Wind shear

About: Wind shear is a research topic. Over the lifetime, 8023 publications have been published within this topic receiving 185373 citations.

Structure and evolution of a severe squall line over Oklahoma

Abstract: The structure of a severe squall line that developed in Oklahoma on May 2, 1979, is presented during its growth and part of its mature period. The line was analyzed using radar, satellite, sounding, and surface data for examining, in particular, the cell propagation mechanisms, the three-dimensional structure of the squall line and individual cells during the mature period, the mass and moisture fluxes, and precipitation efficiency. Comparison of the 1979 Oklahoma squall line with other documented squall lines indicates less well organized down-drafts on the rear side at low levels to midlevels, and an absence of low level to midlevel inflow on the rear side. The magnitude of mass and moisture fluxes was comparable to previous squall line cases. At one point, the motion of the individual cells, initially associated with a synoptic scale cold front, takes a sharp rightward turn. The mechanism of the motion turn is explained in terms of conditions created by a combination of wind shear and moisture convergence and lifting.

Wind measurement system

Abstract: A system for remotely measuring vertical and horizontal winds present in discrete volumes of air at selected locations above the ground. A laser beam is optically focused in range by a telescope, and the output beam is conically scanned at an angle θ about a vertical axis. The backscatter, or reflected light, from the ambient particulates in a volume of air, the focal volume, is detected for shifts in wavelength, and from these, horizontal and vertical wind components are computed.

Wind Shear and Turbulence Effects on Rotor Fatigue and Loads Control

Abstract: The effects of wind shear and turbulence on rotor fatigue and loads control are explored for a large horizontal axis wind turbine in variable speed operation from 4 to 20 m/s. Two and three blade rigid rotors are considered over a range of wind shear exponents up to 1.25 and a range of turbulence intensities up to 17%. RMS blade root flatwise moments are predicted to be very substantially increased at higher wind shear, and resultant fatigue damage is increased by many orders of magnitude. Smaller but similar trends occur with increasing turbulence levels. In-plane fatigue damage is driven by 1P gravity loads and exacerbated by turbulence level at higher wind speeds. This damage is higher by one to two orders of magnitude at the roots of the three blade rotor. Individual blade pitch control of fluctuating flatwise moments markedly reduces flatwise fatigue damage due to this source, and to a lesser degree the in-plane damage due to turbulence. The same is true of fluctuating rotor torque moments driven by turbulence and transmitted to the drive train. Blade root moments out of the plane of rotation aggregate to create rotor pitching and yawing moments transmitted to the turbine structure through the drive train to the yaw drive system and the tower. These moments are predicted to be relatively insensitive to turbulence level and essentially proportional to the wind shear exponent for the two blade rotor. Fluctuating moments are substantially reduced with individual blade pitch control, and addition of a teeter degree of freedom should further contribute to this end. Fluctuating pitching and yawing moments of the three blade rotor are substantially less sensitive to wind shear, more sensitive to turbulence level, and substantially lower than those for the two blade rotor. Mean rotor torque and hence power are essentially the same for both rotors, independent of wind shear, and somewhat reduced with individual blade pitch control of fluctuating flatwise moments. The same is true of mean rotor thrust, however fluctuations in rotor thrust are substantially reduced with individual blade pitch control. It appears, on balance, that higher wind shear coupled with turbulence effects should be accounted for in the fatigue design of large, long life turbines. Much more work is required on this problem.

A Wave-CISK Model of Squall Lines

Abstract: The wave-CISK model of Raymond incorporating lag effects in the updraft and downdraft is implemented as an initial value problem in physical space. Examples of both midlatitude and tropical squall lines are successfully simulated. Diagnosis of the model shows that condensational heating in the updraft is the primary driving mechanism of a mature squall line, with evaporative cooling and convective momentum transfer playing subsidiary roles. However, an instability involving the displacement of boundary layer air by downdrafts apparently plays an important role in squall line initiation. A weakness of the model is its inability to predict the direction of squall line propagation relative to the low level wind shear. This is traced to the insensitivity of the convective parameterization to midlevel entrainment. However, unlike strict two-dimensional squall lint models, the parameterization allows cross-stream mass transfer to occur in the context of overall slab symmetry. Such transfer is dynamical...

Increased shear in the North Atlantic upper-level jet stream over the past four decades

Abstract: Earth’s equator-to-pole temperature gradient drives westerly mid-latitude jet streams through thermal wind balance1. In the upper atmosphere, anthropogenic climate change is strengthening this meridional temperature gradient by cooling the polar lower stratosphere2,3 and warming the tropical upper troposphere4–6, acting to strengthen the upper-level jet stream7. In contrast, in the lower atmosphere, Arctic amplification of global warming is weakening the meridional temperature gradient8–10, acting to weaken the upper-level jet stream. Therefore, trends in the speed of the upper-level jet stream11–13 represent a closely balanced tug-of-war between two competing effects at different altitudes14. It is possible to isolate one of the competing effects by analysing the vertical shear—the change in wind speed with height—instead of the wind speed, but this approach has not previously been taken. Here we show that, although the zonal wind speed in the North Atlantic polar jet stream at 250 hectopascals has not changed since the start of the observational satellite era in 1979, the vertical shear has increased by 15 per cent (with a range of 11–17 per cent) according to three different reanalysis datasets15–17. We further show that this trend is attributable to the thermal wind response to the enhanced upper-level meridional temperature gradient. Our results indicate that climate change may be having a larger impact on the North Atlantic jet stream than previously thought. The increased vertical shear is consistent with the intensification of shear-driven clear-air turbulence expected from climate change18–20, which will affect aviation in the busy transatlantic flight corridor by creating a more turbulent flying environment for aircraft. We conclude that the effects of climate change and variability on the upper-level jet stream are being partly obscured by the traditional focus on wind speed rather than wind shear. The North Atlantic jet stream has become 15 per cent more sheared in the upper atmosphere since 1979, an expected consequence of climate change, and consistent with increased aircraft turbulence.