Wind shear

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

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.

The Minimum Wind Speed for Sustainable Turbulence in the Nocturnal Boundary Layer

Abstract: The collapse of turbulence in the nocturnal boundary layer is studied by means of a simple bulk model that describes the basic physical interactions in the surface energy balance. It is shown that for a given mechanical forcing, the amount of turbulent heat that can be transported downward is limited to a certain maximum. In the case of weak winds and clear skies, this maximum can be significantly smaller than the net radiative loss minus soil heat transport. In the case when the surface has low heat capacity, this imbalance generates rapid surface cooling that further suppresses the turbulent heat transport, so that eventually turbulence largely ceases (positive feedback mechanism). The model predicts the minimum wind speed for sustainable turbulence for the so-called crossing level. At this level, some decameters above the surface, the wind is relatively stationary compared to lower and higher levels. The critical speed is predicted in the range of about 5–7 m s21, depending on radiative forcing and surface properties, and is in agreement with observations at Cabauw. The critical value appears not very sensitive to model details or to the exact values of the input parameters. Finally, results are interpreted in terms of external forcings, such as geostrophic wind. As it is generally larger than the speed at crossing height, a 5 m s21 geostrophic wind may be considered as the typical limit below which sustainable, continuous turbulence under clear-sky conditions is unlikely to exist. Below this threshold emergence of the very stable nocturnal boundary layer is anticipated.

Large-Eddy Simulation of a Stratus-Topped Boundary Layer. Part I: Structure and Budgets

Abstract: The structure of a stratus-topped boundary layer is observed through large-eddy simulation which includes the interaction of longwave radiation and turbulence processes. This simulated boundary layer has a relatively warm and dry overlying inversion, a weak surface buoyancy flux, no solar heating, and an insignificant wind shear across the cloud top. The cloud top height and the layer-averaged buoyancy flux inside the cloud layer define a velocity scale appropriate for this of boundary layer. In the cloud layer, buoyancy generates the vertical component of the turbulent kinetic energy, while pressure effect transfer some of this energy into the horizontal components. In the subcloud layer, the only source of the vertical energy other than the surface buoyancy is import from above and the only source of the horizontal energy other than the mean shear is the vertical energy transferred through pressure effects. The profiles of the vertical velocity variance and kinetic energy flux in the stratus-to...

Dependence of the wind profile power law on stability for various locations

Abstract: Recent environmental regulations have increased the need for construction of meteorological towers at power generation facilities. Due to practical and economic considerations, tower heights are usually lower than effluent release heights. At heights where wind speed data are not available, the wind speed is usually estimated from the measured wind speed using the %th wind profile power law and assuming neutral stability conditions. This study examines published data for many locations and shows that the %th wind profile power law is often unrepresentative of actual conditions because the degree of variation of wind speed with height depends greatly on atmospheric stability. The frequency of neutral stability conditions also varies appreciably by site. These two considerations are especially important in dispersion models which extrapolate wind speed at stack height from low level wind speed data.