scispace - formally typeset
Search or ask a question

Showing papers on "Wind shear published in 2009"


Journal ArticleDOI
03 Jul 2009-Science
TL;DR: Two distinctly different forms of tropical Pacific Ocean warming are shown to have substantially different impacts on the frequency and tracks of North Atlantic tropical cyclones, potentially increasing the predictability of cyclones on seasonal time scales.
Abstract: Two distinctly different forms of tropical Pacific Ocean warming are shown to have substantially different impacts on the frequency and tracks of North Atlantic tropical cyclones. The eastern Pacific warming (EPW) is identical to that of the conventional El Nino, whereas the central Pacific warming (CPW) has maximum temperature anomalies located near the dateline. In contrast to EPW events, CPW episodes are associated with a greater-than-average frequency and increasing landfall potential along the Gulf of Mexico coast and Central America. Differences are shown to be associated with the modulation of vertical wind shear in the main development region forced by differential teleconnection patterns emanating from the Pacific. The CPW is more predictable than the EPW, potentially increasing the predictability of cyclones on seasonal time scales.

358 citations


Journal ArticleDOI
TL;DR: In this article, the authors used a large eddy simulation model with particular wind boundary conditions to simulate and characterize the turbulence generated by the presence of a wind turbine and its evolution downstream the machine.
Abstract: When a wind turbine works in yaw, the wake intensity and the power production of the turbine become slightly smaller and a deflection of the wake is induced. Therefore, a good understanding of this effect would allow an active control of the yaw angle of upstream turbines to steer the wake away from downstream machines, reducing its effect on them. In wind farms where interaction between turbines is significant, it is of interest to maximize the power output from the wind farm as a whole and to reduce fatigue loads on downstream turbines due to the increase of turbulence intensity in wakes. A large eddy simulation model with particular wind boundary conditions has been used recently to simulate and characterize the turbulence generated by the presence of a wind turbine and its evolution downstream the machine. The simplified turbine is placed within an environment in which relevant flow properties like wind speed profile, turbulence intensity and the anisotropy of turbulence are found to be similar to the ones of the neutral atmosphere. In this work, the model is used to characterize the wake deflection for a range of yaw angles and thrust coefficients of the turbine. The results are compared with experimental data obtained by other authors with a particle image velocimetry technique from wind tunnel experiments. Also, a comparison with simple analytical correlations is carried out. Copyright © 2009 John Wiley & Sons, Ltd.

348 citations


Journal ArticleDOI
TL;DR: In this article, the authors systematically assessed the aerosol effects on isolated DCCs based on cloud-resolving model simulations with spectral bin cloud microphysics and found that the decreasing rate of convective strength is greater in the humid air than that in the dry air when wind shear is strong.
Abstract: [1] Aerosol-cloud interaction is recognized as one of the key factors influencing cloud properties and precipitation regimes across local, regional, and global scales and remains one of the largest uncertainties in understanding and projecting future climate changes. Deep convective clouds (DCCs) play a crucial role in the general circulation, energy balance, and hydrological cycle of our climate system. The complex aerosol-DCC interactions continue to be puzzling as more ‘‘aerosol effects’’ unfold, and systematic assessment of such effects is lacking. Here we systematically assess the aerosol effects on isolated DCCs based on cloud-resolving model simulations with spectral bin cloud microphysics. We find a dominant role of vertical wind shear in regulating aerosol effects on isolated DCCs, i.e., vertical wind shear qualitatively determines whether aerosols suppress or enhance convective strength. Increasing aerosols always suppresses convection under strong wind shear and invigorates convection under weak wind shear until this effect saturates at an optimal aerosol loading. We also found that the decreasing rate of convective strength is greater in the humid air than that in the dry air when wind shear is strong. Our findings may resolve some of the seemingly contradictory results among past studies by considering the dominant effect of wind shear. Our results can provide the insights to better parameterize aerosol effects on convection by adding the factor of wind shear to the entrainment term, which could reduce uncertainties associated with aerosol effects on climate forcing.

305 citations


Journal ArticleDOI
TL;DR: In this paper, the radial dependence of the height of the maximum wind speed in a hurricane, which is found to lower with increasing inertial stability (which in turn depends on increasing wind speed and decreasing radius) near the eyewall, is examined.
Abstract: This article examinesthe radial dependence of the height of the maximum wind speed in a hurricane, which is found to lower with increasing inertial stability (which in turn depends on increasing wind speed and decreasing radius) near the eyewall. The leveling off, or limiting value, of the marine drag coefficient in high winds is also examined. The drag coefficient, given similar wind speeds, is smaller for smaller-radii storms; enhanced sea sprayby short or breaking waves is speculatedas a cause. Afitting technique of dropsonde wind profiles is used to model the shape of the vertical profile of mean horizontal wind speeds in the hurricane boundary layer, using only the magnitude and radius of the ‘‘gradient’’ wind. The method slightly underestimates the surface winds in small but intense storms, but errors are less than 5% near the surface. The fit is then applied to a slab layer hurricane wind field model, and combined with a boundary layer transition model to estimate surface winds over both marine and land surfaces.

250 citations


Journal ArticleDOI
TL;DR: In this article, the surface current response to winds is analyzed in a 2-yr time series of a 12-MHz (HF) Wellen Radar (WERA) off the west coast of France.
Abstract: The surface current response to winds is analyzed in a 2-yr time series of a 12-MHz (HF) Wellen Radar (WERA) off the west coast of France. Consistent with previous observations, the measured currents, after filtering tides, are on the order of 1.0%–1.8% of the wind speed, in a direction 10°–40° to the right of the wind, and with systematic trends as a function of wind speed. This Lagrangian current can be decomposed as the vector sum of a quasi-Eulerian current UE, representative of the top 1 m of the water column and part of the wave-induced Stokes drift Uss at the sea surface. Here, Uss is estimated with an accurate numerical wave model using a novel parameterization of wave dissipation processes. Using both observed and modeled wave spectra, Uss is found to be very well approximated by a simple function of the wind speed and significant wave height, generally increasing quadratically with the wind speed. Focusing on a site located 100 km from the mainland, the wave-induced contribution of Uss ...

243 citations


Journal ArticleDOI
TL;DR: In this paper, the influence of wind shear and turbulence on wind turbine performance is analyzed up to a height of 160'm. The analysis is carried out as time series simulations where the electrical power is the primary characterization parameter.
Abstract: To identify the influence of wind shear and turbulence on wind turbine performance, flat terrain wind profiles are analysed up to a height of 160 m. The profiles' shapes are found to extend from no shear to high wind shear, and on many occasions, local maxima within the profiles are also observed. Assuming a certain turbine hub height, the profiles with hub-height wind speeds between 6 m s−1 and 8 m s−1 are normalized at 7 m s−1 and grouped to a number of mean shear profiles. The energy in the profiles varies considerably for the same hub-height wind speed. These profiles are then used as input to a Blade Element Momentum model that simulates the Siemens 3.6 MW wind turbine. The analysis is carried out as time series simulations where the electrical power is the primary characterization parameter. The results of the simulations indicate that wind speed measurements at different heights over the swept rotor area would allow the determination of the electrical power as a function of an ‘equivalent wind speed’ where wind shear and turbulence intensity are taken into account. Electrical power is found to correlate significantly better to the equivalent wind speed than to the single point hub-height wind speed. Copyright © 2008 John Wiley & Sons, Ltd.

186 citations


Journal ArticleDOI
TL;DR: In this paper, the inner-core planetary boundary layer (PBL) representation of the hurricane boundary layer in high-resolution WeatherResearchandForecastModel (WRF) simulation was evaluated by direct comparison to in situ data obtained by research aircraft.
Abstract: This is the second of a two-part study of the representation of the planetary boundary layer (PBL) in highresolutionWeatherResearchandForecastModel(WRF) simulationsofHurricaneIsabel(2003).TheYonsei University (YSU) PBL parameterization and the Mellor‐Yamada‐Janjic´ (MYJ) PBL parameterization are evaluated by direct comparison to in situ data obtained by research aircraft. The numerical model, simulation design, details of the PBL schemes, and the representation of the boundary layer in the outer-core were presented in Part I. This part presents a detailed study of the inner-core PBL, including its axisymmetric and asymmetric structures, and comparisons to analyses of dropsonde data from previous studies. Although neither PBL scheme was designed specifically for hurricane conditions, their simulated boundary layers are reasonably good representations of the observed boundary layer. Both schemes reproduce certain unique features of the hurricane boundary layer, such as the separate depths of the well-mixed layer and the inflow layer, and the pronounced wind speed maxima near the top of the inflow layer. Modification of the original YSU and MYJ schemes to have ocean roughness lengths more in agreement with recent studies considerably improves the results of both schemes. Even with these improvements, the MYJ consistently produces larger frictional tendencies in the boundary layer than the YSU scheme, leading to a stronger lowlevel inflow and a stronger azimuthal wind maximum at the top of the boundary layer. For both schemes, differences in the low-level asymmetries between the simulated and observed wind fields appear to be related to eyewall asymmetries forced by environmental wind shear. The effects of varying horizontal and vertical resolutions are also considered. Increasing the vertical resolution in the PBL results in minor improvements in the inner-core structures. Increasing the horizontal resolution around the eyewall also leads to improved boundary layers, as well as an improvement of the vertical structure of the inner-core wind field. A summary and discussion of the results of both Parts I and II is provided.

151 citations


Journal ArticleDOI
TL;DR: In this article, the wind characteristics of a strong typhoon (Typhoon Maemi 2003) were analyzed on the basis of 10min wind speed samples, where the wind speeds were measured simultaneously by nine vane and seven sonic anemometers at a height of about 15 meters.

149 citations


Journal ArticleDOI
TL;DR: In this paper, a 3D hydrodynamic model is used to investigate how axial wind influences stratification and explore the associated longitudinal salt transport in partially mixed estuaries, and the results confirm that wind straining of the along-channel salinity gradient exerts an important control on stratification.
Abstract: A 3D hydrodynamic model [Regional Ocean Model System (ROMS)] is used to investigate how axial wind influences stratification and to explore the associated longitudinal salt transport in partially mixed estuaries. The model is configured to represent a straight estuarine channel connecting to a shelf sea. The results confirm that wind straining of the along-channel salinity gradient exerts an important control on stratification. Two governing parameters are identified: the Wedderburn number (W) defined as the ratio of wind stress to axial baroclinic pressure gradient force, and the ratio of an entrainment depth to water depth (hs/H). Here W controls the effectiveness of wind straining, which promotes increases (decreases) in stratification during down-estuary (up-estuary) wind. The ratio hs/H determines the portion of the water column affected by direct wind mixing. While stratification is always reduced by up-estuary wind, stratification shows an increase-then-decrease transition when down-estuar...

128 citations


Journal ArticleDOI
TL;DR: In this paper, a minimal complexity model of both the local and remote stationary responses of the atmosphere to tropical heating anomalies is described and demonstrated, where two levels are recast as baroclinic and barotropic components with thermal advection in the tropics neglected.
Abstract: A minimal complexity model of both the local and remote stationary responses of the atmosphere to tropical heating anomalies is described and demonstrated. Two levels are recast as baroclinic and barotropic components with thermal advection in the tropics neglected. The model is linearized about some idealized and realistic background wind fields and forced with a localized heating for illustration. In the tropics, the baroclinic responses are familiar from the Matsuno–Gill model; these excite barotropic responses by advective interactions with vertical background wind shear. The barotropic signals are in turn transmitted to high latitudes only in the presence of barotropic background westerly winds. For an El Nino–like equatorial heating, the barotropic response has anticyclones to the north and south of the heating reinforcing (opposing) the anticyclonic (cyclonic) baroclinic gyres in the upper (lower) troposphere. With realistic background flows, the model reproduces the hemispheric asymmetry ...

119 citations


Journal ArticleDOI
TL;DR: In this article, the authors present a simulation model of a megawatt-level variable-speed wind turbine with a full-scale back-to-back power converter developed in the simulation tool of PSCAD/EMTDC.
Abstract: Grid-connected wind turbines are fluctuating power sources that may produce flicker during continuous operation. This paper presents a simulation model of a megawatt-level variable-speed wind turbine with a full-scale back-to-back power converter developed in the simulation tool of PSCAD/EMTDC. Flicker emission of this system is investigated. Reactive power compensation is mostly adopted for flicker mitigation. However, the flicker mitigation technique shows its limits, when the grid impedance angle is low in some distribution networks. A new method of flicker mitigation by controlling active power is proposed. It smoothes the 3p active power oscillations from wind shear and tower shadow effects of the wind turbine by varying the dc-link voltage of the full-scale converter. Simulation results show that damping the 3p active power oscillation by using the flicker mitigation controller is an effective means for flicker mitigation of variable-speed wind turbines with full-scale back-to-back power converters during continuous operation.

Journal ArticleDOI
TL;DR: In this article, the authors examined thresholds of sand movement, intermittency and the relationship between fluctuating winds and transport intensity based on high frequency measurements of wind speed and saltation and found that sand transport was highly intermittent early in the day when winds were only slightly above threshold and became more continuous towards the upper beach because of increasing fetch distance and decreasing surface moisture.

Journal ArticleDOI
TL;DR: In this paper, the probability of a combination of convective available potential energy (CAPE) and deep tropospheric wind shear being associated with significant severe thunderstorms has been calculated for both areas.

Journal ArticleDOI
TL;DR: In this article, the authors identify two types of coastal mountain gap wind jets that frequently blow across the longitudinal axis of the Red Sea: (1) an eastward-blowing summer daily wind jet originating from the Tokar Gap on the Sudanese Red Sea coast, and (2) wintertime westwardblowing wind-jet bands along the northwestern Saudi Arabian coast, which occur every 10−20 days and can last for several days when occurring.
Abstract: [1] Mesoscale atmospheric modeling over the Red Sea, validated by in-situ meteorological buoy data, identifies two types of coastal mountain gap wind jets that frequently blow across the longitudinal axis of the Red Sea: (1) an eastward-blowing summer daily wind jet originating from the Tokar Gap on the Sudanese Red Sea coast, and (2) wintertime westward-blowing wind-jet bands along the northwestern Saudi Arabian coast, which occur every 10–20 days and can last for several days when occurring. Both wind jets can attain wind speeds over 15 m s−1 and contribute significantly to monthly mean surface wind stress, especially in the cross-axis components, which could be of importance to ocean eddy formation in the Red Sea. The wintertime wind jets can cause significant evaporation and ocean heat loss along the northeastern Red Sea coast and may potentially drive deep convection in that region. An initial characterization of these wind jets is presented.

01 Oct 2009
TL;DR: In this article, the authors present a computationally inexpensive method to apply discrete body forces into the finite-volume flow solver with collocated variable treatment (EllipSys), which avoids the pressure-velocity decoupling issue.
Abstract: This thesis describes the different steps needed to design a steady-state computational fluid dynamics (CFD) wind farm wake model. The ultimate goal of the project was to design a tool that could analyze and extrapolate systematically wind farm measurements to generate wind maps in order to calibrate faster and simpler engineering wind farm wake models. The most attractive solution was the actuator disc method with the steady state k-ε turbulence model. The first step to design such a tool is the treatment of the forces. This thesis presents a computationally inexpensive method to apply discrete body forces into the finite-volume flow solver with collocated variable treatment (EllipSys), which avoids the pressure-velocity decoupling issue. The second step is to distribute the body forces in the computational domain accordingly to rotor loading. This thesis presents a generic flexible method that associates any kind of shapes with the computational domain discretization. The special case of the actuator disc performs remarkably well in comparison with Conway’s heavily loaded actuator disc analytical solution and a CFD full rotor computation, even with a coarse discretization. The third step is to model the atmospheric turbulence. The standard k-ε model is found to be unable to model at the same time the atmospheric turbulence and the actuator disc wake and performs badly in comparison with single wind turbine wake measurements. A comparison with a Large Eddy Simulation (LES) shows that the problem mainly comes from the assumptions of the eddy-viscosity concept, which are deeply invalidated in the wind turbine wake region. Different models that intent to correct the k-ε model’s issues are investigated, of which none of them is found to be adequate. The mixing of the wake in the atmosphere is a deeply non-local phenomenon that is not handled correctly by an eddy-viscosity model such as k-ε . i AAU DCE Thesis No. 22

Journal ArticleDOI
TL;DR: The authors identified and examined the common characteristics of several nocturnal midlatitude mesoscale convective systems (MCSs) that developed near mesoscales convective vortices (MCVs) or cutoff lows.
Abstract: This study identifies and examines the common characteristics of several nocturnal midlatitude mesoscale convective systems (MCSs) that developed near mesoscale convective vortices (MCVs) or cutoff lows. All of these MCSs were organized into convective clusters or lines that exhibited back-building behavior, remained nearly stationary for 6–12 h, and produced locally excessive rainfall (greater than 200 mm in 12 h) that led to substantial flash flooding. Examination of individual events and composite analysis reveals that the MCSs formed in thermodynamic environments characterized by very high relative humidity at low levels, moderate convective available potential energy (CAPE), and very little convective inhibition (CIN). In each case, the presence of a strong low-level jet (LLJ) and weak midlevel winds led to a pronounced reversal of the wind shear vector with height. Most of the MCSs formed without any front or preexisting surface boundary in the vicinity, though weak boundaries were apparent...

Journal ArticleDOI
01 Apr 2009
TL;DR: In this paper, the authors describe a new tool that combines wind observations aloft (from wind profiling radars) with vertically integrated water vapour (IWV) measurements derived from global positioning system (GPS) receivers to estimate the bulk transport of water vapor.
Abstract: The skill of quantitative precipitation forecasts is poor, especially for extreme events. This paper describes a new tool that combines wind observations aloft (from wind profiling radars) with vertically integrated water vapour (IWV) measurements derived from global positioning system (GPS) receivers to estimate the bulk transport of water vapour. This transport strongly influences precipitation enhancement by mountains. Based on earlier research, a controlling wind layer is defined, which has maximum correlation between the horizontal component of the wind directed upslope at the coast and the rainfall measured downwind in the mountains. The altitude of the maximum correlation (∼ 1 km above sea level) often corresponds to the altitude of the low-level jet that typically resides in the region of enhanced water vapour transport ahead of an approaching cyclone's cold front (i.e. in the atmospheric river portion of the storm). The wind at this level usually differs from the wind at the surface, pointing to ...

Journal ArticleDOI
TL;DR: In this article, the diurnal variation of sporadic E layer occurrence frequency in 2006 and 2007 was analyzed using GPS (Global Positioning System) Radio occultation (RO) measurements from CHAMP, GRACE and FORMOSAT-3/COSMIC satellites at Northern Hemisphere midlatitides (50°-55° N) to obtain the diurn variation of sporadically E layer frequency.
Abstract: . GPS (Global Positioning System) Radio occultation (RO) measurements from CHAMP, GRACE and FORMOSAT-3/COSMIC satellites at Northern Hemisphere midlatitides (50°–55° N) are analysed to obtain the diurnal variation of sporadic E layer occurrence frequency in 2006 and 2007. Interconnections with zonal wind shears measured by meteor radar at Collm (51.3° N, 13° E), Germany, are investigated. According to theory, maximum Es occurrence is expected when the zonal wind shear, which is mainly produced by the semidiurnal tide in midlatitudes, is negative. This is confirmed by the present measurements and analysis.


Journal ArticleDOI
01 Mar 2009
TL;DR: The rate of working of surface wind stress on the geostrophic component of the surface flow of the World Ocean is revisited in this article, and the global mean is found to be about 0.85 to 1.0 TW.
Abstract: The rate of working of the surface wind stress on the geostrophic component of the surface flow of the World Ocean is revisited. The global mean is found to be about 0.85 to 1.0 TW. Consistent with previous estimates, about 0.75 to 0.9 TW comes from outside the equatorial region (poleward of 3). The rate of forcing of fluctuating currents integrates to only about 0.02 TW when the equatorial region is included, or close to zero over the extratropical region. Uncertainty in wind power input due to uncertainty in the surface currents is negligible. Results from several different wind stress products are compared, suggesting that uncertainty in wind stress is the dominant source of error. Ignoring the surface currents’ influence upon wind stress leads to a systematic bias in net wind power input; an overestimate of about 10 to 30%. (In previous estimates this positive bias was offset by too weak winds.) Small-scale, zonally elongated structures in the wind power input were found, but have both positive and negative contributions and lead to little net wind power input.

Journal ArticleDOI
TL;DR: In this article, a simple numerical model is constructed that calculates trajectories for raindrops based on their individual sizes, which allows size sorting to occur, and a strong positive correlation between the maximum ZDR in the arc signature and the low-level storm-relative environmental helicity (SRH) in the storm inflow.
Abstract: The dual-polarization radar variables are especially sensitive to the microphysical processes of melting and size sorting of precipitation particles. In deep convective storms, polarimetric measurements of such processes can provide information about the airflow in and around the storm that may be used to elucidate storm behavior and evolution. Size sorting mechanisms include differential sedimentation, vertical transport, strong rotation, and wind shear. In particular, winds that veer with increasing height typical of supercell environments cause size sorting that is manifested as an enhancement of differential reflectivity (ZDR) along the right or inflow edge of the forward-flank downdraft precipitation echo, which has been called the ZDR arc signature. In some cases, this shear profile can be augmented by the storm inflow. It is argued that the magnitude of this enhancement is related to the low-level storm-relative environmental helicity (SRH) in the storm inflow. To test this hypothesis, a simple numerical model is constructed that calculates trajectories for raindrops based on their individual sizes, which allows size sorting to occur. The modeling results indicate a strong positive correlation between the maximum ZDR in the arc signature and the low-level SRH, regardless of the initial drop size distribution aloft. Additional observational evidence in support of the conceptual model is presented. Potential changes in the ZDR arc signature as the supercell evolves and the low-level mesocyclone occludes are described.

Journal ArticleDOI
TL;DR: In this article, a model that qualitatively reproduces the wave-following atmospheric flow and the wave generated wind maximum, as seen from measurements, is proposed. But the model assumes a stationary momentum and turbulent kinetic energy balance and uses the dampening of the waves at the surface to describe the momentum flux from the waves to the atmosphere.
Abstract: Recent field observations and large-eddy simulations have shown that the impact of fast swell on the marine atmospheric boundary layer (MABL) might be stronger than previously assumed. For low to moderate winds blowing in the same direction as the waves, swell propagates faster than the mean wind. The momentum flux above the sea surface will then have two major components: the turbulent shear stress, directed downward, and the swell-induced stress, directed upward. For sufficiently high wave age values, the wave-induced component becomes increasingly dominant, and the total momentum flux will be directed into the atmosphere. Recent field measurements have shown that this upward momentum transfer from the ocean into the atmosphere has a considerable impact on the surface layer flow dynamics and on the turbulence structure of the overall MABL. The vertical wind profile will no longer exhibit a logarithmic shape because an acceleration of the airflow near the surface will take place, generating a low-level wave-driven wind maximum (a wind jet). As waves propagate away from their generation area as swell, some of the wave momentum will be returned to the atmosphere in the form of wave-driven winds. A model that qualitatively reproduces the wave-following atmospheric flow and the wave-generated wind maximum, as seen from measurements, is proposed. The model assumes a stationary momentum and turbulent kinetic energy balance and uses the dampening of the waves at the surface to describe the momentum flux from the waves to the atmosphere. In this study, simultaneous observations of wind profiles, turbulent fluxes, and wave spectra during swell events are presented and compared with the model. In the absence of an established model for the linear damping ratio during swell conditions, the model is combined with observations to estimate the wave damping. For the cases in which the observations showed a pronounced swell signal and almost no wind waves, the agreement between observed and modeled wind profiles is remarkably good. The resulting attenuation length is found to be relatively short, which suggests that the estimated damping ratios are too large. The authors attribute this, at least partly, to processes not accounted for by the model, such as the existence of an atmospheric background wind. In the model, this extra momentum must be supplied by the waves in terms of a larger damping ratio.

Journal ArticleDOI
TL;DR: Simulations of two cases of developing mesoscale convective vortices (MCVs) are examined to determine the dynamics governing the origin and vertical structure of these features in this paper.
Abstract: Simulations of two cases of developing mesoscale convective vortices (MCVs) are examined to determine the dynamics governing the origin and vertical structure of these features. Although one case evolves in strong vertical wind shear and the other evolves in modest shear, the evolutions are remarkably similar. In addition to the well-known mesoscale convergence that spins up vorticity in the midtroposphere, the generation of vorticity in the lower troposphere occurs along the convergent outflow boundary of the parent mesoscale convective system (MCS). Lateral transport of this vorticity from the convective region back beneath the midtropospheric vorticity center is important for obtaining a deep column of cyclonic vorticity. However, this behavior would be only transient without a secondary phase of vorticity growth in the lower troposphere that results from a radical change in the divergence profile favoring lower-tropospheric convergence. Following the decay of the nocturnal MCS, subsequent con...

Journal ArticleDOI
01 Jan 2009
TL;DR: In this article, the authors show that the sea surface temperature change over a long period is a key parameter to control the vertical wind shear over the NIO, an important quantity for cyclone activity.
Abstract: The anomalously strong cyclonic activity in the North Indian Ocean (NIO) during the recent years led to a controversy about the impact of global warming on cyclonic activity in the NIO Tropical Easterly Jet (TEJ) plays an important role in the formation of tropical cyclones (TC) over the NIO during summer monsoon season, but in the recent year it is decreasing. This is because of higher warming on the equatorial side of the TEJ than on the northern side, although on both sides a significant warming trend is seen. This warming seems to be a part of the general warming trend known to be occurring since mid 1970s. The vertical wind shear shows a positive correlation with the number of severe storms suggesting that a decrease in easterly shear is favourable for the formation of severe storms. Here I show that the sea surface temperature change over a long period is a key parameter to control the vertical wind shear over the NIO, an important quantity for cyclone activity. The stronger warming of the tropical North Indian Ocean during recent years drove reduced vertical wind shear Thus if the present decreasing trend of TEJ intensity continues, which is highly probable in view of presently occurring green-house warming, there is a strong likelihood of the formation of tropical cyclones of hurricane intensity even during the summer monsoon. Presently the intense systems are known to form only in the pre and post-monsoon seasons, when the vertical wind shear is small.

Journal ArticleDOI
TL;DR: In this paper, the authors combine simultaneous data from an instrumented Air-Sea Interaction Spar (ASIS) buoy and a 30-m tower, and obtain profiles of wind and turbulence characteristics at several heights from about 1 to 30 m above the water surface during swell conditions.
Abstract: By combining simultaneous data from an instrumented Air–Sea Interaction Spar (ASIS) buoy and a 30-m tower, profiles of wind and turbulence characteristics have been obtained at several heights from about 1 to 30 m above the water surface during swell conditions. Five cases formed as averages over time periods ranging from 2.5 to 9.5 h, representing quasi-steady conditions, have been selected. They represent a range of typical wave age and include wind-following swell cases and cross-swell cases. For relatively large wave age, the wind profile exhibits a well-defined maximum in the height range 5–10 m; for more modest wave age, this maximum turns into a sharp “knee” in the wind profile. Below the maximum (or knee), the wind increases rapidly with height; above that point the wind is very nearly constant up to the highest measuring level on the tower, 30 m. Analysis of balloon data from one day with swell indicates that the layer with constant wind in fact extends to the top of the boundary layer, ...

Journal ArticleDOI
TL;DR: In this article, the authors argue that the measurement of the wind speed over the whole rotor height should be the future preferred approach to minimize the uncertainty in estimating the wind potential of a site.
Abstract: Power curve measurements are encumbered with large uncertainty as wind measurements take place only at hub height. The wind profile over the turbine rotor is an expression of the kinetic energy available to the wind turbine and the evolution of large rotors prohibits the assumption that the hub height wind speed is representative of the wind speed over the whole rotor. Even in the case where measurements cover the lower half of the turbine rotor and extrapolations are attempted, the uncertainties remain considerable. We argue for that the measurement of the wind speed over the whole rotor height should be the future preferred approach. Such a measurement will minimize the uncertainty in estimating the wind potential of a site and the uncertainty in the power curve measurement method and the AEP calculation of wind turbines. To document this, we present wind speed and power curve results from wind and power measurement campaigns, one in flat terrain suffering an energy deficit and one in complex terrain pr...

Journal ArticleDOI
TL;DR: In this paper, the authors compared two simulations, one using a bulk and the other a detailed bin microphysical scheme, of a long-lasting, continental mesoscale convective system with leading convection and trailing stratiform region.
Abstract: Part I of this paper compares two simulations, one using a bulk and the other a detailed bin microphysical scheme, of a long-lasting, continental mesoscale convective system with leading convection and trailing stratiform region. Diagnostic studies and sensitivity tests are carried out in Part II to explain the simulated contrasts in the spatial and temporal variations by the two microphysical schemes and to understand the interactions between cloud microphysics and storm dynamics. It is found that the fixed raindrop size distribution in the bulk scheme artificially enhances rain evaporation rate and produces a stronger near-surface cool pool compared with the bin simulation. In the bulk simulation, cool pool circulation dominates the near-surface environmental wind shear in contrast to the near-balance between cool pool and wind shear in the bin simulation. This is the main reason for the contrasting quasi-steady states simulated in Part I. Sensitivity tests also show that large amounts of fast-...

Journal ArticleDOI
TL;DR: In this paper, the assignment of pressure heights to satellite-derived atmospheric motion vectors (AMVs), commonly known as cloud-drift and water vapor-motion winds, was investigated, and it was shown that vector height assignment is the dominant factor in AMV uncertainty, contributing up to 70% of the error.
Abstract: This study investigates the assignment of pressure heights to satellite-derived atmospheric motion vectors (AMVs), commonly known as cloud-drift and water vapor‐motion winds. Large volumes of multispectral AMV datasets are compared with collocated rawinsonde wind profiles collected by the U.S. Department of Energy Atmospheric Radiation Measurement Program at three geographically disparate sites: the southern Great Plains, the North Slope of Alaska, and the tropical western Pacific Ocean. From a careful analysis of these comparisons, the authors estimate that mean AMV observation errors are ;5‐5.5 m s 21 and that vector height assignment is the dominant factor in AMV uncertainty, contributing up to 70% of the error. These comparisons also reveal that in most cases the RMS differences between matched AMVs and rawinsonde wind values are minimized if the rawinsonde values are averaged over specified layers. In other words, on average, the AMV values better correlate to a motion over a mean tropospheric layer rather than to a traditionally assigned discrete level. The height assignment behavioral characteristics are specifically identified according to AMV height (high cloud vs low cloud), type (spectral bands; clear vs cloudy), geolocation, height assignment method, and amount of environmental vertical wind shear present. The findings have potentially important implications for data assimilation of AMVs, and these are discussed.

Journal ArticleDOI
TL;DR: In this paper, idealized numerical simulations are used to identify the processes responsible for initiating, organizing, and maintaining quasi-stationary convective systems that produce locally extreme rainfall amounts.
Abstract: In this study, idealized numerical simulations are used to identify the processes responsible for initiating, organizing, and maintaining quasi-stationary convective systems that produce locally extreme rainfall amounts. Of particular interest are those convective systems that have been observed to occur near mesoscale convective vortices (MCVs) and other midlevel circulations. To simulate the lifting associated with such circulations, a low-level momentum forcing is applied to an initial state that is representative of observed extreme rain events. The initial vertical wind profile includes a sharp reversal of the vertical wind shear with height, indicative of observed low-level jets. Deep moist convection initiates within the region of mesoscale lifting, and the resulting convective system replicates many of the features of observed systems. The low-level thermodynamic environment is nearly saturated, which is not conducive to the production of a strong surface cold pool; yet the convection qui...

Journal ArticleDOI
TL;DR: In this article, several wind fields developed for Hurricane Katrina (2005) in the US Gulf of Mexico (GOM) were applied with the ADCIRC hydrodynamic model to explore the sensitivity of predictions of coastal surges to wind field developed by alternative methods.
Abstract: Several wind fields developed for Hurricane Katrina (2005) in the US Gulf of Mexico (GOM) are applied with the ADCIRC hydrodynamic model to explore the sensitivity of predictions of coastal surges to wind fields developed by alternative methods. The alternative model predictions are evaluated against water level measurements provided by gages at two coastal locations. It is found that all the post-event analyzed wind fields yield a range of predictions of only ±10% of the available peak surge measurements regardless of whether the wind fields are produced by dynamical boundary layer models, kinematic analysis methods or a blend. However, the richness of meteorological forcing data in the GOM is not typically matched in other basins affected by tropical cyclones and errors may be much larger where storm intensity and size parameters are estimated mainly from satellite data. The attributes and remaining critical deficiencies of current methods for surface wind specification in both data-rich and data-poor environments are reviewed.