8,391 research outputs found
Evidence of crossover phenomena in wind speed data
In this report, a systematic analysis of hourly wind speed data obtained from
three potential wind generation sites (in North Dakota) is analyzed. The power
spectra of the data exhibited a power-law decay characteristic of
processes with possible long-range correlations. Conventional
analysis using Hurst exponent estimators proved to be inconclusive. Subsequent
analysis using detrended fluctuation analysis (DFA) revealed a crossover in the
scaling exponent (). At short time scales, a scaling exponent of
indicated that the data resembled Brownian noise, whereas for
larger time scales the data exhibited long range correlations (). The scaling exponents obtained were similar across the three locations.
Our findings suggest the possibility of multiple scaling exponents
characteristic of multifractal signals
Overshooting thunderheads observed from ATS and Learjet
Overshooting tops of thunderstorms were photographed simultaneously from both ATS and a Learjet during the cloud-truth experiment over the Midwest in the Spring of 1972 and 1973. The characteristics of overshooting tops were studied in various time and space scales, revealing that the horizontal dimensions of overshooting tops vary between 1000 ft and about 10 miles. The period of overshooting turrets with horizontal dimensions of less than 1 mile is found to be comparable to the Brunt-Vaisalla frequency of gravity waves at the lowermost stratosphere. The up-and-down motion of an overshooting dome, consisting of a number of turrets, is much slower than that of individual turrets. It is assumed that the height of a dome is closely related to the intensity of the up and downdrafts beneath the dome. Emphasis is placed upon the importance of the investigation of overshooting domes toward the identification of severe storm characteristics from satellites
Quantifying Eulerian Eddy Leakiness in an Idealized Model
An idealized eddyâresolving ocean basin, closely resembling the North Pacific Ocean, is simulated using MITgcm. We identify rotationally coherent Lagrangian vortices (RCLVs) and sea surface height (SSH) eddies based on the Lagrangian and Eulerian framework, respectively. General statistical results show that RCLVs have a much smaller coherent core than SSH eddies with the ratio of radius is about 0.5. RCLVs are often enclosed by SSH anomaly contours, but SSH eddy identification method fails to detect more than half of RCLVs. Based on their locations, two types of eddies are classified into three categories: overlapping RCLVs and SSH eddies, nonoverlapping SSH eddies, and nonoverlapping RCLVs. Using Lagrangian particles, we examine the processes of leakage and intrusion around SSH eddies. For overlapping SSH eddies, over the lifetime, the material coherent core only accounts for about 25% and about 50% of initial water leak from eddy interior. The remaining 25% of water can still remain inside the boundary, but only in the form of filaments outside the coherent core. For nonoverlapping SSH eddies, more water leakage (about 60%) occurs at a faster rate. Guided by the number and radius of SSH eddies, fixed circles and moving circles are randomly selected to diagnose the material flux around these circles. We find that the leakage and intrusion trends of moving circles are quite similar to that of nonoverlapping SSH eddies, suggesting that the material coherence properties of nonoverlapping SSH eddies are not significantly different from random pieces of ocean with the same size
Kinetic-scale magnetic turbulence and finite Larmor radius effects at Mercury
We use a nonstationary generalization of the higher-order structure function
technique to investigate statistical properties of the magnetic field
fluctuations recorded by MESSENGER spacecraft during its first flyby
(01/14/2008) through the near Mercury's space environment, with the emphasis on
key boundary regions participating in the solar wind -- magnetosphere
interaction. Our analysis shows, for the first time, that kinetic-scale
fluctuations play a significant role in the Mercury's magnetosphere up to the
largest resolvable time scale ~20 s imposed by the signal nonstationarity,
suggesting that turbulence at this planet is largely controlled by finite
Larmor radius effects. In particular, we report the presence of a highly
turbulent and extended foreshock system filled with packets of ULF
oscillations, broad-band intermittent fluctuations in the magnetosheath,
ion-kinetic turbulence in the central plasma sheet of Mercury's magnetotail,
and kinetic-scale fluctuations in the inner current sheet encountered at the
outbound (dawn-side) magnetopause. Overall, our measurements indicate that the
Hermean magnetosphere, as well as the surrounding region, are strongly affected
by non-MHD effects introduced by finite sizes of cyclotron orbits of the
constituting ion species. Physical mechanisms of these effects and their
potentially critical impact on the structure and dynamics of Mercury's magnetic
field remain to be understood.Comment: 46 pages, 5 figures, 2 table
Active current sheets and hot flow anomalies in Mercury's bow shock
Hot flow anomalies (HFAs) represent a subset of solar wind discontinuities
interacting with collisionless bow shocks. They are typically formed when the
normal component of motional (convective) electric field points toward the
embedded current sheet on at least one of its sides. The core region of an HFA
contains hot and highly deflected ion flows and rather low and turbulent
magnetic field. In this paper, we report first observations of HFA-like events
at Mercury identified over a course of two planetary years. Using data from the
orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and
Ranging (MESSENGER) mission, we identify a representative ensemble of active
current sheets magnetically connected to Mercury's bow shock. We show that some
of these events exhibit unambiguous magnetic and particle signatures of HFAs
similar to those observed earlier at other planets, and present their key
physical characteristics. Our analysis suggests that Mercury's bow shock does
not only mediate the flow of supersonic solar wind plasma but also provides
conditions for local particle acceleration and heating as predicted by previous
numerical simulations. Together with earlier observations of HFA activity at
Earth, Venus and Saturn, our results confirm that hot flow anomalies are a
common property of planetary bow shocks, and show that the characteristic size
of these events is of the order of one planetary radius.Comment: 39 pages, 15 figures, 2 table
Long-term correlations in hourly wind speed records in Pernambuco, Brazil
AbstractWe study the statistical properties of hourly wind speed time series detected at four weather stations in the state of Pernambuco, Brazil, in the period 2008â2009. We find that the average and maximum hourly wind speeds deviate from a mutual linear relationship, and that they may be well explained individually by a Weibull distribution, however, with different shape parameter values. On the other hand, the long-term correlations of both of these observables obey the same power-law behavior, with two distinct scaling regimes. Our results agree with previous studies on wind speed series correlations in other regions of the world, which is suggestive of universal behavior
A Multifractal Description of Wind Speed Records
In this paper, a systematic analysis of hourly wind speed data obtained from
four potential wind generation sites in North Dakota is conducted. The power
spectra of the data exhibited a power law decay characteristic of
processes with possible long range correlations. The temporal
scaling properties of the records were studied using multifractal detrended
fluctuation analysis {\em MFDFA}. It is seen that the records at all four
locations exhibit similar scaling behavior which is also reflected in the
multifractal spectrum determined under the assumption of a binomial
multiplicative cascade model
Analysis of time dynamics in wind records by means of multifractal detrended fluctuation analysis and Fisher-Shannon information plane
The time structure of more than 10 years of hourly wind data measured in one
site in northern Italy from April 1996 to December 2007 is analysed. The data
are recorded by the Sodar Rass system, which measures the speed and the
direction of the wind at several heights above the ground level. To investigate
the wind speed time series at seven heights above the ground level we used two
different approaches: i) the Multifractal Detrended Fluctuation Analysis
(MF-DFA), which permits the detection of multifractality in nonstationary
series, and ii) the Fisher-Shannon (FS) information plane, which allows to
discriminate dynamical features in complex time series. Our results point out
to the existence of multifractal time fluctuations in wind speed and to a
dependence of the results on the height of the wind sensor. Even in the FS
information plane a height-dependent pattern is revealed, indicating a good
agreement with the multifractality. The obtained results could contribute to a
better understanding of the complex dynamics of wind phenomenon
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