viscous Damping for Low Frequency Motions of Floating Structures

2011 ◽  
Author(s):  
Z. J. Huang ◽  
B. J. O’Donnell ◽  
T. W. Yung ◽  
S. T. Slocum
Keyword(s):  
Forced Oscillation ◽  
Low Frequency ◽  
Test Method ◽  
Irregular Waves ◽  
Viscous Damping ◽  
Damping Force ◽  
Near Resonance ◽  
Research Company ◽  
Decay Tests ◽  
Scale Motion

ExxonMobil Upstream Research Company developed an advanced model test method to determine reliable damping values for predicting low frequency motions of an FLNG barge and an LNG carrier [1]. An inertial compensation system was introduced in the test to confidently isolate the relatively very small viscous damping force from the total measured forces in the forced oscillation tests. In the system, the spring stiffness in the restoring mechanism was tuned such that the test was done near resonance. This method has been successfully applied to ExxonMobil forced oscillation tests to measure damping of deeply submerged, double body models. Three types of motions were generated in the tests: sinusoidal motions, decay motions and motions with multiple frequencies. In this paper, the authors attempt to correlate the damping obtained from decay tests and from tests with motions of multiple frequency components. Findings from this work help determine damping for predictions of full scale motion in irregular waves.

scholarly journals On the new modes of planetary-scale electromagnetic waves in the ionosphere

2004 ◽  
Vol 22 (4) ◽  
pp. 1203-1211 ◽  
Author(s):  
G. D. Aburjania ◽  
K. Z. Chargazia ◽  
G. V. Jandieri ◽  
A. G. Khantadze ◽  
O. A. Kharshiladze
Keyword(s):  
Electromagnetic Waves ◽  
Large Scale ◽  
Low Frequency ◽  
Planetary Scale ◽  
Phase Velocities ◽  
F Region ◽  
E Region ◽  
General Dispersion ◽  
Fast Waves ◽  
The Waves

Abstract. Using an analogy method the frequencies of new modes of the electromagnetic planetary-scale waves (with a wavelength of 103 km or more), having a weather forming nature, are found at different ionospheric altitudes. This method gives the possibility to determine spectra of ionospheric electromagnetic perturbations directly from the dynamic equations without solving the general dispersion equation. It is shown that the permanently acting factor-latitude variation of the geomagnetic field generates fast and slow weakly damping planetary electromagnetic waves in both the E- and F-layers of the ionosphere. The waves propagate eastward and westward along the parallels. The fast waves have phase velocities (1–5)km s–1 and frequencies (10–1–10–4), and the slow waves propagate with velocities of the local winds with frequencies (10–4–10–6)s–1 and are generated in the E-region of the ionosphere. Fast waves having phase velocities (10-1500)km s–1 and frequencies (1–10–3)s–1 are generated in the F-region of the ionosphere. The waves generate the geomagnetic pulsations of the order of one hundred nanoTesla by magnitude. The properties and parameters of the theoretically studied electromagnetic waves agree with those of large-scale ultra-low frequency perturbations observed experimentally in the ionosphere. Key words. Ionosphere (ionospheric disturbances; waves propagation; ionosphere atmosphere interactions)

scholarly journals Eddy-Induced Instability for Low-Frequency Variability

2010 ◽  
Vol 67 (6) ◽  
pp. 1947-1964 ◽  
Author(s):  
F-F. Jin
Keyword(s):  
Dynamic Instability ◽  
Low Frequency ◽  
Basic Flow ◽  
Mean Flow ◽  
Eddy Activity ◽  
Planetary Scale ◽  
Synoptic Eddy ◽  
Low Frequency Variability ◽  
Eddy Structures ◽  
Scale Perturbation

Abstract Synoptic eddy–mean flow interaction has been recognized as one of the key sources for extratropical low-frequency variability. In this paper, the underlying dynamics of this interaction are examined from the perspective of a synoptic eddy-induced dynamic instability. To delineate this instability, a barotropic model is used that is linearized with respect to a stochastic basic flow prescribed with both climatologic-mean flow and synoptic eddy statistics. This linear model captures the dynamics of feedback between synoptic eddy and low-frequency flow through a dynamic closure that relates the anomalous eddy vorticity forcing to low-frequency flow anomalies. After reducing this dynamic closure to its fundamental components, this stability is elucidated with analytical results under the most idealized consideration of basic flow. It is shown that through systematic alteration of the synoptic eddy structures in the basic flow, a low-frequency planetary-scale perturbation generates anomalous eddy vorticity forcing positively proportional to the vorticity of the perturbation. Such a perturbation amplifies itself; the energy source for its growth comes from the reservoir residing in the basic synoptic eddy activity. Thus, the growth rate of the synoptic eddy-induced dynamic instability depends primarily on the kinetic energy level of the basic synoptic eddy activity. Moreover, this instability is scale selective with preference for zonal symmetric and asymmetric planetary-scale modes, whose meridional and zonal scales are roughly in the range of those of the observed leading low-frequency patterns. Analysis of this synoptic eddy-induced instability provides insight into the origin of extratropical low-frequency variability.

scholarly journals Planetary-scale Coherent Structures of Tropical Moist Convection

1998 ◽  
Vol 51 (5) ◽  
pp. 865 ◽  
Author(s):  
Jun-Ichi Yano
Keyword(s):  
Coherent Structures ◽  
Tropical Atmosphere ◽  
Moist Convection ◽  
Initial Attempt ◽  
Planetary Scale ◽  
Characteristic Scales

Two competing pictures for planetary-scale moist convective coherency in the tropical atmosphere exist. The nonlinear turbulent picture emphasises the scaling nature of the system, whereas the traditional picture emphasises the characteristic scales associated with the variability. Some idealised simulations were performed as an initial attempt to reconcile these two competing pictures.

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