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Wind shear

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


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Journal ArticleDOI
TL;DR: In this article, the medium range spread of a cloud of material released in a turbulent wind is considered theoretically, and the effect of systematic wind shear with height is investigated to see whether this may be important.
Abstract: The medium range spread of a cloud of material released in a turbulent wind is considered theoretically. Attention is directed to the effect of systematic wind shear with height to see whether this may be important. It is shown that there are two cases, according to whether the height to which the material may rise is effectively bounded or not. In the first case when the height is bounded, the situation is in essentials the same as for flow in a pipe (Taylor 1953, 1954). After a sufficient length of time, the horizontal spread is described by a diffusivity which is the linear sum of two components, one representing the effect of horizontal turbulent fluctuations of the wind, and the other the effect of an interaction between the systematic wind shear and the vertical transport of material by the turbulence. For given mean wind profile, the latter effect is proportional in an inverse way to the coefficient of vertical diffusion. In the second case, when the material may rise to any height, it is shown that the horizontal spread cannot be described, in general, by a constant diffusivity, and that the horizontal mean-square spread or dispersion does not increase linearly with the time. However, the horizontal fluctuations and the interaction of wind shear with vertical transport still contribute independently to the horizontal dispersion. It is found that the interaction effect now depends in a direct way on the coefficient of vertical diffusion. It is also concluded that for medium-range diffusion the effect of the interaction of wind shear with vertical transport is dominant.

121 citations

Journal ArticleDOI
TL;DR: In this article, a set of two-dimensional simulations of density currents within a dry, statically neutral environment is used to quantify the dependence of lifting along an idealized cold pool on the upper-level shear.
Abstract: Recent observational studies have shown that strong midlatitude mesoscale convective systems (MCSs) tend to decay as they move into environments with less instability and smaller deep-layer vertical wind shear. These observed shear profiles that contain significant upper-level shear are often different from the shear profiles considered to be the most favorable for the maintenance of strong, long-lived convective systems in some past idealized simulations. Thus, to explore the role of upper-level shear in strong MCS environments, a set of two-dimensional (2D) simulations of density currents within a dry, statically neutral environment is used to quantify the dependence of lifting along an idealized cold pool on the upper-level shear. A set of three-dimensional (3D) simulations of MCSs is produced to gauge the effects of the upper-level shear in a more realistic framework. Results from the 2D experiments show that the addition of upper-level shear to a wind profile with weak to moderate low-level shear increases the vertical displacement of parcels despite a decrease in the vertical velocity along the cold pool interface. Parcels that are elevated above the surface (1–2 km) overturn and are responsible for the deep lifting in the deep-shear environments, while the surface-based parcels typically are lifted through the cold pool region in a rearward-sloping path. This deep overturning helps to maintain the leading convection and greatly increases the size and total precipitation output of the convective systems in more complex 3D simulations, even in the presence of 3D structures. These results show that the shear profile throughout the entire troposphere must be considered to gain a more complete understanding of the structure and maintenance of strong midlatitude MCSs.

121 citations

Journal ArticleDOI
TL;DR: In this paper, a 240-year run of the ECHAM4/OPYC3 coupled ocean-atmosphere model with transient greenhouse gas (GHG) forcing according to the IPCC IS92a scenario is examined with respect to sim- ulated changes in boreal winter cyclone activity and 10 m wind speeds over Europe, the North Atlantic and Eastern North America.
Abstract: A 240 yr run of the ECHAM4/OPYC3 coupled ocean-atmosphere model with transient greenhouse gas (GHG) forcing according to the IPCC IS92a scenario is examined with respect to sim- ulated changes in boreal winter cyclone activity and 10 m wind speeds over Europe, the North Atlantic and Eastern North America. It is found that simulated cyclone activity undergoes a pronounced north- and eastward shift over Europe and the Northeast Atlantic. This shift is accompanied by a decrease in the number of weak cyclones and an increase in deep cyclones (with core pressures below 970 hPa) in this area. The cyclone signal corresponds to the changes in storm track activity and upper-tropospheric baroclinicity. Increases of mean wind speeds and of wind speed extremes are identified over Northern Europe and parts of the East Atlantic. The wind signal is due to an increase in wind speed variability and an intensification of the westerly mean current connected with an enhanced mean pressure gradi- ent. It is shown that the rising number of extreme wind events in the GHG simulation is connected to the augmented occurrence of deep cyclones over Northern Europe and the adjacent ocean areas. There are also strong wind speed increases over Hudson Bay and the Greenland Sea. They are restricted to the planetary boundary layer and appear to be connected to the reduction in winter mean sea-ice cover, which leads to locally decreased static stability and —over the Greenland Sea —also to a reduction in surface roughness.

121 citations

Journal ArticleDOI
TL;DR: In this paper, the authors examined the dynamics of entrainment and its effects on the evolution of the dry atmospheric convective boundary layer (CBL) when wind shear is present.
Abstract: The reported study examines the dynamics of entrainment and its effects on the evolution of the dry atmospheric convective boundary layer (CBL) when wind shear is present. The sheared CBL can be studied by means of direct measurements in the atmosphere, laboratory studies, and numerical techniques. The advantages and disadvantages of each technique are discussed in the present paper, which also describes the methodological background for studying the dynamics of entrainment in sheared CBLs. For the reported study, large-eddy simulation (LES) was chosen as the primary method of convective entrainment investigation. Twenty-four LES runs were conducted for CBLs growing under varying conditions of surface buoyancy flux, free-atmospheric stratification, and wind shear. The simulations were divided into three categories: CBL with no mean wind (NS), CBL with a height-constant geostrophic wind of 20 m s−1 (GC), and CBL with geostrophic wind shear (GS). In the simulated cases, the sheared CBLs grew fastes...

121 citations

Journal ArticleDOI
TL;DR: In this article, the entrainment velocities observed over the course of the mission, which took place during July 2001, ranged from 0.12 to 0.72 cm s−1, and appear to outpace the estimated large-scale subsidence as the boundary layer advects over warmer sea surface temperatures.
Abstract: Fast measurements of three scalars, ozone, dimethyl sulfide (DMS), and total water, are used to investigate the entrainment process in the stratocumulus-topped boundary layer (STBL) observed over the eastern subtropical Pacific during the second Dynamics and Chemistry of Marine Stratocumulus Experiment (DYCOMS-II). Direct measurement of the flux profiles by eddy covariance is used to estimate the entrainment velocity, the average rate at which the boundary layer grows diabatically via incorporation of overlying free tropospheric air. The entrainment velocities observed over the course of the mission, which took place during July 2001, ranged from 0.12 to 0.72 cm s−1, and appear to outpace the estimated large-scale subsidence as the boundary layer advects over warmer sea surface temperatures. Observed entrainment velocities display only a weak correlation with the buoyancy Richardson number defined at the inversion, which suggests that processes other than inversion strength, such as wind shear, m...

121 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023155
2022347
2021165
2020157
2019187
2018165