scholarly journals Record statistics for random walks and Lévy flights with resetting

Author(s):  
Satya N Majumdar ◽  
Philippe Mounaix ◽  
Sanjib Sabhapandit ◽  
Gregory Schehr

Abstract We compute exactly the mean number of records $\langle R_N \rangle$ for a time-series of size $N$ whose entries represent the positions of a discrete time random walker on the line with resetting. At each time step, the walker jumps by a length $\eta$ drawn independently from a symmetric and continuous distribution $f(\eta)$ with probability $1-r$ (with $0\leq r < 1$) and with the complementary probability $r$ it resets to its starting point $x=0$. This is an exactly solvable example of a weakly correlated time-series that interpolates between a strongly correlated random walk series (for $r=0$) and an uncorrelated time-series (for $(1-r) \ll 1$). Remarkably, we found that for every fixed $r \in [0,1[$ and any $N$, the mean number of records $\langle R_N \rangle$ is completely universal, i.e., independent of the jump distribution $f(\eta)$. In particular, for large $N$, we show that $\langle R_N \rangle$ grows very slowly with increasing $N$ as $\langle R_N \rangle \approx (1/\sqrt{r})\, \ln N$ for $0<r <1$. We also computed the exact universal crossover scaling functions for $\langle R_N \rangle$ in the two limits $r \to 0$ and $r \to 1$. Our analytical predictions are in excellent agreement with numerical simulations.

Fractals ◽  
1996 ◽  
Vol 04 (02) ◽  
pp. 161-168 ◽  
Author(s):  
S. HAVLIN ◽  
A. BUNDE ◽  
H. LARRALDE ◽  
Y. LEREAH ◽  
M. MEYER ◽  
...  

The number of distinct sites visited by a random walker after t steps is of great interest, as it provides a direct measure of the territory covered by a diffusing particle. We review the analytical solution to the problem of calculating SN(t), the mean number of distinct sites visited by N random walkers on a d-dimensional lattice, for d=1, 2, 3 in the limit of large N. There are three distinct time regimes for SN(t). A remarkable transition, for dimension ≥2, in the geometry of the set of visited sites is found. This set initially grows as a disk with a relatively smooth surface until it reaches a certain size, after which the surface becomes increasingly rough. We also review the results for a model for migration and spreading of populations and diseases. The model is based on N diffusing species, where each species has a probability α- of dying (or recovery from a disease) and a probability α+ to give birth (or to infect another species). It is found analytically that when α+ ≈ α- ≠ 0, after a crossover time t× ~ N/2α-, the territory covered by the population is localized around its center of mass while the center of mass diffuses regularly. When α+ > α-, the localization breaks down after a second crossover time and the species diffuse and spread around their center of mass. These results may explain the phenomena of migration and spreading of diseases and population appearing in nature.


1989 ◽  
Vol 46 (1) ◽  
pp. 16-27 ◽  
Author(s):  
Dennis L. Scarnecchia ◽  
Árni Ísaksson ◽  
S. E. White

Investigations were conducted on the effects of oceanic variations (as measured by sea temperatures) and catches by the West Greenland salmon fishery on the sea age composition of Atlantic salmon (Salmo salar) stocks from 21 Icelandic west coast rivers. Annual ratios of grilse to two-sea-winter (2SW) salmon were strongly correlated among the 21 rivers. All eight rivers with time series extending back before the expansion of the West Greenland fishery showed lower ratios during the earlier period. Only 2 of the 21 rivers, however, had significantly declining ratios over their time series. In addition, for only one river was West Greenland catch significantly related to the ratios (P < 0.05), and for only one river did ratios increase when the expanded West Greenland fishery was active. Overall, the effects of the fishery on stock composition are evidently minimal. The mean April–May temperature when the smolts were to migrate out of rivers was significantly and positively related to subsequent ratios for five of the rivers, which, along with correlations among the ratios, indicated that more rapid growth of smolts in their first summer may have increased the ratios of grilse to 2SW salmon on several rivers.


2015 ◽  
Vol 45 (5) ◽  
pp. 1302-1324 ◽  
Author(s):  
Lisa M. Beal ◽  
Shane Elipot ◽  
Adam Houk ◽  
Greta M. Leber

AbstractThe volume transport of the Agulhas Current was measured over a 3-yr period by an array of seven current meter moorings and four current- and pressure-recording inverted echo sounders (CPIES) deployed at 34°S. CPIES extended the array farther offshore in order to capture, for the first time, the full Agulhas Current during meander events. Transports derived from CPIES are well correlated with overlapping current meter transports (0.89). The Eulerian mean current is 219 km wide and 3000 m deep, with peak surface speeds of 1.8 m s−1 and a weak northward undercurrent on the continental slope below 1200 m. A new algorithm to capture the western boundary jet transport at each time step T is defined as the poleward transport out to the first maximum of the vertically integrated velocity beyond the half-width of the mean jet. The mean transport of the Agulhas Current jet, so defined, is −84 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) with a standard error of 2 Sv. Sampling and instrumental errors are explicitly estimated and amount to an additional 9 Sv. A more traditional estimate, based on net transport integrated to a fixed distance offshore Tbox, gives a mean transport of −77 ± 5 Sv. This transport is 10 Sv greater than an equivalent transport at 32°S, corresponding to a latitudinal increase equal to that predicted by Sverdrup dynamics. The time series of T and Tbox show important differences during solitary meander events and at longer time scales. In terms of an annual cycle, the Agulhas Current appears strongest during austral summer, a similar phase to the Gulf Stream and Kuroshio.


Author(s):  
Julian Talbot ◽  
Charles Antoine

Abstract We consider a minimal model of random pan stacking. A single pan consists of a V-shaped object characterized by its internal angle α. The stack is constructed by piling up N pans with different angles in a given, random order. The set of pans is generated by sampling from various kinds of distributions of the pan angles: discrete or continuous, uniform or optimized. For large N the mean height depends principally on the average of the distance between the bases of two consecutive pans, and the effective compaction of the stack, compared with the unstacked pans, is 2 log 2/π. We also obtain the discrete and continuous distributions that maximize the mean stack height. With only two types of pans, the maximum occurs for equal probabilities, while when many types of pans are available, the optimum distribution strongly favours those with the most acute and the most obtuse angles. With a continuous distribution of angles, while one never finds two identical pans, the behaviour is similar to a system with a large number of discrete angles.


1989 ◽  
Vol 54 (1) ◽  
pp. 101-105 ◽  
Author(s):  
J. Bruce Tomblin ◽  
Cynthia M. Shonrock ◽  
James C. Hardy

The extent to which the Minnesota Child Development Inventory (MCDI), could be used to estimate levels of language development in 2-year-old children was examined. Fifty-seven children between 23 and 28 months were given the Sequenced Inventory of Communication Development (SICD), and at the same time a parent completed the MCDI. In addition the mean length of utterance (MLU) was obtained for each child from a spontaneous speech sample. The MCDI Expressive Language scale was found to be a strong predictor of both the SICD Expressive scale and MLU. The MCDI Comprehension-Conceptual scale, presumably a receptive language measure, was moderately correlated with the SICD Receptive scale; however, it was also strongly correlated with the expressive measures. These results demonstrated that the Expressive Language scale of the MCDI was a valid predictor of expressive language for 2-year-old children. The MCDI Comprehension-Conceptual scale appeared to assess both receptive and expressive language, thus complicating its interpretation.


2004 ◽  
Vol 155 (5) ◽  
pp. 142-145 ◽  
Author(s):  
Claudio Defila

The record-breaking heatwave of 2003 also had an impact on the vegetation in Switzerland. To examine its influences seven phenological late spring and summer phases were evaluated together with six phases in the autumn from a selection of stations. 30% of the 122 chosen phenological time series in late spring and summer phases set a new record (earliest arrival). The proportion of very early arrivals is very high and the mean deviation from the norm is between 10 and 20 days. The situation was less extreme in autumn, where 20% of the 103 time series chosen set a new record. The majority of the phenological arrivals were found in the class «normal» but the class«very early» is still well represented. The mean precocity lies between five and twenty days. As far as the leaf shedding of the beech is concerned, there was even a slight delay of around six days. The evaluation serves to show that the heatwave of 2003 strongly influenced the phenological events of summer and spring.


Author(s):  
Andrew Gelman ◽  
Deborah Nolan

Descriptive statistics is the typical starting point for a statistics course, and it can be tricky to teach because the material is more difficult than it first appears. The activities in this chapter focus more on the topics of data displays and transformations, rather than the mean, median, and standard deviation, which are covered easily in a textbook and on homework assignments. Specific topics include: distributions and handedness scores; extrapolation of time series and world record times for the mile run; linear combinations and economic indexes; scatter plots and exam scores; and logarithmic transformations and metabolic rates.


2009 ◽  
Vol 27 (1) ◽  
pp. 1-30 ◽  
Author(s):  
P. Prikryl ◽  
V. Rušin ◽  
M. Rybanský

Abstract. A sun-weather correlation, namely the link between solar magnetic sector boundary passage (SBP) by the Earth and upper-level tropospheric vorticity area index (VAI), that was found by Wilcox et al. (1974) and shown to be statistically significant by Hines and Halevy (1977) is revisited. A minimum in the VAI one day after SBP followed by an increase a few days later was observed. Using the ECMWF ERA-40 re-analysis dataset for the original period from 1963 to 1973 and extending it to 2002, we have verified what has become known as the "Wilcox effect" for the Northern as well as the Southern Hemisphere winters. The effect persists through years of high and low volcanic aerosol loading except for the Northern Hemisphere at 500 mb, when the VAI minimum is weak during the low aerosol years after 1973, particularly for sector boundaries associated with south-to-north reversals of the interplanetary magnetic field (IMF) BZ component. The "disappearance" of the Wilcox effect was found previously by Tinsley et al. (1994) who suggested that enhanced stratospheric volcanic aerosols and changes in air-earth current density are necessary conditions for the effect. The present results indicate that the Wilcox effect does not require high aerosol loading to be detected. The results are corroborated by a correlation with coronal holes where the fast solar wind originates. Ground-based measurements of the green coronal emission line (Fe XIV, 530.3 nm) are used in the superposed epoch analysis keyed by the times of sector boundary passage to show a one-to-one correspondence between the mean VAI variations and coronal holes. The VAI is modulated by high-speed solar wind streams with a delay of 1–2 days. The Fourier spectra of VAI time series show peaks at periods similar to those found in the solar corona and solar wind time series. In the modulation of VAI by solar wind the IMF BZ seems to control the phase of the Wilcox effect and the depth of the VAI minimum. The mean VAI response to SBP associated with the north-to-south reversal of BZ is leading by up to 2 days the mean VAI response to SBP associated with the south-to-north reversal of BZ. For the latter, less geoeffective events, the VAI minimum deepens (with the above exception of the Northern Hemisphere low-aerosol 500-mb VAI) and the VAI maximum is delayed. The phase shift between the mean VAI responses obtained for these two subsets of SBP events may explain the reduced amplitude of the overall Wilcox effect. In a companion paper, Prikryl et al. (2009) propose a new mechanism to explain the Wilcox effect, namely that solar-wind-generated auroral atmospheric gravity waves (AGWs) influence the growth of extratropical cyclones. It is also observed that severe extratropical storms, explosive cyclogenesis and significant sea level pressure deepenings of extratropical storms tend to occur within a few days of the arrival of high-speed solar wind. These observations are discussed in the context of the proposed AGW mechanism as well as the previously suggested atmospheric electrical current (AEC) model (Tinsley et al., 1994), which requires the presence of stratospheric aerosols for a significant (Wilcox) effect.


2019 ◽  
Vol 23 (10) ◽  
pp. 4323-4331 ◽  
Author(s):  
Wouter J. M. Knoben ◽  
Jim E. Freer ◽  
Ross A. Woods

Abstract. A traditional metric used in hydrology to summarize model performance is the Nash–Sutcliffe efficiency (NSE). Increasingly an alternative metric, the Kling–Gupta efficiency (KGE), is used instead. When NSE is used, NSE = 0 corresponds to using the mean flow as a benchmark predictor. The same reasoning is applied in various studies that use KGE as a metric: negative KGE values are viewed as bad model performance, and only positive values are seen as good model performance. Here we show that using the mean flow as a predictor does not result in KGE = 0, but instead KGE =1-√2≈-0.41. Thus, KGE values greater than −0.41 indicate that a model improves upon the mean flow benchmark – even if the model's KGE value is negative. NSE and KGE values cannot be directly compared, because their relationship is non-unique and depends in part on the coefficient of variation of the observed time series. Therefore, modellers who use the KGE metric should not let their understanding of NSE values guide them in interpreting KGE values and instead develop new understanding based on the constitutive parts of the KGE metric and the explicit use of benchmark values to compare KGE scores against. More generally, a strong case can be made for moving away from ad hoc use of aggregated efficiency metrics and towards a framework based on purpose-dependent evaluation metrics and benchmarks that allows for more robust model adequacy assessment.


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