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Showing papers on "Wind shear published in 2007"


Journal ArticleDOI
TL;DR: In this paper, the authors used a combination of basic theory and empirical statistical analysis to find that much of the variability in both ocean basins can be explained by variations in potential intensity, low-level vorticity and vertical wind shear.
Abstract: Revised estimates of kinetic energy production by tropical cyclones in the Atlantic and western North Pacific are presented. These show considerable variability on interannual-to-multidecadal time scales. In the Atlantic, variability on time scales of a few years and more is strongly correlated with tropical Atlantic sea surface temperature, while in the western North Pacific, this correlation, while still present, is considerably weaker. Using a combination of basic theory and empirical statistical analysis, it is shown that much of the variability in both ocean basins can be explained by variations in potential intensity, low-level vorticity, and vertical wind shear. Potential intensity variations are in turn factored into components related to variations in net surface radiation, thermodynamic efficiency, and average surface wind speed. In the Atlantic, potential intensity, low-level vorticity, and vertical wind shear strongly covary and are also highly correlated with sea surface temperature, at least during the period in which reanalysis products are considered reliable. In the Pacific, the three factors are not strongly correlated. The relative contributions of the three factors are quantified, and implications for future trends and variability of tropical cyclone activity are discussed.

312 citations


Journal ArticleDOI
TL;DR: In this article, power losses and turbulence increase due to wind turbine wake interactions in large offshore wind farms are quantified based on observations from Middelgrunden and state-of-the-art models.
Abstract: Understanding of power losses and turbulence increase due to wind turbine wake interactions in large offshore wind farms is crucial to optimizing wind farm design. Power losses and turbulence increase due to wakes are quantified based on observations from Middelgrunden and state-of-the-art models. Observed power losses due solely to wakes are approximately 10% on average. These are relatively high for a single line of wind turbines due in part to the close spacing of the wind farm. The wind farm model Wind Analysis and Application Program (WAsP) is shown to capture wake losses despite operating beyond its specifications for turbine spacing. The paper describes two methods of estimating turbulence intensity: one based on the mean and standard deviation (SD) of wind speed from the nacelle anemometer, the other from mean power output and its SD. Observations from the nacelle anemometer indicate turbulence intensity which is around 9% higher in absolute terms than those derived from the power measurements. For comparison, turbulence intensity is also derived from wind speed and SD from a meteorological mast at the same site prior to wind farm construction. Despite differences in the measurement height and period, overall agreement is better between the turbulence intensity derived from power measurements and the meteorological mast than with those derived from data from the nacelle anemometers.The turbulence in wind farm model indicates turbulence increase of the order 20% in absolute terms for flow directly along the row which is in good agreement with the observations. Copyright © 2007 John Wiley & Sons, Ltd.

283 citations


Journal ArticleDOI
TL;DR: In this article, a probabilistic clustering method based on a regression mixture model was used to describe tropical cyclone propagation in the western North Pacific (WNP) and seven clusters were obtained and described in Part I of this two-part study.
Abstract: A new probabilistic clustering method, based on a regression mixture model, is used to describe tropical cyclone (TC) propagation in the western North Pacific (WNP). Seven clusters were obtained and described in Part I of this two-part study. In Part II, the present paper, the large-scale patterns of atmospheric circulation and sea surface temperature associated with each of the clusters are investigated, as well as associations with the phase of the El Nino–Southern Oscillation (ENSO). Composite wind field maps over the WNP provide a physically consistent picture of each TC type, and of its seasonality. Anomalous vorticity and outgoing longwave radiation indicate changes in the monsoon trough associated with different types of TC genesis and trajectory. The steering winds at 500 hPa are more zonal in the straight-moving clusters, with larger meridional components in the recurving ones. Higher values of vertical wind shear in the midlatitudes also accompany the straight-moving tracks, compared to...

277 citations


Journal ArticleDOI
TL;DR: In this article, a suite of state-of-the-art global climate model experiments is used to project changes in vertical wind shear (Vs) over the tropical Atlantic during hurricane season, which has been historically associated with diminished hurricane activity and intensity.
Abstract: [1] To help understand possible impacts of anthropogenic greenhouse warming on hurricane activity, we assess model-projected changes in large-scale environmental factors tied to variations in hurricane statistics. This study focuses on vertical wind shear (Vs) over the tropical Atlantic during hurricane season, the increase of which has been historically associated with diminished hurricane activity and intensity. A suite of state-of-the-art global climate model experiments is used to project changes in Vs over the 21st century. Substantial increases in tropical Atlantic and East Pacific shear are robust features of these experiments, and are shown to be connected to the model-projected decrease in the Pacific Walker circulation. The relative changes in shear are found to be comparable to those of other large-scale environmental parameters associated with Atlantic hurricane activity. The influence of these Vs changes should be incorporated into projections of long-term hurricane activity.

250 citations


Journal ArticleDOI
TL;DR: In this article, the authors explored the relationship between the likelihood of tropical cyclogenesis and external environmental forcings in the simplest idealized modelling framework possible: radiative-convective equilibrium on a doubly periodic f-plane.
Abstract: In this study, the relationship between the likelihood of tropical cyclogenesis and external environmental forcings is explored in the simplest idealized modelling framework possible: radiative-convective equilibrium on a doubly periodic f-plane. In such an environment, control of the equilibrium environmental sounding is reduced to three parameters: the sea-surface temperature, the Coriolis parameter, and the imposed background surface wind speed. Cloud-resolving mesoscale model simulations are used to generate environments of radiative-convective equilibrium determined by these three factors. The favourability of these environments for tropical cyclogenesis is measured in three ways: in terms of the maximum potential intensity (MPI) of the sounding, based on the thermodynamic theory of Emanuel; in terms of the ‘genesis potential’ determined by an empirical genesis parameter; and in terms of the propensity of weak initial vortices in these environments to form into tropical cyclones. The simulated environments of radiative—convective equilibrium with no vertical wind shear are found to be very favourable for tropical cyclogenesis. Weak initial vortices always transition to a tropical cyclone, even for rather low sea-surface temperatures. However, the time required for these vortices to make the transition from a weak, mid-level vortex to a rapidly developing tropical cyclone decreases as the MPI increases, indicating the importance of MPI in enhancing the frequency of cyclogenesis. The relationship between this ‘time to genesis’ and the thermodynamic parameters is explored. The time to genesis is found to be very highly (negatively) correlated to MPI, with little or no relationship to convective instability, Coriolis parameter, mid-level humidity, or the empirical genesis parameter. In some cases, tropical cyclones are found to form spontaneously from random convection. This formation is due to a cooperative interaction between large-scale moisture, long-wave radiation, and locally enhanced sea-surface fluxes, similar to the ‘aggregation’ of convection found in previous studies. Copyright © 2007 Royal Meteorological Society

164 citations


Journal ArticleDOI
TL;DR: In this article, the local covariability of these variables is documented over intermonthly and interannual time scales, using global precipitation products and atmospheric reanalysis from 1979 to 2004, and the spatial correspondence between the precipitation and two indices of synoptic activity in the extratropics is also documented.
Abstract: Extratropical precipitation is primarily produced by cold and warm fronts associated with surface cyclones and upper-level troughs. The growth of these midlatitude storms is partially controlled by the dry baroclinicity of the troposphere, which in turn can be roughly quantified by the intensity of the upper-level zonal flow. Orographic rainfall, an important component of the precipitation in several midlatitude regions, is also partially determined by the intensity of the cross-mountain midlevel winds. Thus, at monthly and longer time scales, variations of precipitation and zonal flow aloft (as well as wind shear) at a given location should exhibit some degree of coherence. In this work the local covariability of these variables is documented over intermonthly and interannual time scales, using global precipitation products and atmospheric reanalysis from 1979 to 2004. The spatial correspondence between the precipitation and two indices of synoptic activity in the extratropics is also documented...

151 citations


Journal ArticleDOI
07 Jun 2007-Nature
TL;DR: A record of the frequency of major Atlantic hurricanes over the past 270 years is constructed using proxy records of vertical wind shear and sea surface temperature from corals and a marine sediment core, indicating that the average frequency ofmajor hurricanes decreased gradually from the 1760s until the early 1990s.
Abstract: The frequency of major hurricanes over the Atlantic Ocean has increased significantly since 1995, but it is still not clear whether this is due to global warming or natural variability. One way to address this question is to consider changes in hurricane frequency in the past, but reliable observations of Atlantic hurricane activity only cover a few decades. Nyberg et al. use proxy records from corals and a marine sediment core that appear to reflect changes in the two main parameters that influence hurricane activity — vertical wind shear and sea surface temperature — to reconstruct the frequency of major hurricanes over the Atlantic since 1730. The results indicate that the frequency was anomalously low during the 1970s and 1980s compared with the past 270 years, and that the phase of increased hurricane frequency since 1995 represents a recovery to 'normal' hurricane activity. These trends appear to be related to wind shear, but what caused this parameter to change remains uncertain. Proxy records from corals and marine sediment core that reflect changes in vertical wind shear and sea surface temperature (the two main parameters that influence hurricane activity) are used to reconstruct the frequency of major hurricanes over the Atlantic since 1730. The results indicate that the frequency of major hurricanes was anomalously low during the 1970s and 1980s compared to the past 270 years. Hurricane activity in the North Atlantic Ocean has increased significantly since 1995 (refs 1, 2). This trend has been attributed to both anthropogenically induced climate change3 and natural variability1, but the primary cause remains uncertain. Changes in the frequency and intensity of hurricanes in the past can provide insights into the factors that influence hurricane activity, but reliable observations of hurricane activity in the North Atlantic only cover the past few decades2. Here we construct a record of the frequency of major Atlantic hurricanes over the past 270 years using proxy records of vertical wind shear and sea surface temperature (the main controls on the formation of major hurricanes in this region1,3,4,5) from corals and a marine sediment core. The record indicates that the average frequency of major hurricanes decreased gradually from the 1760s until the early 1990s, reaching anomalously low values during the 1970s and 1980s. Furthermore, the phase of enhanced hurricane activity since 1995 is not unusual compared to other periods of high hurricane activity in the record and thus appears to represent a recovery to normal hurricane activity, rather than a direct response to increasing sea surface temperature. Comparison of the record with a reconstruction of vertical wind shear indicates that variability in this parameter primarily controlled the frequency of major hurricanes in the Atlantic over the past 270 years, suggesting that changes in the magnitude of vertical wind shear will have a significant influence on future hurricane activity.

150 citations


Journal ArticleDOI
Abstract: Breaking waves on the tropopause are viewed as potential vorticity (PV) streamers on middle-world isentropic levels. A Northern Hemisphere winter climatology of the streamers’ spatial distribution and meridional orientation is derived from the 40-yr ECMWF Re-Analysis (ERA-40) dataset, and used to assess the nature and frequency of occurrence of breaking synoptic-scale waves. The streamers are grouped into two classes related to the so-called cyclonic (LC2) and anticyclonic (LC1) patterns, and the ambient wind strength and wind shear is also noted. It is shown that the occurrence of cyclonic and anticyclonic PV streamers exhibits a distinct spatial variability in the horizontal and the vertical. The majority of cyclonic PV streamers are found on lower isentropic levels that intersect the tropopause at more poleward latitudes, whereas anticyclonic streamers predominate at higher elevations in the subtropics. An analysis of the streamer patterns for the two phases of the North Atlantic Oscillation (...

134 citations


Journal ArticleDOI
TL;DR: In this article, the effects of two environmental dynamical factors, namely, the transitional speed and vertical wind shear, on tropical cyclone intensification, intensity, and lifetime peak intensity were analyzed based on observations in the western North Pacific during 1981-2003.
Abstract: The effects of two environmental dynamical factors, namely, the transitional speed and vertical wind shear, on tropical cyclone (TC) intensification, intensity, and lifetime peak intensity were analyzed based on observations in the western North Pacific during 1981–2003. In general, both the fast translation and strong vertical shear are negative to both TC intensification and the lifetime peak intensity. Both the very intense TCs and the TCs with rapid intensification rate are found only to occur in a narrow range of translational speeds between 3 and 8 m s−1, and in relatively weak vertical shear. The overwhelming majority of western North Pacific TCs reach their lifetime peak intensity just prior to recurvature where their environmental steering flow and vertical shear are both weak. The results show that few TCs intensified when they moved faster than 15 m s−1, or when their large-scale environmental vertical shear is larger than 20 m s−1. The intensification rate of TCs is found to increase ...

127 citations


Journal ArticleDOI
TL;DR: In this article, the potential value of parameters derived from radiosonde data or data from numerical atmospheric models for the forecasting of severe weather associated with convective storms is presented focusing on the possible value of parameter derived from soundings in the proximity of large hail, tornadoes (including tornadoes over water: waterspouts) and thunderstorms.

123 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the effect of convection wave propagation on convectively active boundary layers (CBLs) in the presence of vertical wind shear and found that these convection waves are ubiquitous over fields of shallow fair weather cumuli or clear air thermals.
Abstract: Widespread gravity wave systems have been found to exist over convectively active boundary layers (CBLs) in the presence of vertical wind shear. In contrast to mountain waves, these ‘convection waves’ occur over flat terrain and are ubiquitous over fields of shallow fair weather cumuli or clear air thermals. They extend vertically to at least 9km a.m.s.l., probably to the tropopause. First discovered by glider pilots as ‘thermal waves’, they have now been systematically investigated by research aircraft during the NCAR Convection Wave Project, which also incorporates an effort in numerical simulation. the flight results are reported in this paper, while the numerical simulations are described in a companion paper. Typical wavelengths of convection waves were found to range from 5 to 15 km, (average 9 km) and typical vertical motion amplitudes from ± 1 to ± 3 m s−1. In all cases vertical wind shear exceeded 3×10−3s−1. Under these conditions cloud tops were measured to move with relative velocities of 8 to 10 m s−1 against their environment, hence they present an obstacle to the surrounding airflow. the analogy to mountainous obstacles launching gravity waves into the atmosphere is suggestive and borne out by the aforementioned numerical simulations, which describe this process step by step. The interaction between the CBL and the overlying stable layers is studied by spectral and cross-spectral analysis of aircraft data including coherence and phase relations between wave motions and cloud population underneath. Consideration is given to other potential sources of the observed gravity waves. At this stage the conclusions of our pilot study may be considered tentative. Some implications for further research are also discussed.

Journal ArticleDOI
TL;DR: In this article, five nights with weak-wind, clear-sky, very stable boundary layers during the Cooperative Atmosphere-Surface Exchange Study (CASES-99) are investigated.
Abstract: The light-wind, clear-sky, very stable boundary layer (vSBL) is characterized by large values of bulk Richardson number. The light winds produce weak shear, turbulence, and mixing, and resulting strong temperature gradients near the surface. Here five nights with weak-wind, very stable boundary layers during the Cooperative Atmosphere–Surface Exchange Study (CASES-99) are investigated. Although the winds were light and variable near the surface, Doppler lidar profiles of wind speed often indicated persistent profile shapes and magnitudes for periods of an hour or more, sometimes exhibiting jetlike maxima. The near-surface structure of the boundary layer (BL) on the five nights all showed characteristics typical of the vSBL. These characteristics included a shallow traditional BL only 10–30 m deep with weak intermittent turbulence within the strong surface-based radiation inversion. Above this shallow BL sat a layer of very weak turbulence and negligible turbulent mixing. The focus of this paper i...

Journal ArticleDOI
TL;DR: In this paper, the evolution and spatial distribution of the energetics of African waves are studied and complete eddy energy equations for an open system are derived for the computation of energy transformations during wave generation and dissipation.
Abstract: The evolution and spatial distribution of the energetics of African waves are studied. Complete eddy energy equations for an open system are derived for the computation of energy transformations during wave generation and dissipation. It is found that baroclinic overturning is the dominant energy source, although barotropic conversions can be almost equally important when there is concentrated moist convection south of the jet or shallow cumulus convection beneath the jet. The generation of active waves usually results from the nearly in-phase evolution of baroclinic and barotropic conversions, which are associated with significant rainfall over Africa. Significant barotropic instability associated with the horizontal shear is usually induced by concentrated deep convection on the southern flank of the jet. Barotropic conversions associated with the vertical wind shear may attain even greater magnitudes than that associated with the horizontal shear when shallow cumulus convection beneath the jet...

Journal ArticleDOI
TL;DR: In this paper, a high-resolution numerical simulation of Hurricane Erin (2001) is used to examine the organization of vertical motion in the eyewall and how that organization responds to a large and rapid increase in the environmental vertical wind shear and subsequent decrease in shear.
Abstract: A high-resolution numerical simulation of Hurricane Erin (2001) is used to examine the organization of vertical motion in the eyewall and how that organization responds to a large and rapid increase in the environmental vertical wind shear and subsequent decrease in shear. During the early intensification period, prior to the onset of significant shear, the upward motion in the eyewall was concentrated in small-scale convective updrafts that formed in association with regions of concentrated vorticity (herein termed mesovortices) with no preferred formation region around the eyewall. Asymmetric flow within the eye was weak. As the shear increased, an azimuthal wavenumber-1 asymmetry in storm structure developed with updrafts tending to occur on the downshear to downshear-left side of the eyewall. Continued intensification of the shear led to increasing wavenumber-1 asymmetry, large vortex tilt, and a change in eyewall structure and vertical motion organization. During this time, the eyewall structure was dominated by a vortex couplet with a cyclonic (anticyclonic) vortex on the downtilt-left (downtilt-right) side of the eyewall and strong asymmetric flow across the eye that led to strong mixing of eyewall vorticity into the eye. Upward motion was concentrated over an azimuthally broader region on the downtilt side of the eyewall, upstream of the cyclonic vortex, where low-level environmental inflow converged with the asymmetric outflow from the eye. As the shear diminished, the vortex tilt and wavenumber-1 asymmetry decreased, while the organization of updrafts trended back toward that seen during the weak shear period. Based upon the results for the Erin case, as well as that for a similar simulation of Hurricane Bonnie (1998), a conceptual model is developed for the organization of vertical motion in the eyewall as a function of the strength of the vertical wind shear. In weak to moderate shear, higher wavenumber asymmetries associated with eyewall mesovortices dominate the wavenumber-1 asymmetry associated with the shear so that convective-scale updrafts form when the mesovortices move into the downtilt side of the eyewall and dissipate on the uptilt side. Under strong shear conditions, the wavenumber-1 asymmetry, characterized by a prominent vortex couplet in the eyewall, dominates the vertical motion organization so that mesoscale ascent (with embedded convection) occurs over an azimuthally broader region on the downtilt side of the eyewall. Further research is needed to determine if these results apply more generally.

Journal ArticleDOI
TL;DR: In this article, the effect of vertical shear in the lower stratosphere on baroclinic instability in the tropospheric midlatitude jet is examined using a hierarchy of models and observations.
Abstract: Using a hierarchy of models, and observations, the effect of vertical shear in the lower stratosphere on baroclinic instability in the tropospheric midlatitude jet is examined. It is found that increasing stratospheric shear increases the phase speed of growing baroclinic waves, increases the growth rate of modes with low synoptic wavenumbers, and decreases the growth rate of modes with higher wavenumbers. The meridional structure of the linear modes, and their acceleration of the zonal mean jet, changes with increasing stratospheric shear, but in a way that apparently contradicts the observed stratosphere–troposphere northern annular mode (NAM) connection. This contradiction is resolved at finite amplitude. In nonlinear life cycle experiments it is found that increasing stratospheric shear, without changing the jet structure in the troposphere, produces a transition from anticyclonic (LC1) to cyclonic (LC2) behavior at wavenumber 7. All life cycles with wavenumbers lower than 7 are LC1, and all with wavenumber greater than 7 are LC2. For the LC1 life cycles, the effect of increasing stratospheric shear is to increase the poleward displacement of the zonal mean jet by the eddies, which is consistent with the observed stratosphere–troposphere NAM connection. Finally, it is found that the connection between high stratospheric shear and high-tropospheric NAM is present by NCEP–NCAR reanalysis data.

Journal ArticleDOI
01 Jul 2007
TL;DR: In this article, the effect of wake interaction for a row of three wind turbines in a wind farm is analyzed using the actuator line technique, with the aim of deriving the optimal pitch setting of the foremost turbine, with respect to the total power from the row.
Abstract: The effect of wake interaction for a row of three wind turbines in a wind farm is analysed using the actuator line technique. Both full wake and half wake situations are considered with the aim of deriving the optimal pitch setting of the foremost turbine, with respect to the total power from the row. The mutual distance between the turbines is 5 diameters and the turbines are considered to operate in a wind shear with an exponent of 0.15, with the rotor centre located at 1.4 radii from the ground. The main findings reveal clear effects of reducing the loading on the foremost turbine towards increased production of turbine 2 and 3 in a row

Journal ArticleDOI
TL;DR: In this paper, the work done by the wind over the northwest Atlantic Ocean is examined using a realistic high-resolution ocean model driven by synoptic wind forcing, and two model runs are conducted with the difference only in the way the wind stress is calculated.
Abstract: The work done by the wind over the northwest Atlantic Ocean is examined using a realistic high-resolution ocean model driven by synoptic wind forcing. Two model runs are conducted with the difference only in the way the wind stress is calculated. Our results show that the effect of including ocean surface currents in the wind stress formulation is to reduce the total wind work integrated over the model domain by about 17%. The reduction is caused by a sink term in the wind work calculation associated with the presence of ocean currents. In addition, the modelled eddy kinetic energy decreases by about 10%, in response to direct mechanical damping by the surface stress. A simple scaling argument shows that the latter can be expected to be more important than bottom friction in the energy budget.

Journal ArticleDOI
15 Nov 2007-Icarus
TL;DR: In this article, a simple force balance equation is applied to quantify the wind shear stress required for removal of glass spheres from a sand bed and the results agree well with observations of dust removal and wind speed measurements made by the NASA Viking landers at the martian surface.

Journal ArticleDOI
TL;DR: In this paper, meteorological variables derived from sounding observations taken in the environment of quasi-linear MCSs were examined and a set of 186 soundings sampled from the beginning and mature stages of the mesoscale convective systems were categorized by their production of severe surface winds into weak, severe, and derecho-producing MCS.
Abstract: The prediction of the strength of mesoscale convective systems (MCSs) is a major concern to operational meteorologists and the public. To address this forecast problem, this study examines meteorological variables derived from sounding observations taken in the environment of quasi-linear MCSs. A set of 186 soundings that sampled the beginning and mature stages of the MCSs are categorized by their production of severe surface winds into weak, severe, and derecho-producing MCSs. Differences in the variables among these three MCS categories are identified and discussed. Mean low- to upper-level wind speeds and deep-layer vertical wind shear, especially the component perpendicular to the convective line, are excellent discriminators among all three categories. Low-level inflow relative to the system is found to be an excellent discriminator, largely because of the strong relationship of system severity to system speed. Examination of the mean wind and shear vectors relative to MCS motion suggests th...

Journal ArticleDOI
TL;DR: In this article, the authors investigated the formation and evolution of a heavy sea fog episode that occurred over the Yellow Sea on 9 March 2005, using satellite images, surface observations and soundings at islands and coasts, and analyses from the Japan Meteorology Agency (JMA) are used to describe and analyze this event.
Abstract: In this paper, a heavy sea fog episode that occurred over the Yellow Sea on 9 March 2005 is investigated. The sea fog patch, with a spatial scale of several hundred kilometers at its mature stage, reduced visibility along the Shandong Peninsula coast to 100 m or much less at some sites. Satellite images, surface observations and soundings at islands and coasts, and analyses from the Japan Meteorology Agency (JMA) are used to describe and analyze this event. The analysis indicates that this sea fog can be categorized as advection cooling fog. The main features of this sea fog including fog area and its movement are reasonably reproduced by the Fifth-generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5). Model results suggest that the formation and evolution of this event can be outlined as: (1) southerly warm/moist advection of low-level air resulted in a strong sea-surface-based inversion with a thickness of about 600 m; (2) when the inversion moved from the warmer East Sea to the colder Yellow Sea, a thermal internal boundary layer (TIBL) gradually formed at the base of the inversion while the sea fog grew in response to cooling and moistening by turbulence mixing; (3) the sea fog developed as the TIBL moved northward and (4) strong northerly cold and dry wind destroyed the TIBL and dissipated the sea fog. The principal findings of this study are that sea fog forms in response to relatively persistent southerly warm/moist wind and a cold sea surface, and that turbulence mixing by wind shear is the primary mechanism for the cooling and moistening the marine layer. In addition, the study of sensitivity experiments indicates that deterministic numerical modeling offers a promising approach to the prediction of sea fog over the Yellow Sea but it may be more efficient to consider ensemble numerical modeling because of the extreme sensitivity to model input.

Journal ArticleDOI
TL;DR: This article examined the strength and statistical behavior of mesoscale motions on time scales up to 1 h using eight data sets over different surface types, including internal gravity waves, microfront-like structures, horizontal modes and a complex variety of other signatures, perhaps resulting from superposition of different modes.
Abstract: This study examines the strength and statistical behavior of mesoscale motions on time scales up to 1 h using eight data sets over different surface types. The mesoscale motions include internal gravity waves, microfront-like structures, horizontal modes, and a complex variety of other signatures, perhaps resulting from superposition of different modes. With weak large-scale flow, the mesoscale motions lead to meandering of the wind direction, as found in previous studies. However, the meandering often takes the form of sudden wind shifts rather than oscillation of wind direction.

Patent
15 Mar 2007
TL;DR: In this article, the pitch of the blades is controlled by a conventional manner by a command component, a rotor blade pitch command signal, which includes compensation for instantaneous moment deviations of the wind turbine.
Abstract: A wind turbine rotor load control. The pitch of the blades is controlled in a conventional manner by a command component, a rotor blade pitch command signal. A storage contains stored values of a set of moments for various wind speeds. A moment sensor provides a moment signal output. An instantaneous wind speed indicator provides an instantaneous wind speed value output. A conversion logic connected to the moment signal and to the instantaneous wind speed value, provides a calculated pitch modulation command. Combining logic connected to the calculated blade pitch modulation command and to the collective pitch command, provides a combined blade pitch command, which includes compensation for instantaneous moment deviations of the wind turbine.

Journal ArticleDOI
01 Oct 2007-Icarus
TL;DR: In this article, a reanalysis of an extended set of images obtained at two wavelengths, 418 nm (violet) and 986 nm (near infrared), that sense different altitude levels in the upper cloud is presented.

Journal ArticleDOI
TL;DR: In this paper, a comparative study of the conventional stationary wind speed model and a newly proposed non-stationary model using field measurements is presented, and the differences between, the two wind models are briefly reviewed.
Abstract: We present a comparative study of the conventional stationary wind speed model and a newly proposed non-stationary wind speed model using field measurements. The concept of, and the differences between, the two wind models are briefly reviewed. Wind data recorded by a field measurement system for wind turbulence parameters (FMS-WTP) of 1-year duration are analyzed using the two wind models. Comparisons were made between the wind characteristics obtained from the two models, including hourly mean wind speed, turbulence intensity, the wind spectrum, integral length scale, root coherence function and probability density function. The effects of wind types (monsoon or typhoon), statistical properties (stationary or non-stationary), and surface roughness (open-sea fetch or overland fetch) on wind characteristics are discussed. The comparative study demonstrates that the non-stationary wind model appears to be more appropriate than the conventional stationary wind speed model for characterizing turbulent winds of one-hour duration over complex terrain.

Journal ArticleDOI
TL;DR: A packet of equatorial Rossby (ER) waves that lasted 2.5 months is identified in the lower troposphere of the northwest Pacific as discussed by the authors, which is associated with the development of at least 8 of the 13 tropical cyclones that formed during the period.
Abstract: A packet of equatorial Rossby (ER) waves that lasted 2.5 months is identified in the lower troposphere of the northwest Pacific. Waves within the packet had a period of 22 days, a wavelength of 3600 km, a westward phase speed of 1.9 m s−1, and a near-zero group speed. The wave properties followed the ER wave dispersion relation with an equivalent depth near 25 m. The packet was associated with the development of at least 8 of the 13 tropical cyclones that formed during the period. A composite was constructed around the genesis locations. Tropical cyclones formed east of the center of the composite ER wave low in a region of strong convection and a separate 850-hPa vorticity maximum. The background flow during the life of the packet was characterized by 850-hPa zonal wind convergence and easterly vertical wind shear. Wave amplitude peaked at the west end of the convergent region, suggesting that wave accumulation played a significant role in the growth of the packet. The presence of easterly verti...

Journal ArticleDOI
TL;DR: In this paper, the authors presented calculated values of wind shear coefficients (WSE) using measured values at 20, 30, and 40m above ground level (AGL) for Dhahran, Saudi Arabia, for a period of almost 5 years between 4 October 1995 and 30 November 2000.

Journal ArticleDOI
TL;DR: In this article, the authors measured flow past simulated sinusoidal hills in an ABLWT that modeled typical full-scale complex terrain for many wind turbine locations in the Altamont Pass, California, USA.

Journal ArticleDOI
TL;DR: In this paper, the Quadrics Bologna Limited Area Model (QBOLAM) is used for surface wind forecasting from a Quick Scatterometer (QuikSCAT) wind observations.
Abstract: Surface wind forecasts from a limited-area model [the Quadrics Bologna Limited-Area Model (QBOLAM)] covering the entire Mediterranean area at 0.1° grid spacing are verified against Quick Scatterometer (QuikSCAT) wind observations. Only forecasts within the first 24 h in coincidence with satellite overpasses are used. Two years of data, from 1 October 2000 to 31 October 2002, have been considered, allowing for an adequate statistical assessment under different wind conditions. This has been carried out by analyzing the fields of the mean wind vectors, wind speed bias, correlation, difference standard deviation, steadiness, gustiness, and mean wind direction difference, in order to investigate spatial variability. Statistics have been computed on a seasonal basis. A comparison of satellite and forecast winds with measurements from three buoys was also performed. Some critical areas of the Mediterranean Sea where wind forecast quality is lower than average have been identified. Such areas correspond to semienclosed basins surrounded by important orography and to small regions at the lee side of the main islands. In open-sea regions the model underestimates wind strength from about 0.5 m s 1 in spring and summer to 1.0 m s 1 in winter, as evidenced by the existing biases against scatterometer data. Also, a wind direction bias (scatterometer minus model) generally between 5° and 15° exists. A survey of the identified and likely sources of forecast error is performed, indicating that orography representation plays an important role. Numerical damping is identified as a likely factor reducing forecast wind strength. The need for a correction scheme is envisaged to provide more accurate forcing for numerical sea state forecasting models, wind energy evaluation, and latent and/or sensible heat exchanges.

Journal ArticleDOI
TL;DR: In this article, the performance of QuikSCAT-derived wind vectors is evaluated using in-situ data from moored buoys over the Indian Ocean, and the results show that the mean differences for wind speed and wind direction are 0.37 millimms-1 and 5.8 millimm-1, respectively.
Abstract: The performance of QuikSCAT-derived wind vectors is evaluated using in-situ data from moored buoys over the Indian Ocean. The results show that the mean differences for wind speed and wind direction are 0.37 ms-1 and 5.8°, root mean square deviations are 1.57 ms-1 and 44.1° and corresponding coefficients of correlation are 0.87 and 0.75, respectively. The matching between in-situ and satellite estimates seems to be better in the North Indian Ocean than in the Equatorial Indian Ocean. The effects of sea surface temperature and air-sea temperature difference on wind residuals were also investigated. In general, QuikSCAT is found to overestimate the winds. It is speculated that low wind speed during rain-free conditions and high wind speed, normally associated with rain, may be the reason for the less accurate estimation of the wind vector from QuikSCAT over the Indian Ocean.

Journal ArticleDOI
TL;DR: In this paper, the authors used satellite-borne passive microwave radiometer estimates of the rain fields of Atlantic tropical cyclones to identify a preference for rainfall enhancement in the directions that are down shear and also to the left of the shear vector (in the Northern Hemisphere).
Abstract: [1] When tropical cyclones interact with environmental vertical wind shear, they often take on an asymmetric structure. Prior observational and modeling studies have identified a preference for rainfall enhancement in the directions that are down shear and also to the left of the shear vector (in the Northern Hemisphere). This study composites hundreds of snapshots of the rain fields for Atlantic tropical cyclones between 1988–2004, taken from satellite-borne passive microwave radiometer estimates. Quadrant mean rain rates are fairly symmetrical in weakly sheared hurricanes. For cases with shear greater than 10 m s−1, the favored downshear and left-of-shear quadrants have a factor of two greater rain rates in the inner core than the non-favored quadrants. Beyond 100 km from the center, this asymmetry increases to a factor of four or more. The magnitude of the asymmetry is greater for tropical storms, although the mean rain rates are less.