Showing papers on "Wind shear published in 2003"

Reduced drag coefficient for high wind speeds in tropical cyclones

Abstract: The transfer of momentum between the atmosphere and the ocean is described in terms of the variation of wind speed with height and a drag coefficient that increases with sea surface roughness and wind speed. But direct measurements have only been available for weak winds; momentum transfer under extreme wind conditions has therefore been extrapolated from these field measurements. Global Positioning System sondes have been used since 1997 to measure the profiles of the strong winds in the marine boundary layer associated with tropical cyclones. Here we present an analysis of these data, which show a logarithmic increase in mean wind speed with height in the lowest 200 m, maximum wind speed at 500 m and a gradual weakening up to a height of 3 km. By determining surface stress, roughness length and neutral stability drag coefficient, we find that surface momentum flux levels off as the wind speeds increase above hurricane force. This behaviour is contrary to surface flux parameterizations that are currently used in a variety of modelling applications, including hurricane risk assessment and prediction of storm motion, intensity, waves and storm surges.

Gas transfer velocities measured at low wind speed over a lake

Abstract: The relationship between gas transfer velocity and wind speed was evaluated at low wind speeds by quantifying the rate of evasion of the deliberate tracer, SF 6, from a small oligotrophic lake. Several possible relationships between gas transfer velocity and low wind speed were evaluated by using 1-min-averaged wind speeds as a measure of the instantaneous wind speed values. Gas transfer velocities in this data set can be estimated virtually equally well by assuming any of three widely used relationships between k600 and winds referenced to 10-m height, U10: (1) a bilinear dependence with a break in the slope at ; 3.7 ms 21 , which resulted in the best fit; (2) a power dependence; and (3) a constant transfer velocity for U10 ,; 3.7 ms 21 , with a linear dependence on wind speed at higher wind speeds. The lack of a unique relationship between transfer velocity and wind speed at low wind speeds suggests that other processes, such as convective cooling, contribute significantly to gas exchange when the wind speeds are low. All three proposed relationships clearly show a strong dependence on wind for winds .3.7 m s 21 which, coupled with the typical variability in instantaneous wind speeds observed in the field, leads to average transfer velocity estimates that are higher than those predicted for steady wind trends. The transfer velocities predicted by the bilinear steady wind relationship for U10 ,; 3.7 ms 21 are virtually identical to the theoretical predictions for transfer across a smooth surface.

The Relationship between Storm Motion, Vertical Wind Shear, and Convective Asymmetries in Tropical Cyclones

Abstract: The influence of the direction of storm motion on the azimuthal distribution of electrified convection in 35 Atlantic basin tropical cyclones from 1985 to 1999 was examined using data from the National Lightning Detection Network. In the inner 100 km, flashes most often occurred in the front half of storms, with a preference for the right-front quadrant. In the outer rainbands (r = 100–300 km), flashes occurred predominantly to the right of motion, although the maximum remained in the right-front quadrant. The results are shown to be consistent with previous studies of asymmetries in rainfall, radar reflectivity, and vertical motion with respect to tropical cyclone motion. The motion effect has been attributed to the influence of asymmetric friction in the tropical cyclone boundary layer. The authors previously found a strong signature in the azimuthal distribution of lightning with respect to vertical wind shear. Because both effects show clearly, vertical wind shear and storm motion must themse...

Spatial and temporal distributions of U.S. winds and wind power at 80 m derived from measurements

Abstract: windprofilesfromthesoundings,resultedin80-mwindspeedsthatare,onaverage,1.3–1.7 m/s faster than those obtained from the most common methods previously used to obtain elevated data for U.S. wind power maps, a logarithmic law and a power law, both with constant coefficients. The results suggest that U.S. wind power at 80 m may be substantially greater than previously estimated. It was found that 24% of all stations (and 37% of all coastal/offshore stations) are characterized by mean annual speeds � 6.9 m/s at 80 m, implying that the winds over possibly one quarter of the United States are strong enough to provide electric power at a direct cost equal to that of a new natural gas or coal power plant. ThegreatestpreviouslyunchartedreservoirofwindpowerinthecontinentalUnitedStatesis offshore and nearshore along the southeastern and southern coasts. When multiple wind sites are considered, the number of days with no wind power and the standard deviation of the wind speed, integrated across all sites, are substantially reduced in comparison with when one wind site is considered. Therefore a network of wind farms in locations with high annual mean wind speeds may provide a reliable and abundant source of electric power. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 3399 Meteorology and Atmospheric Dynamics: General or miscellaneous; 9350 Information Related to Geographic Region: North America;KEYWORDS:U.S. wind power, least squares, global warming, air pollution, energy, wind speed Citation: Archer, C. L., and M. Z. Jacobson, Spatial and temporal distributions of U.S. winds and wind power at 80 m derived from measurements, J. Geophys. Res., 108(D9), 4289, doi:10.1029/2002JD002076, 2003.

A Numerical Study of the Impact of Vertical Shear on the Distribution of Rainfall in Hurricane Bonnie (1998)

Abstract: Despite the significant impacts of torrential rainfall from tropical cyclones at landfall, quantitative precipitation forecasting (QPF) remains an unsolved problem A key task in improving tropical cyclone QPF is understanding the factors that affect the intensity and distribution of rainfall around the storm These include the storm motion, topography, and orientation of the coast, and interactions with the environmental flow The combination of these effects can produce rainfall distributions that may be nearly axisymmetric or highly asymmetric and rainfall amounts that range from 1 or 2 cm to >30 cm This study investigates the interactions between a storm and its environmental flow through a numerical simulation of Hurricane Bonnie (1998) that focuses on the role of vertical wind shear in governing azimuthal variations of rainfall The simulation uses the high-resolution nonhydrostatic fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) to simulate the storm between 0000

Low-Level Mesovortices within Squall Lines and Bow Echoes. Part II: Their Genesis and Implications

Abstract: This two-part study proposes a fundamental explanation of the genesis, structure, and implications of lowlevel, meso-g-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such ‘‘mesovortices’’ are observed frequently, at times in association with tornadoes. Idealized experiments with a numerical cloud model show that significant low-level mesovortices develop in simulated QLCSs, especially when the environmental vertical wind shear is above a minimum threshold and when the Coriolis forcing is nonzero. As illustrated by a QLCS simulated in an environment of moderate vertical wind shear, mesovortexgenesis is initiated at low levels by the tilting, in downdrafts, of initially crosswise horizontal baroclinic vorticity. Over a 30-min period, the resultant vortex couplet gives way to a dominant cyclonic vortex as the relative and, more notably, planetary vorticity is stretched vertically; hence, the Coriolis force plays a direct role in the low-level mesovortexgenesis. A downward-directed vertical pressure-gradient force is subsequently induced within the mesovortices, effectively segmenting the previously (nearly) continuous convective line. In moderate-to-strong environmental shear, the simulated QLCSs evolve into bow echoes with ‘‘straight line’’ surface winds found at the bow-echo apex and additionally in association with, and in fact induced by, the lowlevel mesovortices. Indeed, the mesovortex winds tend to be stronger, more damaging, and expand in area with time owing to a mesovortex amalgamation or ‘‘upscale’’ vortex growth. In weaker environmental shear—in which significant low-level mesovortices tend not to form—damaging surface winds are driven by a rear-inflow jet that descends and spreads laterally at the ground, well behind the gust front.

Modeling circulation in lakes: Spatial and temporal variations

Abstract: The influence of spatial and temporal variations in wind forcing on the circulation in lakes is investigated using field data and the three-dimensional Estuary and Lake Computer Model (ELCOM) applied to Lake Kinneret. Lake Kinneret field data from six thermistor chains and eight wind anemometers deployed during July 2001 are presented. Internal wave motions are well reproduced by the numerical model when forced with a spatially uniform wind taken from a station near the lake center; however, simulated seiche amplitudes are too large (especially vertical mode 2) and lead observations by 3‐10 h (for a 24-h period wave) at different locations around the lake. Consideration of the spatial variation of the wind field improves simulated wave amplitude, and phase error at all stations is reduced to less than 1.5 h. This improvement is attributable to a better representation of the horizontally averaged wind stress and can be reproduced with a spatially uniform wind that has the same horizontally averaged wind stress as the spatially varying wind field. However, a spatially varying wind field is essential for simulating mean surface circulation, which is shown to be predominantly directly forced by the surface-layer‐averaged wind stress moment.

Low-Level Mesovortices within Squall Lines and Bow Echoes. Part I: Overview and Dependence on Environmental Shear

Abstract: This two-part study proposes fundamental explanations of the genesis, structure, and implications of lowlevel meso-g-scale vortices within quasi-linear convective systems (QLCSs) such as squall lines and bow echoes. Such ‘‘mesovortices’’ are observed frequently, at times in association with tornadoes. Idealized simulations are used herein to study the structure and evolution of meso- g-scale surface vortices within QLCSs and their dependence on the environmental vertical wind shear. Within such simulations, significant cyclonic surface vortices are readily produced when the unidirectional shear magnitude is 20 m s 21 or greater over a 0‐2.5- or 0‐5-km-AGL layer. As similarly found in observations of QLCSs, these surface vortices form primarily north of the apex of the individual embedded bowing segments as well as north of the apex of the larger-scale bow-shaped system. They generally develop first near the surface but can build upward to 6‐8 km AGL. Vortex longevity can be several hours, far longer than individual convective cells within the QLCS; during this time, vortex merger and upscale growth is common. It is also noted that such mesoscale vortices may be responsible for the production of extensive areas of extreme ‘‘straight line’’ wind damage, as has also been observed with some QLCSs. Surface vortices are also produced for weaker shears but remain shallow, weak, and short-lived. Although similar in size and strength to mesocyclones associated with supercell storms, and also sometimes producing similar hooklike structures in the rain field, it is also shown that the present vortices are quite distinct, structurally and dynamically. Most critically, such vortices are not associated with long-lived, rotating updrafts at midlevels and the associated strong, dynamically forced vertical accelerations, as occur within supercell mesocyclones.

Satellite observations of modulation of surface winds by typhoon-induced upper ocean cooling

Abstract: [1] Two remote sensing data sets, the Tropical Rainfall Measurement Mission Sea Surface Temperature (SST) and the NASA QuikSCAT ocean surface wind vectors, are analysed to study ocean-atmosphere interactions in cold SST regions formed in the trail of two typhoon events. Anomalously cold SST patches up to 6°C below the surrounding warm tropical ocean SST are found along the trail of typhoon tracks as cold, deep waters are entrained up to the mixed layer due to typhoon forcing. In both typhoon events, significant and systematic weakening of surface wind speed is found over cold SST patches relative to surface wind speed in surrounding regions. The wind speed anomalies disappear as the patches recover to the level of the surrounding SST. The results are consistent with the mechanism proposed by Wallace et al. that surface winds are modulated by SST via stability. As wind within the well-mixed boundary layer moves over the cold patch, boundary layer stability increases, vertical mixing is suppressed, and the vertical wind shear increases; reduction in surface wind speed is caused. In particular, our result shows that this mechanism can act on relatively small spatial (≈100 km) and short (≈1 day) time scales.

Numerical simulation of Martian dust devils

Abstract: [1] Large eddy simulations of vertical convective vortices and dust devils in the Martian convective boundary layer are presented, employing a version of the Mars MM5 mesoscale model, adapted to use periodic boundary conditions and run at resolutions of 10 to 100 m. The effects of background horizontal wind speed and shear on dust devil development are studied in four simulations, each extending over the daytime portion of one Martian day. The general vorticity development in all cases is similar, with roughly equal positive and negative vorticity extrema. Two dust devils were found to develop in the highest wind speed case and in a case run without background wind. The dust devil structures were found to agree well qualitatively with terrestrial dust devil observations, including the prediction of greatly diminished vertical velocities in the vortex core. Thermodynamic scaling theory of dust devils was found to provide good prediction of the relationship between central pressure and temperature in the modeled vortices. Examination of the turbulent kinetic energy budgets suggests balance between buoyancy generation and loss through dissipation and transport. The vorticity for the dust devils is provided by twisting of horizontal vorticity into the vertical. The horizontal vorticity originates from horizontal variations in temperature at the lower boundary (thermal buoyancy). While the horizontal winds generated by the modeled dust devils were likely insufficient to lift dust, this study provides a solid starting point for dynamic modeling of what may be an important component of the Martian dust cycle.

Offshore Wind Turbine Wakes Measured by Sodar

Abstract: A ship-mounted sodar was used to measure wind turbine wakes in an offshore wind farm in Denmark. The wake magnitude and vertical extent were determined by measuring the wind speed profile behind an operating turbine, then shutting down the turbine and measuring the freestream wind profile. These measurements were compared with meteorological measurements on two offshore and one coastal mast at the same site. The main purposes of the experiment were to evaluate the utility of sodar for determining wind speed profiles offshore and to provide the first offshore wake measurements with varying distance from a wind turbine. Over the course of a week, 36 experiments were conducted in total. After quality control of the data (mainly to exclude rain periods), 13 turbine-on, turbine-off pairs were analyzed to provide the velocity deficit at hub height as a function of the distance from the turbine. The results are presented in the context of wake measurements at other coastal locations. The velocity defici...

Wind Stress Vector over Ocean Waves

Abstract: Previous investigations of the wind stress in the marine surface layer have primarily focused on determining the stress magnitude (momentum flux) and other scalar variables (e.g., friction velocity, drag coefficient, roughness length). However, the stress vector is often aligned with a direction different from that of the mean wind flow. In this paper, the focus is on the study of the stress vector direction relative to the mean wind and surface-wave directions. Results based on measurements made during three field campaigns onboard the R/P Floating Instrument Platform (FLIP) in the Pacific are discussed. In general, the wind stress is a vector sum of the 1) pure shear stress (turbulent and viscous) aligned with the mean wind shear, 2) wind-wave-induced stress aligned with the direction of the pure wind-sea waves, and 3) swell-induced stress aligned with the swell direction. The direction of the wind-wave-induced stress and the swell-induced stress components may coincide with, or be opposite to,...

Characteristics of Vertical Wind Profiles near Supercells Obtained from the Rapid Update Cycle

Abstract: Over 400 vertical wind profiles in close proximity to nontornadic and tornadic supercell thunderstorms are examined. The profiles were obtained from the Rapid Update Cycle (RUC) model/analysis system. Ground-relative wind speeds throughout the lower and middle troposphere are larger, on average, in tornadic supercell environments than in nontornadic supercell environments. The average vertical profiles of storm-relative wind speed, vertical wind shear, hodograph curvature, crosswise and streamwise vorticity, and storm-relative helicity are generally similar above 1 km in the tornadic and nontornadic supercell environments, with differences that are either not statistically significant or not what most would regard as meteorologically significant. On the other hand, considerable differences are found in these average vertical profiles within 1 km of the ground, with environments associated with significantly tornadic supercells (those producing tornadoes of at least F2 intensity) having substantia...

NOTES AND CORRESPONDENCE Characteristics of Vertical Wind Profiles near Supercells Obtained from the Rapid Update Cycle

Abstract: Over 400 vertical wind profiles in close proximity to nontornadic and tornadic supercell thunderstorms are examined. The profiles were obtained from the Rapid Update Cycle (RUC) model/analysis system. Groundrelative wind speeds throughout the lower and middle troposphere are larger, on average, in tornadic supercell environments than in nontornadic supercell environments. The average vertical profiles of storm-relative wind speed, vertical wind shear, hodograph curvature, crosswise and streamwise vorticity, and storm-relative helicity are generally similar above 1 km in the tornadic and nontornadic supercell environments, with differences that are either not statistically significant or not what most would regard as meteorologically significant. On the other hand, considerable differences are found in these average vertical profiles within 1 km of the ground, with environments associated with significantly tornadic supercells (those producing tornadoes of at least F2 intensity) having substantially larger low-level vertical wind shear, streamwise vorticity, and storm-relative helicity compared to environments associated with nontornadic supercells and weakly tornadic supercells (those producing F0 or F1 tornadoes). These findings may partly explain the extraordinary difficulty in discriminating between tornadic and nontornadic supercell environments in a forecasting setting, given the low temporal and spatial frequency of wind observations in the lowest 1 km. It is believed that it would be a worthwhile investment to augment low-level wind profiling capabilities, in addition to taking a closer look at the dynamical sensitivities of supercell storms to near-surface wind shear by way of high-resolution numerical simulations.

Meteorological predictions for 2003 Mars Exploration Rover high‐priority landing sites

Abstract: [1] The Mars Regional Atmospheric Modeling System is used to predict meteorological conditions that are likely to be encountered by the Mars Exploration Rovers at several proposed landing sites during entry, descent, and landing. Seven areas, five of which contain specific high-priority landing ellipses, are investigated: Hematite (two sites), Isidis Planitia, Elysium Planitia (two sites), Valles Marineris, and Gusev Crater. The last two locations are in regions of extreme topography, and the local and regional thermal circulations that result are equally extreme. Horizontal wind speeds near the floor of Valles Marineris exceed 50 ms−1. Vertical velocities near the walls exceed 40 ms−1 and penetrate 10 km in altitude above the rim of the canyon. Thermal convection is suppressed within Valles Marineris by subsidence that forms in response to the upslope flows along the canyon walls. Wind magnitudes at Gusev crater are approximately one third of those at the canyon, but horizontal wind shear is greater. Deep convective thermals are noted at the relatively flat Hematite site, where 10 ms−1 updrafts rising to heights of 5 km are not uncommon during the midafternoon. Linearly organized convective updrafts superimposed on upslope circulations dominate over most of Isidis Planitia. Hexagonal and linearly organized convection predominates at Elysium Planitia. Afternoon circulations at all sites pose some risk (significant risk in some cases) to entry, descent, and landing. Most of the atmospheric hazards are not evident in current observational data and general circulation model simulations and can only be ascertained through mesoscale modeling of the region.

Ocean wind fields retrieved from radar‐image sequences

Abstract: [1] The dependency of radar backscatter on the surface wind field is investigated using a nautical X-band radar operating at grazing incidence and horizontal polarization in transmit and receive. This resulted in development of an algorithm for wind retrieval from nautical radar image sequences. The algorithm consists of two parts: In the first part, wind directions are extracted from wind-induced streaks that are in line with the mean surface wind direction. These streaks are visible in the temporal integrated radar images at scales between 100 and 500 m. The orientation of the streaks is determined from the local gradients, which are derived from the radar images smoothed and reduced to the appropriate scales. In the second part, wind speeds are derived from the backscatter of the temporal integrated radar image sequence and the radar retrieved wind direction. The dependency of the radar backscatter on the local surface wind and geometry of the radar is parameterized by training a neural network. The algorithm is applied to radar image sequences acquired by a nautical X-band radar mounted aboard an offshore platform in the North Sea. The radar-derived winds are validated by comparison to in situ wind data measured at the platform. The comparison of wind directions resulted in a correlation of 0.99 with a standard deviation of 14.2°; for wind speeds the correlation is 0.97 with a standard deviation of 0.85 m s−1. In contrast to traditional offshore wind sensors, the retrieval of the wind field from the backscatter of the ocean surface makes the system independent of the sensors motion and installation height and reduces the effects due to platform-induced blockage and turbulence effects.

Meteorological predictions for 2003 Mars Exploration Rover high-priority landing sites : Mars exploration rover mission and landing sites

Abstract: The Mars Regional Atmospheric Modeling System is used to predict meteorological conditions that are likely to be encountered by the Mars Exploration Rovers at several proposed landing sites during entry, descent, and landing. Seven areas, five of which contain specific high-priority landing ellipses, are investigated: Hematite (two sites), Isidis Planitia, Elysium Planitia (two sites), Valles Marineris, and Gusev Crater. The last two locations are in regions of extreme topography, and the local and regional thermal circulations that result are equally extreme. Horizontal wind speeds near the floor of Valles Marineris exceed 50 ms -1 . Vertical velocities near the walls exceed 40 ms -1 and penetrate 10 km in altitude above the rim of the canyon. Thermal convection is suppressed within Valles Marineris by subsidence that forms in response to the upslope flows along the canyon walls. Wind magnitudes at Gusev crater are approximately one third of those at the canyon, but horizontal wind shear is greater. Deep convective thermals are noted at the relatively flat Hematite site, where 10 ms -1 updrafts rising to heights of 5 km are not uncommon during the midafternoon. Linearly organized convective updrafts superimposed on upslope circulations dominate over most of Isidis Planitia. Hexagonal and linearly organized convection predominates at Elysium Planitia. Afternoon circulations at all sites pose some risk (significant risk in some cases) to entry, descent, and landing. Most of the atmospheric hazards are not evident in current observational data and general circulation model simulations and can only be ascertained through mesoscale modeling of the region.

A multidiagnostic investigation of the mesospheric bore phenomenon

Abstract: [1] Imaging measurements of a bright wave event in the nighttime mesosphere were made on 14 November 1999 at two sites separated by over 500 km in the southwestern United States. The event was characterized by a sharp onset of a series of extensive wavefronts that propagated across the entire sky. The waves were easily visible to the naked eye, and the entire event was observed for at least 5 hours. The event was observed using three wide-angle imaging systems located at the Boston University field station at McDonald Observatory (MDO), Fort Davis, Texas, and the Starfire Optical Range (SOR), Albuquerque, New Mexico. The spaced imaging measurements provided a unique opportunity to estimate the physical extent and time history of the disturbance. Simultaneous radar neutral wind measurements in the 82 to 98 km altitude region were also made at the SOR which indicated that a strong vertical wind shear of 19.5 ms−1km−1 occurred between 80 and 95 km just prior to the appearance of the disturbance. Simultaneous lidar temperature and density measurements made at Fort Collins, Colorado, ∼1100 km north of MDO, show the presence of a large (∼50 K) temperature inversion layer at the time of the wave event. The observations indicated that the event was most probably due to an undular mesospheric bore, a relatively uncommon disturbance which has only recently been reported [Taylor et al., 1995a]. Evidence is also shown to suggest that a large east-west tropospheric frontal system lying over the northern United States was the origin of the disturbance.

Environmental Vertical Wind Shear with Hurricane Bertha (1996)

Abstract: Hurricane Bertha (1996) was influenced by vertical wind shear with highly variable direction and magnitude. The paper describes a unique method for determining the vertical tilt of a tropical cyclone vortex using satellite and aircraft data. Hurricane Bertha's vortex tracks at three levels are shown during a period of intensification just prior to landfall. During this period, the hurricane vortex becomes more closely aligned in the vertical. Changes in asymmetries of satellite infrared (IR) cold cloud areas are shown to be related to the vortex alignment. Environmental vertical shear measurements throughout Hurricane Bertha's life cycle are presented using IR cloud asymmetries and numerical model analyses. Intensification periods are associated with more symmetric IR cloud measurements. The directions of the IR cloud asymmetric orientations are compared with numerical-model-derived vertical shear directions. The changes in the vertical shear analyses are discussed with respect to observed intens...

Modelling of Offshore Wind Turbine Wakes with the Wind Farm Program FLaP

Abstract: The wind farm layout program FLaP estimates the wind speed at any point in a wind farm and the power output of the turbines The ambient flow conditions and the properties of the turbines and the farm are used as input The core of the program is an axisymmetric wake model describing the wake behind one rotor Here an approach based on the simplified Reynolds equation with eddy viscosity closure is chosen The single-wake model is combined with a model for the vertical wind speed profile and a wind farm model, which takes care of the interaction of all wakes in a wind farm The wake model has been extended to improve the description of wake development in offshore conditions, especially the low ambient turbulence and the effect of atmospheric stability Model results are compared with measurements from the Danish offshore wind farm Vindeby Vertical wake profiles and mean turbulence intensities in the wake are compared for single-, double- and quintuple-wake cases with different mean wind speed, turbulence intensity and atmospheric stability It is found that within the measurement uncertainties the results of the wake model compare well with the measurements for the most important ambient conditions The effect of the low turbulence intensity offshore on the wake development is modelled well for Vindeby wind farm Deviations are found when atmospheric stability deviates from near-neutral conditions For stable atmospheric conditions both the free vertical wind speed profile and the wake profile are not modelled satisfactorily Copyright © 2003 John Wiley & Sons, Ltd

Entrainment Processes in the Convective Boundary Layer with Varying Wind Shear

Abstract: Large-eddy simulations (LES) are performed to investigate the entrainment andthe structure of the inversion layer of the convective boundary layer (CBL) withvarying wind shears. Three CBLs are generated with the constant surface kinematicheat flux of 0.05 K m s-1 and varying geostrophic wind speeds from 5 to 15m s-1. Heat flux profiles show that the maximum entrainment heat flux as afraction of the surface heat flux increases from 0.13 to 0.30 in magnitude withincreasing wind shear. The thickness of the entrainment layer, relative to the depthof the well-mixed layer, increases substantially from 0.36 to 0.73 with increasingwind shear. The identification of vortices and extensive flow visualizations nearthe entrainment layer show that concentrated vortices perpendicular to the meanboundary-layer wind direction are identified in the capping inversion layer for thecase of strong wind shear. These vortices are found to develop along the mean winddirections over strong updrafts, which are generated by convective rolls and to appearas large-scale wavy motions similar to billows generated by the Kelvin–Helmholtzinstability. Quadrant analysis of the heat flux shows that in the case of strong windshear, large fluctuations of temperature and vertical velocity generated by largeamplitude wavy motions result in greater heat flux at each quadrant than that inthe weak wind shear case.

Wind and Stream Flow Induced Reaeration

Abstract: Reaeration in combined wind/stream driven flows was experimentally studied in a laboratory wind-water tunnel. Wind velocities, stream flow properties, and oxygen transfer rates were measured under a variety of cocurrent and countercurrent flow conditions. For pure open channel flows and pure wind-driven flows, respectively, the results of oxygen transfer measurement can be scaled by the simple forcing parameters and show good agreement with other laboratory studies. For wind/stream combined flows, the transfer velocity depends on the stream flow condition and wind shear velocity. Based on the magnitude of the bottom-shear induced transfer and the wind-shear induced transfer velocity, a criterion was proposed to determine the dominant mechanism for gas transfer in combined wind/stream driven flows. A simple additive model is developed for the reaeration coefficient in combined wind and stream flow-influenced environments. Also, the effects of tidal flow on reaeration in tidal estuaries are briefly discussed.

The nature of the mistral: Observations and modelling of two MAP events

Abstract: Two mistral events observed in 1999 during the Mesoscale Alpine Programme (MAP) are studied using observational data and high-resolution mesoscale-model simulations from the US Navy's Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPS). Radiosondes suggested that both mistral wind events were associated with the passage of cold fronts and post-frontal air descent. EuroSat Rapidscan images indicated a stationary and persistent cloud edge along the lee of the Massif Central. The cloud edge marks the beginning of mistral flow acceleration and descent according to COAMPS simulations. Narrow persistent cloud banners were identified trailing from the Mont Lozere in the Massif Central during both mistral events. COAMPS indicated that these cloud banners were associated with a deeper turbulent boundary layer in wakes induced by hydraulic jumps. The cloud banner and wake defined the western boundary of the mistral. Analysis of dropsonde data and in situ flight data indicated the presence of a sharp shear line separating the cold severe mistral wind from the warm calm wake flow to the lee of the French Alps. Trajectory analysis of model data suggested that air parcels on the two sides of the shear line had different histories. Copyright © 2003 Royal Meteorological Society.

Model Simulations Examining the Relationship of Lake-Effect Morphology to Lake Shape, Wind Direction, and Wind Speed

Abstract: Idealized model simulations with an isolated elliptical lake and prescribed winter lake-effect environmental conditions were used to examine the influences of lake shape, wind speed, and wind direction on the mesoscale morphology. This study presents the first systematic examination of variations in lake shape and the interplay between these three parameters. The array of 21 model simulations produced cases containing each of the three classic lake-effect morphologies (i.e., vortices, shoreline bands, and widespread coverage), and, in some instances, the mesoscale circulations were composed of coexisting morphologies located over the lake, near the downwind shoreline, or inland from the downwind shore. As with lake-effect circulations simulated over circular lakes, the ratio of wind speed (U) to maximum fetch distance (L) was found to be a valuable parameter for determining the morphology of a lake-effect circulation when variations of lake shape, wind speed, and wind direction were introduced. F...

Field measurements of wind speed and reconfiguration in Arundo donax (Poaceae) with estimates of drag forces

Abstract: The giant reed (Arundo donax) is well known as a species that can withstand high wind loads without mechanical damage. To examine wind impact, profiles of vertical wind speeds in the plant's natural habitat (southern France) were measured at the edge and within a stand in the main wind direction. Wind speed was recorded simultaneously at five heights. For 75 measurements of within-canopy wind speed profiles, the attenuation coefficient was 4.4 ± 0.5, a value typical for plant stands with very dense canopies. Video recordings proved that A. donax becomes streamlined with increasing wind speed, reducing the projected surface area of leaves and stem. The total projected surface area is a function of wind speed and can be characterized by a second-order polynomial regression curve. For small wind velocities up to 1 m/s, the calculated drag force is proportional to the square of the wind speed. However, when A. donax plants are subjected to higher wind speeds (1.5-10 m/s), the drag force becomes directly proportional to the wind speed. Streamlining is a potentially important adaptation for withstanding high wind loads, especially for individual plants and plants at the edge of stands, whereas in dense stands streamlining probably plays a minor role.