Fast Wind Speed Research Articles

Glacier mass balances (1993–2001), Taylor Valley, McMurdo Dry Valleys, Antarctica

AbstractMass balances were measured on four glaciers in Taylor Valley, Antarctica, from 1993 to 2001. We used a piecewise linear regression, which provided an objective assessment of error, to estimate the mass balance with elevation. Missing measurements were estimated from linear regressions between points and showed a significant improvement over other methods. Unlike temperate glaciers the accumulation zone of these polar glaciers accumulates mass in summer and winter and the ablation zone loses mass in both seasons. A strong spatial trend of smaller mass-balance values with distance inland (r2 = 0.80) reflects a climatic gradient to warmer air temperatures, faster wind speeds and less precipitation. Annual and seasonal mass-balance values range only several tens of millimeters in magnitude and no temporal trend is evident. The glaciers of Taylor Valley, and probably the entire McMurdo Dry Valleys, are in equilibrium with the current climate, and contrast with glacier trends elsewhere on the Antarctic Peninsula and in temperate latitudes.

The impact of broadleaved woodland on water resources in lowland UK: II. Evaporation estimates from sensible heat flux measurements over beech woodland and grass on chalk sites in Hampshire

Abstract. The impact on recharge to the Chalk aquifer of substitution of broadleaved woodland for pasture is a matter of concern in the UK. Hence, measurements of energy balance components were made above beech woodland and above pasture, both growing on shallow soils over chalk in Hampshire. Latent heat flux (evaporation) was calculated as the residual from these measurements of energy balances in which sensible heat flux was measured with an eddy correlation instrument that determined fast response vertical wind speeds and associated temperature changes. Assessment of wind turbulence statistics confirmed that the eddy correlation device performed satisfactorily in both wet and dry conditions. There was excellent agreement between forest transpiration measurements made by eddy correlation and stand level tree transpiration measured with sap flow devices. Over the period of the measurements, from March 1999 to late summer 2000, changes in soil water content were small and grassland evaporation and transpiration estimated from energy balance-eddy flux measurements were in excellent agreement with Penman estimates of potential evaporation. Over the 18-month measurement period, the cumulative difference between broadleaved woodland and grassland was small but evaporation from the grassland was 3% higher than that from the woodland. In the springs of 1999 and 2000, evaporation from the grassland was greater than that from the woodland. However, following leaf emergence in the woodland, the difference in cumulative evaporation diminished until the following spring.

Flow over Heated Terrain. Part II: Generation of Convective Precipitation

Abstract Previous studies have shown that thunderstorms in the Rocky Mountain region have preferred areas in which to form. There has been some indication that these areas depend on the midtropospheric wind direction. A nonhydrostatic model with a terrain-following horizontal grid is employed to investigate the initiation of precipitating convection over heated topography. Horizontally homogeneous meteorological conditions with no directional shear in the vertical wind profile are used. The numerical simulations indicate that precipitating convection was more likely to be generated downwind of ridges than upwind of them. Initiation of these storms was more likely downwind of ridges with their long axis parallel to the wind direction than downwind of ridges with their long axis perpendicular to the wind direction. In Part I of this study it was shown that heating-induced convergence is larger downwind of a ridge with its longer axis parallel to the wind direction. For the orographic configuration of the Rocky Mountains, total precipitation is maximized for southerly and northwesterly winds. Slower wind speeds are more likely and faster wind speeds are less likely to produce convective storms. Soundings with larger instability are more likely to produce convection. The soundings with a greater temperature lapse rate produce more initiation locations, and soundings with greater moisture produce greater amounts of precipitation. Even though a number of assumptions were made for this study, the authors believe the results explain a significant amount of the observed variability in the initiation locations of precipitating convection in the Rocky Mountains during the summer. Because of the theoretical basis for this work, detailed in Part I of this study, the authors believe it should explain convective initiation in other mountainous areas that are subject to strong solar heating.

Acceleration of Weakly Collisional Solar-Type Winds

One of the basic properties of the solar wind, that is the high speed of the fast wind, is still not satisfactorily explained. This is mainly due to the theoretical difficulty of treating weakly collisional plasmas. The fluid approach implies that the medium is collision dominated and that the particle velocity distributions are close to Maxwellians. However the electron velocity distributions observed in the solar wind depart significantly from Maxwellians. Recent kinetic collisionless models (called exospheric) using velocity distributions with a suprathermal tail have been able to reproduce the high speeds of the fast solar wind. In this letter we present new developments of these models by generalizing them over a large range of corona conditions. We also present new results obtained by numerical simulations that include collisions. Both approaches calculate the heat flux self-consistently without any assumption on the energy transport. We show that both approaches - the exospheric and the collisional one - yield a similar variation of the wind speed with the basic parameters of the problem; both produce a fast wind speed if the coronal electron distribution has a suprathermal tail. This suggests that exospheric models contain the necessary ingredients for the powering of a transonic stellar wind, including the fast solar one.

04/02921 A standard method for specifying the response ofhydroelectric plant in frequency-control mode: Jones, D. I. et al. Electric Power Systems Research, 2004, 68, (1), 19–32

The power from wind varies depending on the environmental factors. Many methods have been proposed to locate and track the maximum power point (MPPT) of the wind, such as the fuzzy logic (FL), artificial neural network (ANN) and neuro-fuzzy. In this paper, a variable-speed wind-generator maximum-power-point-tracking (MPPT) based on adaptative neuro-fuzzy inference system (ANFIS) is presented. It is designed as a combination of the Sugeno fuzzy model and neural network. The ANFIS model is used to predict the optimal speed rotation using the variation of the wind speed as the input. The wind energy conversion system (WECS) employing a permanent magnet synchronous generator connected to a DC bus using a power converter is presented. A wind speed step model was used in the design phase. The performance of the WECS with the proposed ANFIS controller is tested for fast wind speed variation. Simulation results showed the possibility of achieving maximum power tracking for the wind and output voltage regulation for the DC bus simultaneously with the ANFIS controller. The results also proved the good response and robustness of the control system proposed.

The kinetic shell model of coronal heating and acceleration by ion cyclotron waves: 3. The proton halo and dispersive waves

The kinetic shell model seeks to test the collisionless kinetic response of coronal hole protons to the quasi‐linear resonant cyclotron dissipation of outwardly directed, parallel propagating ion cyclotron waves, which is often invoked as the mechanism responsible for generating the fast solar wind. In this model, we make the approximation that the resonant cyclotron interaction proceeds much faster than any other process affecting the protons, so that the resonant portions of the distribution are maintained in a state of marginal stability with respect to this interaction. Under this approximation, the resonant protons are organized on nested shells of constant density in velocity space, defined by the condition that the proton energy is conserved in the rest frame moving with the phase speed of the resonant wave. These shells then evolve on the slower nonresonant timescale, responding to the shell‐averaged forces of gravity, charge‐separation electric field, mirror force, and an inertial force due to the fact that the waves are propagating in the accelerating plasma frame. Since these kinetic shell distributions are held in a marginally stable state, they cannot exchange any more energy with the waves than that taken to maintain that state. Thus their evolution corresponds to the effect of the maximum possible dissipation of outward propagating waves. Our earlier work was simplified in two major respects: we considered only dispersionless waves and we assumed that all the antisunward protons were resonantly interacting with self‐generated inward propagating waves. In this paper, we first present an improved treatment of the antisunward protons which yields more plausible speeds and temperatures than obtained previously. We then incorporate the effects of ion cyclotron dispersion into the model. Dispersion creates resonant shells which are broader in υ∥ than our previous dispersionless distributions. We find that the nonresonant forces acting on these dispersive shell distributions invariably produce only weak acceleration and result in perpendicular cooling rather than the heating that is generally required for fast solar wind speeds. These effects are attributed to the weaker inertial force on the sunward dispersive shells and are essentially independent of the behavior of the antisunward particles. Since these results correspond to the maximum possible dissipation by this mechanism, we conclude that the heating of coronal hole protons and the generation of the fast solar wind are not caused by the collisionless dissipation of parallel propagating ion cyclotron waves.

Large‐amplitude Alfvén waves in open and closed coronal structures: A numerical study

We present the first simulations of the coronal response to Alfvén wave injection using transparent boundaries in a classical one‐fluid, isothermal axisymmetric model including both closed and open magnetic field structures. The aim of the work is first to study how Alfvén waves change the contrast between the equatorial and high‐latitude wind, and second, how they modify the geometry of the wind and its global stability. We integrate the full time‐dependent MHD equations, and inject large‐amplitude (150 km/s), low‐frequency (20 min period) waves at 1.8 Rs, both in open and in closed field line regions, except within narrow regions around the poles and the equator. The domain considered extends up to 16 Rs. Our principal results are the following: (1) The assumption of a latitude‐independent Alfvén wave amplitude compatible with observations leads to a large acceleration both of the high‐latitude and equatorial wind; as a consequence, the contrast between slow and fast wind speeds at 16 Rs is not as large as the observed values if extrapolated to 1 AU, a result which could potentially change with the use of better resolved, less dissipative simulations; (2) an initial delay in the Alfvén flux onset in one hemisphere generates a stable global circulation in the closed loops region, which after a long enough time produces a global north‐south asymmetry and changes the structure of the corona as a whole.

Coordinated ground-based and Cluster observations of large amplitude global magnetospheric oscillations during a fast solar wind speed interval

Abstract. We present magnetospheric observations of very large amplitude global scale ULF waves, from 9 and 10 December 2000 when the upstream solar wind speed exceeded 600 km/s. We characterise these ULF waves using ground-based magnetometer, radar and optical instrumentation on both the dawn and dusk flanks; we find evidence to support the hypothesis that discrete frequency field line resonances (FLRs) were being driven by magnetospheric waveguide modes. During the early part of this interval, Cluster was on an outbound pass from the northern dusk side magnetospheric lobe into the magnetosheath, local-time conjugate to the Canadian sector. In situ magnetic fluctuations, observed by Cluster FGM, show evidence of quasi-periodic motion of the magnetosheath boundary layer with the same period as the ULF waves seen on the ground. Our observations represent the first simultaneous magnetometer, radar and optical observations of the characteristics of FLRs, and confirm the potential importance of ULF waves for magnetosphere-ionosphere coupling, particularly via the generation and modulation of electron precipitation into the ionosphere. The in situ Cluster measurements support the hypothesis that, during intervals of fast solar wind speed, the Kelvin-Helmholtz instability (KHI) can excite magnetospheric waveguide modes which bathe the flank magnetosphere with discrete frequency ULF wave power and drive large amplitude FLRs. Paper submitted to the special issue devoted to "Cluster: First scientific results", Ann. Geophysicae, 19, 10/11/12, 2001.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; MHD waves and instabilities; solar wind-magnetosphere interactions)

Analysis of fine-scale canopy turbulence within and above an Amazon forest using Tsallis’ generalized thermostatistics

[1] We analyzed the probability density function (PDF) of velocity and temperature differences in the canopy sublayer of Amazonia based on Tsallis' generalized thermostatistics theory. We show that such a theory provides an accurate framework for modeling the statistical behavior of the inertial subrange above and below the canopy. For this, we compared the experimental PDFs with the theoretically predicted ones. The data were measured during the wet season of the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA), which was carried out during the months of January–March 1999 in the southwestern part of Amazonia region. Measurements were made simultaneously at different heights in a 60 m micrometeorological tower located in the Biological Reserve of Jaru (10°04′S, 61°56′W), Brazil. The fast response wind speed measurements, sampled at 60 Hz rate, were made using three-dimensional sonic anemometers at the heights of 66 m (above the canopy) and 21 m (below the canopy). The results showed good agreement between experimental data measured above the canopy forest and Tsallis' generalized thermostatistics theory. For below canopy data, the agreement between experimental and theoretical PDFs was fairly good, but some distortion was observed. This is probably due to some peculiar characteristics of turbulent momentum transfer process inside the forest crown. Discussion is presented to explain these results. Conclusions regarding the absence of “universal scaling” in the inertial subrange are also presented in the context of the entropic parameter of Tsallis' theory.

The Bastille day Magnetic Clouds and Upstream Shocks: Near-Earth Interplanetary Observations

The energetic charged particle, interplanetary magnetic field, and plasma characteristics of the 'Bastille Day' shock and ejecta/magnetic cloud events at 1 AU occurring over the days 14 - 16 July 2000 are described. Profiles of MeV (WIND/LEMT) energetic ions help to organize the overall sequence of events from the solar source to 1 AU. Stressed are analyses of an outstanding magnetic cloud (MC2) starting late on 15 July and its upstream shock about 4 hours earlier in WIND magnetic field and plasma data. Also analyzed is a less certain, but likely, magnetic cloud (MC1) occurring early on 15 July; this was separated from MC2 by its upstream shock and many heliospheric current sheet (HCS) crossings. Other HCS crossings occurred throughout the 3-day period. Overall this dramatic series of interplanetary events caused a large multi-phase magnetic storm with min Dst lower than −300 nT. The very fast solar wind speed (≥ 1100 km s −1 ) in and around the front of MC2 (for near average densities) was responsible for a very high solar wind ram pressure driving in the front of the magnetosphere to geocentric distances estimated to be as low as ≈ 5 RE ,m uch lower than the geosynchronous orbit radius. This was consistent with magnetic field observations from two GOES satellites which indicated they were in the magnetosheath for extended times. A static force-free field model is used to fit the two magnetic cloud profiles providing estimates of the clouds' physical and geometrical properties. MC2 was much larger than MC1, but their axes were nearly antiparallel, and their magnetic fields had the same left-handed helicity. MC2's axis and its upstream shock normal were very close to being perpendicular to each other, as might be expected if the cloud were driving the shock at the time of observation. The estimated axial magnetic flux carried by MC2 was 52 × 10 20 Mx, which is about 5 times the typical magnetic flux estimated for other magnetic clouds in the WIND data over its first 4 years and is 17 times the flux of MC1. This large flux is due to both the strong axially-directed field of MC2 (46.8 nT on the axis) and the large radius (R0 = 0.189 AU) of the flux tube. MC2's average speed is consistent with the expected transit time from a halo-CME to which it is apparently related.

A correlation between extended intervals of Ulf wave power and storm‐time geosynchronous relativistic electron flux enhancements

Recent research has highlighted the possibility that large‐amplitude ULF pulsations may act as an acceleration mechanism for generating relativistic electron populations in the outer zone magnetosphere during storm‐time. We examine solar wind characteristics, Pc5 ULF wave power and outer magnetospheric measurements of high energy electron flux during the recurrent fast solar wind speed streams which occurred during the first half of 1995. We find a close correlation between extended intervals of significant pulsation power and observations of enhanced relativistic (>2 MeV) electron flux in the outer zone magnetosphere, suggesting that these two features may be causally related. We demonstrate that significant electron flux increases at geosynchronous orbit are only observed in response to ULF wave power which is sustained at high levels over a number of days following storm onset. Our observations suggest that ULF pulsations may be the likely acceleration mechanism for generating storm‐time MeV “killer” electrons in the magnetosphere.

A two-dimensional air quality model in an urban street canyon: evaluation and sensitivity analysis

In order to predict the air quality impact by vehicle emissions within an urban street canyon, a two-dimensional air quality numerical model was developed based on atmospheric convection diffusion equations and a k– ε turbulent model. The numerical model has been evaluated using the database from a set of street canyon air tracer experiments carried out near the crossing of Aoyama ichome, Minato-ku, Tokyo in December 1980, by the Japan Environmental Management Association of Industry (JEMAI). Twenty-four cases have been studied for the sensitivity analysis, including more practical cases when the inflow wind has an inclination with the horizontal road and the two buildings have different heights. As a result, it has been shown that the concentration distributions of pollutants emitted from the street are governed by both the inflow wind and the street canyon geometry. A stable vortex was formed within the street canyon, which agrees with other researchers. Pollutant concentrations were predicted to have higher values on the leeward side compared to the windward side. It was concluded that the released pollutants from street canyon become more diluted in the following cases: a lower height of the street canyon, a faster wind speed, a higher height of the leeward building than the windward building and an inflow wind direction towards the street. It is also suggested that the numerical model is useful for predicting the air quality within a typical urban street canyon.

Contribution of intermittent temperature peaks to the generation of the solar wind flow

Observations of frequent, intermittent, density, velocity, and temperature variations, called jets, in the solar corona have led some authors to propose these jets as possible acceleration mechanisms for the fast solar wind seen at large distances. In the search of possible dynamical effects, we look here at the implications of intermittent temperature fluctuations at the coronal level on the subsonic and supersonic solar wind properties, in a spherically symmetric (single fluid) wind with polytropic index close to unity and no magnetic field. Sudden temperature pulses are applied periodically at the coronal level, with no pressure variation, the velocity being free. One finds that (1) the advected perturbations take the form of a mixture of upward propagating shock waves and advected pressure equilibrium structures and (2) the mean temperature increases due to the larger expansion rate of the fluid with peak temperature. As a result, a main part of the wind flow at large distance appears to be generated by intermittent coronal events with peak temperature. We propose thus to reduce the discrepancy between the fast wind speed observed at 1 AU and the observed coronal temperature by taking into account not the average temperature, but the peak coronal temperature, still to be determined.