Dominant Time Scales Research Articles

Attractive Instability of Oppositely Charged Membranes Induced by Charge Density Fluctuations

We predict the conditions under which two oppositely charged membranes show a dynamic, attractive instability. Two layers with unequal charges of opposite sign can repel or be stable when in close proximity. However, dynamic charge density fluctuations can induce an attractive instability and thus facilitate fusion. We predict the dominant instability modes and time scales and show how these are controlled by the relative charge and membrane viscosities. These dynamic instabilities may be the precursors of membrane fusion in systems where artificial vesicles are engulfed by biological cells of opposite charge.

Practical planet prospecting

(Abridged) Space missions to search for exo-planets via the transit method, such as COROT, Eddington and Kepler, will need to address problems associated with the automated and efficient detection of planetary transits in light curves affected by a variety of noise sources, including stellar variabilility. Starting from a general purpose maximum likelihood approach we discuss the links between a variety of period and transit finding methods. The natural endpoint of this hierarchy of methods is shown to be a fast, robust and statistically efficient least-squares algorithm based on box-shaped transits. This approach is predicated on the assumption of periodic transits hidden in random noise, usually assumed to be superposed on a flat continuum with regular continuous sampling. We next show how to generalise the transit finding method to the more realistic scenario where complex stellar (micro) variability, irregular sampling and long gaps in the data are all present. Tests of this methodology on simulated Eddington light curves including realistic stellar micro-variability, irregular sampling and data gaps, are used to quantify the performance. In the case where transit durations are short compared to the dominant timescales for stellar variability and data record segments, it is possible to decouple the transit signal from the remainder. We conclude that even with realistic contamination from stellar variability, irregular sampling, and gaps in the data record, it is still possible to detect transiting planets with an efficiency close to the idealised theoretical bound. In particular, space missions have the potential to approach the regime of detecting earth-like planets around G2V-type stars.

Interannual-Interdecadal Variation in Large-Scale Atmospheric Circulation and Extremely Wet and Dry Summers in China/Japan during 1951-2000 Part II: Dominant Timescales

Based on the results from the SVD analysis in Part I, here we use wavelet and composite analyses to identify the dominant timescales on which the extremely wet and dry summers in China/Japan, influenced by the large-scale regional atmospheric circulation, may have occurred in 1951-2000. An apparent trend is found in the temporal coefficient of SVD1, with a less significant trend in SVD2. Both trends change sign in the late 1970s. Associated with these trends, the 500 hPa continental high over Eurasia is greatly intensified, with increased 850 hPa northerly wind trends between 100°E-120°E. This circulation change is associated with wet trends in the Yangtze River valley, northern Kyushu and the Pacific side of Tohoku, and dry trends in northern and southern China, Kinki and the Nansei Islands of Japan. All three modes display interdecadal variability, but their cycle lengths and intensities change considerably before and after the late 1970s. The quasi-biennial (QB) signal dominates over the ENSO signal on interannual timescales in all three modes, except for SVD3 after the late 1970s. In SVD2, the QB signal is predominant and modulated on bidecadal and longer timescales. The QB's amplitude peaks in the 1990s. During this decade, the extremely wet and dry summers occurred alternately in the two countries are mainly on the QB timescale of SVD2, but also modified by other timescales and other modes. Our results suggest that the timing and the location for an extremely wet/dry summer to occur be largely determined by the interaction and reinforcement among the dominant timescales of the dominant SVD modes. Since the regional large-scale circulation is better predicted at one season lead than summer rainfall, the spatial and temporal relationships found in this work may help explore possible deterministic causes of spatial/temporal covariability in circulation and rainfall, leading to improved regional climate forecast.

Interannual-Interdecadal Variation in Large-Scale Atmospheric Circulation and Extremely Wet and Dry Summers in China/Japan during 1951-2000 Part I: Spatial Patterns

This is the first part of a two-part study that seeks links between summer rainfall variability in China/Japan and the large-scale circulation over the East Asia/western Pacific region, in both space and time, for the period of 1951-2000. Part I focuses on the spatial patterns, while Part II on the dominant timescales with which some extremely wet and dry summers in these two countries may have occurred. In this part, we use the singular value decomposition (SVD) method to find the dominant covariance patterns of summer rainfall anomalies over 160 stations in China and 72 stations in Japan, and the regional 500 hPa geopotential height anomaly over 60°E-160°W, 20°S-70°N. The associated 850 hPa horizontal wind patterns are studied by linear regression against the temporal coefficients of these modes. For a positive temporal coefficient, SVD1 represents a north-south wave pattern: a blocking high over eastern Siberia, a southwestward advanced and intensified subtropical high, and an elongated mid-latitude low in between. This mode mainly represents wet/dry trends over the 50-year period in the following regions: wetter summers in the Yangtze River valley and southwestern Japan, and drier summers in northern China and the Kinki area of Japan after the climate regime shift in the late 1970s. SVD2 and SVD3 are a pair of north-south wave patterns over the East Asia/western Pacific region with their phases in quadrature. The dominance of SVD2 or SVD3 implies a shift of the main rain band in China across the Yangtze River in the north-south direction, in association with very different rainfall patterns in Japan—either most of Japan experiences above- or below-normal rainfall in the same summer due to SVD2, or rainfall anomalies are opposite in sign between the Pacific side and the Sea of Japan side due to SVD3. These three dominant SVD spatial patterns, with their positive and negative phases, represent six dominant patterns of the western Pacific subtropical high and the mid-latitude wave systems in the regional large-scale circulation, in association with six main rain patterns in the two countries. The temporal behaviors of these modes will be further studied with wavelet and composite analyses in Part II.

Kuroshio Extension Variability and Forcing of the Pacific Decadal Oscillations: Responses and Potential Feedback

Abstract A forcing mechanism is sought for the large-scale circulation changes in the Kuroshio Extension region of the western North Pacific Ocean as inferred by TOPEX/Poseidon sea surface height (SSH) data. The low-frequency signal of the Kuroshio Extension over the last decade was characterized by a modulation in its zonal mean flow intensity: the mean Kuroshio Extension jet weakened progressively from 1993 to 1996 and this trend reversed after 1997. The ability to simulate the major trends in the observed SSH signals with linear vorticity dynamics leads the authors to conclude that the modulation in the zonal mean jet was remotely forced by wind stress curl anomalies in the eastern North Pacific Ocean related to the Pacific decadal oscillations (PDOs). To be specific, the weakening (strengthening) trend in 1993–96 (1997–2001) was caused by westward expansions of negative (positive) SSH anomalies south of the Kuroshio Extension and positive (negative) SSH anomalies north of the Kuroshio Extension. Emergence of oppositely signed SSH anomalies on the two sides of the Kuroshio Extension jet is due to the different propagating speeds of the baroclinic Rossby waves, which carry the wind-induced SSH anomalies generated in the eastern North Pacific at different phases of the PDOs. Hindcasting the Kuroshio Extension jet strength over the last 45 years reveals that the jet modulation has a dominant timescale of ∼12 yr. Given the location of the Kuroshio Extension jet relative to the maximum atmospheric forcing, it is found that this dominant timescale is consistent with the preferred timescale under a stochastic white-noise atmospheric forcing. It is hypothesized that this connection between the Kuroshio Extension strength and the latitudinally dependent baroclinic adjustment contributes to an increase in variance and persistence of the North Pacific midlatitude coupled system on the decadal timescale.

Kuroshio Extension Variability and Forcing of the Pacific Decadal Oscillations: Responses and Potential Feedback

A forcing mechanism is sought for the large-scale circulation changes in the Kuroshio Extension region of the western North Pacific Ocean as inferred by TOPEX/Poseidon sea surface height (SSH) data. The low-frequency signal of the Kuroshio Extension over the last decade was characterized by a modulation in its zonal mean flow intensity: the mean Kuroshio Extension jet weakened progressively from 1993 to 1996 and this trend reversed after 1997. The ability to simulate the major trends in the observed SSH signals with linear vorticity dynamics leads the authors to conclude that the modulation in the zonal mean jet was remotely forced by wind stress curl anomalies in the eastern North Pacific Ocean related to the Pacific decadal oscillations (PDOs). To be specific, the weakening (strengthening) trend in 1993–96 (1997–2001) was caused by westward expansions of negative (positive) SSH anomalies south of the Kuroshio Extension and positive (negative) SSH anomalies north of the Kuroshio Extension. Emergence of oppositely signed SSH anomalies on the two sides of the Kuroshio Extension jet is due to the different propagating speeds of the baroclinic Rossby waves, which carry the wind-induced SSH anomalies generated in the eastern North Pacific at different phases of the PDOs. Hindcasting the Kuroshio Extension jet strength over the last 45 years reveals that the jet modulation has a dominant timescale of ∼12 yr. Given the location of the Kuroshio Extension jet relative to the maximum atmospheric forcing, it is found that this dominant timescale is consistent with the preferred timescale under a stochastic white-noise atmospheric forcing. It is hypothesized that this connection between the Kuroshio Extension strength and the latitudinally dependent baroclinic adjustment contributes to an increase in variance and persistence of the North Pacific midlatitude coupled system on the decadal timescale.

Frequency response of polarization switching in vertical-cavity surface-emitting lasers

We present an experimental study of the current-driven polarization modulation properties of VCSELs. In some VCSELs, abrupt polarization switching (PS) between two polarization modes is observed at a particular value of the pump current. We investigate the dynamics and the associated dominating time scales of PS as these features are strongly linked with the underlying physical mechanism causing the PS. To this end, we measure both for gain- and index-guided VCSELs the critical modulation amplitude necessary to steadily force PS back and forth across the PS point as a function of the modulation frequency. This yields the current-driven polarization modulation frequency response, which we compare with the thermal frequency response of the studied devices. The dynamic behavior turns out to be strikingly different for the different VCSEL types. Thermal effects only play a minor role in the PS in our index-guided VCSELs, while they really seem to lie at the origin of PS in the gain-guided VCSELs. By implementing this in a rate-equation based theoretical model, we are able to explain the peculiarities of the measured response curves and to reproduce the experimental findings.

A simple model of real-time flow aggregation

The IETF's integrated services (IntServ) architecture, together with reservation aggregation, provides a mechanism to support the quality-of-service demands of real-time flows in a scalable way, i.e., without requiring that each router be signaled with the arrival or departure of each new flow for which it forwards data. However, reserving resources in "bulk" implies that the reservation does not precisely match the true demand. Consequently, if the flows' demanded bandwidth varies rapidly and dramatically, aggregation can incur significant performance penalties of under-utilization and unnecessarily rejected flows. On the other hand, if demand varies moderately and at slower time scales, aggregation can provide an accurate and scalable approximation to IntServ. We develop a simple analytical model and perform extensive trace-driven simulations to explore the effectiveness of aggregation under a broad class of factors. Example findings include: 1) a simple single-time-scale model with random noise can capture the essential behavior of surprisingly complex scenarios; 2) with a two-order-of-magnitude separation between the dominant time scale of demand and the time scale of signaling and moderate levels of secondary noise, aggregation achieves a performance that closely approximates that of IntServ.

A measurement-analytic approach for QoS estimation in a network based on the dominant time scale

In this paper, we describe a measurement-analytic approach for estimating the overflow probability, an important measure of the quality of service (QoS), at a given multiplexing point in the network. A multiplexing point in the network could be a multiplexer or an output port of a switch or router where resources such as bandwidth and buffers are shared. Our approach impinges on using the notion of the dominant time scale (DTS), which corresponds to the most probable time scale over which overflow occurs. The DTS provides us with a measurement window for the statistics of the traffic, but is in fact itself defined in terms of the statistics of the traffic over all time. This, in essence, results in a chicken-and-egg type of unresolved problem. For the DTS to be useful for on-line measurements, we need to be able to break this chicken-and-egg cycle, and to estimate the DTS with only a bounded window of time over which the statistics of the traffic are to be measured. In this paper, we present a stopping criterion to successfully break this cycle and find a bound on the DTS. Thus, the result has significant implications for network measurements. Our approach is quite different from other works in the literature that require off-line measurements of the entire trace of the traffic. In our case, we need to measure only the statistics of the traffic up to a bound on the DTS. We also investigate the characteristics of this upper bound on the DTS, and provide numerical results to illustrate the utility of our measurement analytic approach.

Effect of Unsteady Wake Passing Frequency on Boundary Layer Transition, Experimental Investigation, and Wavelet Analysis

Detailed experimental and theoretical investigations were carried out to study the effect of unsteady wake passing frequency on the boundary layer transition along the concave surface of a curved plate under a zero longitudinal pressure gradient. Periodic unsteady flow with different passing frequencies is generated utilizing an unsteady flow research facility with a rotating cascade of rods positioned upstream of the curved plate. Extensive unsteady boundary layer measurements are carried out. The data are analyzed using conventional and wavelet-based methods. Local time scales are defined as those of the most energetic fluctuations, and are calculated from wavelet transforms of the velocity signals. The dominant time scales are mapped as functions of the distance to the plate, the downstream location, and the phase relative to the wake-passing. Furthermore, conditional sampling is applied, laminar and turbulent time scales are calculated and the effects of wake passing frequency on these scales are shown.

Characterizing Some of the Influences of the General Circulation on Subtropical Marine Boundary Layer Clouds

Abstract The seasonal and intraseasonal variability of boundary layer cloud in the subtropical eastern oceans is studied using combined data from the International Satellite Cloud Climatology Project and the European Centre for Medium-Range Weather Forecasts reanalysis. Spectral analysis reveals that most of the time variability of cloud properties occurs on seasonal to annual timescales. The variance decreases by one to two orders of magnitude for each decade of timescale decrease, indicating that daily to monthly timescales and their spatial extent have smaller, although nonnegligible, variability. The length of these dominant timescales suggests that the majority of the variability is influenced by the general circulation and its interaction with boundary layer turbulence, rather than being a product of local boundary layer turbulence alone. Although the dominance of seasonal to annual periods in the temporal power spectra of low-cloud fraction—TAU and CTP—justifies the previous focus of effort on seas...

The spectral description of climate change including the 100 ky energy

Core records, both ice and deep-sea, suggest that the dominant character is that of a red-noise process or random walk. Examination of a few typical records supports the inference that the contribution of the Milankovitch frequencies to climate change at most represents only a small fraction of total climate variance. Most spectral densities are sufficiently "flat" that rates of change will be dominated by the highest frequencies present in the forcing. A broad maximum near 100 ky period can be readily rationalized without invoking an oscillator. One need only suppose that there is an approximate threshold beyond which the climate system collapses. The quasiperiodicity is then governed by a combination of the collapse threshold, the system memory time scale, and the intensity of the stochastic forcing. Changes in the forcing intensity would lead to a shift in the dominant time scale. Some inferred spectral power laws may be inaccurate owing to undersampling of the records.

Onset of the Summer Monsoon over the Indochina Peninsula: Climatology and Interannual Variations*

The temporal and spatial structures of the atmospheric circulation associated with the climatology and interannual variations of the summer monsoon onset over the Indochina Peninsula were studied using the observed daily rainfall at 30 stations and the NCEP‐NCAR reanalysis from 1951 to 1996. The climatological monsoon onset over Indochina is on 9 May, with a standard deviation of 12 days. The monsoon onset is characterized by the pronounced northeastward progression of the low-level southwesterlies over the Indian Ocean and the intensification and northward extension of the tropical convection from Sumatra. It coincides with the weakening of the midlatitude westerly over south Asia, and the westward propagation of the intraseasonal oscillation (ISO) originated in the South China Sea (SCS) and the western Pacific with a dominant timescale of 12‐25 days.

Dominate time scale characteristics of the turbulence across canopy-atmosphere interface of a mixed broadleaved-Korean pine forest in Changbai Mountains

The measurement and observation for this study were carried out at Forest Ecosystem Opened Research Station of Changbai Mountains (128°28′E and 42°24′ N, Jilin Province, P. R. China). Characteristics of Dominant time scales (DTSes) and dominant time scales contribution ratios (DCRs) across canopy-atmosphere interface and controlling factors were analyzed with multi-scale method. DTS of stream wise and lateral velocity components were larger than that of vertical velocity component. While DCR of vertical velocity component was larger than that of the stream wise and lateral velocity components. Effect of atmospheric stability on DCR and DTS was nonlinear. DTS under unstable conditions was larger than that under stable conditions; DTS and DCR were determined upon the original scales characteristics of upwind flow under strong stability. Canopy structure would influence DTS and DCR across canopy-atmosphere interface remarkably.

Unsteady air bubble entrainment and detrainment at a plunging breaker: dominant time scales and similarity of water level variations

At plunging breakers, air bubbles are entrained at the impingement of the water jet, formed at the top of the wave, with the water free surface in front. During the present study, air bubble entrainment at a pseudo-plunging breaker was investigated at near full-scale and further experimental work studied the bubble detrainment process. Experimental observations included the generation and propagation of waves downstream of the plunge point. Experimental results highlighted a number of unsteady air–water flow patterns and emphasise high levels of aeration: i.e., depth-averaged void fraction of more than 10% next to jet impact in shallow waters. Unsteady bubble injection experiments showed a strong vortical motion induced by the rising bubbles. Altogether, the results suggest that a dominant time scale is the bubble rise time d 1/ u r, which cannot be scaled properly with an undistorted Froude model. The study contributes to a better understanding of unsteady bubble entrainment at a pseudo-plunging breaker and the associated vortical circulation.

Traffic equivalence and substitution in a multiplexer with applications to dynamic available capacity estimation

For a multiplexer fed by a large number of sources, we derive conditions under which a given subset of the sources can be substituted for a single source while preserving the buffer overflow probability and the dominant timescales of buffer overflows. This notion of traffic equivalence is stronger than simple effective bandwidth equality and depends on the multiplexing context. We propose several applications of the above traffic substitution conditions. First, we show that fractional Brownian motion as a single source substitute can effectively model a large number of multiplexed sources using information obtained purely from traffic traces; this has direct application to simple but accurate traffic generation. Second, we focus on dynamic (i.e., on-line) estimation of available capacity and buffer overflow probability. This requires the solution of a double optimization problem expressed in terms of functions whose values are obtained from time averages of the traffic traces over a large range of timescales. We show how to solve this problem on-line by reducing it to the calculation of a fixed-point equation that can be solved iteratively by combining traffic substitution using fractional Brownian motion with dynamic measurements of the actual traffic. We have validated this approach by extensive experimentation with large numbers of real traffic sources that are fed to a high bandwidth link, and comparing our on-line estimation of available capacity and the resulting dynamic call admission control with other existing approaches. The superior accuracy of our approach also suggests that taking the buffer size into account, as does our on-line algorithm, may be vital for achieving approximations of practical interest.

The ultrasonic velocity profile measurement of flow structure in the near field of a square free jet

Coherent structures in the near field of a three-dimensional jet have been investigated. Experiments were carried out for a free jet issuing from a square nozzle using a water channel. Instantaneous velocity profiles were obtained in the axial and radial directions by using an ultrasonic velocity profile (UVP) monitor. Axial variations of dominant time-scales of vortex structures were examined from one-dimensional wavelet spectra. Wavenumber-frequency spectra were calculated by two-dimensional Fourier transform along the axial direction in a mixing layer, and it was found that a convective velocity of flow structures was nearly constant independently of their scales in space and time. Coherent structures in the axial direction were investigated in terms of proper orthogonal decomposition (POD). Eigenfunctions are similar to a sinusoidal wave, and reconstructed velocity fields by the lower-order and higher-order POD modes demonstrate large-scale and smaller-scale coherent structures, respectively.

Spatial and Temporal Variations of the Rainy Season over Indonesia and their Link to ENSO.

Regional and interannual variations of the rainy season over Indonesia are investigated using daily rainfall data during 1961-90. Pentad-mean rainfall data, with a relatively better continuity have been obtained for 46 stations, and the annual and semi-annual cycles of rainfall at these stations have been objectively analyzed by harmonic analysis. The onset of southern-hemispheric spring/summer (SON/ DJF) rainy season starts from the Indian Ocean side of Jawa in the middle September, and propagates northward (in Jawa) and eastward (to Nusa Tenggara in middle December). Another route of rainy sea-son propagation from Irian Jaya to Nusa Tenggara is observed. The withdrawal of the rainy season starts from western Nusa Tenggara in March, and goes eastward (to eastern Nusa Tenggara), and westward (to Jawa), until late May. Rainy season onset comes later (earlier) in El Nino (La Nina) years than the average at most stations (particularly in the south-eastern part of Jawa). Correlations between rainfall amounts at those stations, and the southern oscillation index in SON, are significantly high. However, the rainfall amount throughout a rainy season is not dependent upon the length of the rainy season (between onset and withdrawal) in many areas (except southern Sulawesi). The dominant time scale of interannual variations of rainy season onset during 1961-90 is 2-3 years, which looks somewhat shorter than that during 1910-41.

Active / break cycles: diagnosis of the intraseasonal variability of the Asian Summer Monsoon

In this study, various diagnostics have been applied to daily observed outgoing longwave radiation (OLR) and ECMWF ReAnalysis (ERA) products to provide a comprehensive description of the active/break cycles associated with the Asian Summer Monsoon and to address the differing behaviour of the two dominant time scales of intraseasonal variability, 10–20 days and 30–60 days. Composite analysis of OLR based on filtered daily All-India rainfall (AIR) for the 40 day (30–60 days) intraseasonal mode indicates that during active phases, convection is significantly enhanced over the Indian continent, extending over the Bay of Bengal, Maritime continent and equatorial west Pacific, while convection is suppressed over the equatorial Indian Ocean and northwest tropical Pacific, resulting in a ‘quadrapole’ structure over the Asian monsoon domain. In response to this heating pattern, the large-scale Hadley (lateral) and the two east-west (transverse) tropical circulations are enhanced. There is also a significant impact on the extra-tropical circulation through excitation and propagation of Rossby waves. In contrast, the 15-day mode is more regional to the monsoon domain and has a prominent east-west orientation in convection. Only the local Hadley circulation over the monsoon region is modulated by this mode. The evolution of these two modes as revealed by POP (principal oscillation pattern) analysis, shows that the 40-day mode originates over the equatorial Indian Ocean. Once formed it has poleward propagation on either side of the equator, and eastward propagation into the equatorial west Pacific. From the equatorial west Pacific, northward propagation over the west Pacific and westward propagation into the Indian longitudes are prominent. The propagative features are complex and interactive and are responsible for the ‘quadrapole’ structure in convection seen from the composites. The interannual variability, assessed from the POP coefficient time series, indicates that the 40-day mode is strong during the onset phase of the monsoon in all the years but systematic propagation over the entire season depends crucially on the activity of the oceanic TCZ (tropical convergence zone). The POP analysis of the 15-day mode indicates that this event originates over the␣equatorial west Pacific, associated with westward propagating Rossby waves, amplifies over the northwest tropical Pacific and modulates both the continental and oceanic TCZs over Indian longitudes simultaneously. This mode is pronounced during the established phase of the monsoon. Due to the complexity in the propagational features of both the intraseasonal modes, it is concluded that understanding the subseasonal variability of one regional component of the Asian Summer Monsoon (ASM), requires understanding the entire ASM system.

The Kuroshio East of Taiwan: Moored Transport Observations from the WOCE PCM-1 Array

Observations from the WOCE PCM-1 moored current meter array east of Taiwan for the period September 1994 to May 1996 are used to derive estimates of the Kuroshio transport at the entrance to the East China Sea. Three different methods of calculating the Kuroshio transport are employed and compared. These methods include 1) a “direct” method that uses conventional interpolation of the measured currents and extrapolation to the surface and bottom to estimate the current structure, 2) a “dynamic height” method in which moored temperature measurements from moorings on opposite sides of the channel are used to estimate dynamic height differences across the current and spatially averaged baroclinic transport profiles, and 3) an “adjusted geostrophic” method in which all moored temperature measurements within the array are used to estimate a relative geostrophic velocity field that is referenced and adjusted by the available direct current measurements. The first two methods are largely independent and are shown to produce very similar transport results. The latter two methods are particularly useful in situations where direct current measurements may have marginal resolution for accurate transport estimates. These methods should be generally applicable in other settings and illustrate the benefits of including a dynamic height measuring capability as a backup for conventional direct transport calculations. The mean transport of the Kuroshio over the 20-month duration of the experiment ranges from 20.7 to 22.1 Sv (1 Sv ≡ 106 m3 s−1) for the three methods, or within 1.3 Sv of each other. The overall mean transport for the Kuroshio is estimated to be 21.5 Sv with an uncertainty of 2.5 Sv. All methods show a similar range of variability of ±10 Sv with dominant timescales of several months. Fluctuations in the transport are shown to have a robust vertical structure, with over 90% of the transport variance explained by a single vertical mode. The moored transports are used to determine the relationship between Kuroshio transport and sea-level difference between Taiwan and the southern Ryukyu Islands, allowing for long-term monitoring of the Kuroshio inflow to the East China Sea.