Wind shear

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

The slopes of cumulus clouds in relation to external wind shear

Abstract: An equation for the slope of cumulus clouds is derived. It is shown to depend upon the vertical ascent rate, the external wind shear and the horizontal frictional forces between the cloud and its surroundings. These frictional forces are of two origins: one due to entrainment or mixing with the outside air, and the other due to form or body drag. Four observational cases are presented, one concerning trade cumulus, and three involving clouds in mid-latitudes. In two of these, measurement of updraught speed, wind shear, and cloud slope permit an estimate of the magnitude of the friction forces. The minimum contribution due to entrainment is approximately equal to that inferred by Stommel for trade cumulus.

A Theoretical Study of the St. Louis Heat Island: The Wind and Temperature Distribution

Abstract: A three-dimensional primitive equation model was used to study the St. Louis heat island. In this paper, the influence of synoptic wind speed and wind direction on the heat island is presented. With respect to the synoptic wind speed, it was found that the temperature and wind distribution associated with the St. Louis heat island changed markedly as the wind speed increased. When the synoptic wind speed was small, the intensity of the heat island was independent of the wind direction. However, for large synoptic wind speeds, the intensity of the heat island changed, and the change was dependent on the wind direction. These changes were due to the influence of the local topography.

Wave-Coherent Airflow and Critical Layers over Ocean Waves

Abstract: An analysis of coherent measurements of winds and waves from data collected during the Office of Naval Research (ONR) High-Resolution air–sea interaction (HiRes) program, from the Floating Instrument Platform (R/P FLIP), off the coast of northern California in June 2010 is presented. A suite of wind and wave measuring systems was deployed to resolve the modulation of the marine atmospheric boundary layer by waves. Spectral analysis of the data provided the wave-induced components of the wind velocity for various wind–wave conditions. The power spectral density, the amplitude, and the phase (relative to the waves) of these wave-induced components are computed and bin averaged over spectral wave age c/U(z) or c/u*, where c is the linear phase speed of the waves, U(z) is the mean wind speed measured at the height z of the anemometer, and u* is the friction velocity in the air. Results are qualitatively consistent with the critical layer theory of Miles. Across the critical height zc, defined such tha...

Response of a Vertical Axis Wind Turbine to Time Varying Wind Conditions Found within the Urban Environment

Abstract: Experimental testing of a vertical axis wind turbine within the urban environment was used to assess the effects that unsteady wind conditions have on the aerodynamic performance of the turbine. Temporal variation of the wind with respect to the direction and velocity fluctuations was quantified and the corresponding performance of the experimental turbine was compared to the base case wind tunnel performance.Theoretical analysis indicated that the performance of the turbine should depend on wind speed fluctuations, while remaining relatively independent of direction fluctuations. Experimental testing confirmed the independence of the direction fluctuations and quantified the impact of the wind speed fluctuations on the turbine performance. The wind speed fluctuations ( based on a 10 s averaging interval) exhibited minimal impact on the performance for Iv < 0.15, while for greater fluctuations the performance decreased with a roughly linear relationship with increasing Iv.

Correction and downscaling of NWP wind speed forecasts

Abstract: NWP models typically parametrize the effects of unresolved orography, often through use of an effective (orographic) roughness. Whilst this parametrization realistically models the orographic drag on the synoptic-scale flow, it creates two problems for the assimilation of wind observations from high ground. First, the artificially increased surface stress causes a reduction in the predicted wind speed at the standard wind observing height of 10 m, and second, the speed-up over the unresolved summits is not modelled. A method is described for reconciling observed and modelled wind speeds. The method is based on the linear theory of neutral boundary-layer flow over hills and includes a resolution of both the problems described above. The method is applied to both the assimilation of observations and the creation of an improved 10 m wind analysis. The method has been on trial in the Met Office's nowcasting system; significant improvements are demonstrated, particularly during strong wind events. The simplified model presented here is not claimed to represent the full complexities of the boundary layer, but nevertheless produces computationally cheap, low-level wind forecasts, which are a significant improvement on the existing output from the Unified Model. © Crown Copyright 2007. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd.