Influence of mesoscale winds on the turbulent structure of the urban boundary layer over St. Louis
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Keywords:
Urban climatology
Convective Boundary Layer
Thermal wind
Roughness length
Maximum sustained wind
Thermal wind
Log wind profile
Shear velocity
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The longitudinal equations of motion with wind shear terms were used to analyze the stability and motions of a jet transport. A positive wind shear gives a decreasing head wind or changes a head wind into a tail wind. A negative wind shear gives a decreasing tail wind or changes a tail wind into a head wind. It was found that wind shear had very little effect on the short period mode and that negative wind shear, although it affected the phugoid, did not cause stability problems. On the other hand, it was found that positive wind shear can cause the phugoid to become aperiodic and unstable. In this case, a stability boundary for the phugoid was found that is valid for most aircraft at all flight speeds. Calculations of aircraft motions confirmed the results of the stability analysis. It was found that a flight path control automatic pilot and an airspeed control system provide good control in all types of wind shear. Appendixes give equations of motion that include the effects of downdrafts and updrafts and extend the longitudinal equations of motion for shear to six degrees of freedom.
Thermal wind
Longitudinal static stability
Airplane
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Study on the Wind Profiles and Turbulent Characteristics in the Baroclinic Convective Boundary Layer
Abstract Based on the data of STORM‐FEST (STorm‐scale Operational and Research Meteorology‐Front Experiment System Test) carried out near the boundary between Kansas and Nebraska, U. S. in 1992, the vertical distributions of temperature, humidity and wind velocity in the baroclinic convective boundary layer were analysed. The results showed that the temperature and humidity were well mixed in the convective boundary layer. The wind velocity was also well mixed, but there existed wind shear sometime. Under conditions with and without wind shear the turbulent kinetic energy budget was calculated. Finally. The possible reasons of wind shear formation in the convective boundary layer were discussed.
Convective Boundary Layer
Thermal wind
Log wind profile
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A stability wind shear term of logarithmic wind profile based on the terms of turbulent kinetic energy equation is proposed. The fraction influenced by thermal stratification is considered in the shear production term. This thermally affected shear is compared with buoyant term resulting in a stability wind shear term. It is also considered Reynolds stress as a sum of two components associated with wind shear from mechanical and thermal stratification process. The stability wind shear is responsible to Reynolds stress of thermal stratification term, and also to Reynolds stress of mechanical term at no neutral condition. The wind profile and its derivative are validated with data from Pedra do Sal experiment in a flat terrain and 300m from shoreline located in northeast coast of Brazil. It is close to the Equator line, so the meteorological condition are strongly influenced by trade winds and sea breeze. The site has one 100m tower with five instrumented levels, one 3D sonic anemometer, and a medium-range wind lidar profiler up 500m. The dataset are processed and filter from September to November of 2013 which results in about 550 hours of data available. The results show the derivative of wind profile with R^2 of 0.87 and RMSE of 0.08 m/s. The calculated wind profile performances well up to 400m at unstable condition and up to 280m at stable condition with R^2 better than 0.89. The proposed equation is valid for this specific site and is limited to a stead state condition with constant turbulent fluxes in the surface layer.
Log wind profile
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Theprobabilitydensityfunction(pdf)oflandsurfacewindspeedsischaracterizedusingaglobalnetworkof observations. Daytime surface wind speeds are shown to be broadly consistent with the Weibull distribution, while nighttime surface wind speeds are generally more positively skewed than the corresponding Weibull distribution (particularly in summer). In the midlatitudes, these strongly positive skewnesses are shown to be generally associated with conditions of strong surface stability and weak lower-tropospheric wind shear. Long-term tower observations from Cabauw, the Netherlands, and Los Alamos, New Mexico, demonstrate that lower-tropospheric wind speeds become more positively skewed than the corresponding Weibull distribution only in the shallow (;50 m) nocturnal boundary layer. This skewness is associated with two populations of nighttime winds: (i) strongly stably stratified with strong wind shear and (ii) weakly stably or unstably stratified with weak wind shear. Using an idealized two-layer model of the boundary layer momentumbudget,itisshownthattheobservedvariabilityofthedaytimeandnighttimesurfacewindspeedscan be accounted for through a stochastic representation of intermittent turbulent mixing at the nocturnal boundary layer inversion.
Thermal wind
Log wind profile
Surface layer
Middle latitudes
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Abstract Large, rapid, and intermittent changes in wind direction were commonly observed in low–wind speed conditions in the very stable boundary layer during the phase 2 of the Project Sagebrush field tracer study. This paper investigates the occurrence and magnitude of these wind direction changes in the very stable boundary layer and explores their associated meteorological factors. The evidence indicates that these wind direction changes occur mainly at wind speeds of less than 1.5 m s −1 and are associated with momentum and sensible heat fluxes approaching zero in low–wind shear conditions. This results in complete vertical decoupling. They are only weakly dependent on the magnitude of turbulence. The magnitude of the wind direction changes is generally greatest near the surface, because of the greater prevalence of low wind speeds there, and decreases upward.
Log wind profile
Thermal wind
Decoupling (probability)
Wind Stress
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The growth of the atmospheric convective boundary layer (CBL) is forced mostly by buoyancy production at the surface. However, wind shear has a significant impact on the turbulence struc- ture within the CBL and can contribute significantly to CBL growth when mean winds or wind shear in the lower atmosphere are strong and buoyancy flux from the surface and stratification in the free atmos- phere above are both weak. Regimes of CBL growth in calm (shear-free), windy, and strong wind shear cases were studied using large eddy simulation (LES). The study evaluated the effects of shear on the CBL growth rate, the evolution of mean wind profiles within the CBL, and semi-organized and turbulent flow structures.
Convective Boundary Layer
Thermal wind
Stratification (seeds)
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Based on the data of STORM-FEST (STorm-scale Operational and Research Meteorology-Front Experiment System Test) carried out near the boundary of Kansas and Nebraska, U. S. in 1992, the vertical distribution of temperature, humidity and wind velocity in the baroclinic convective boundary layer was analysed. The results show that the temperature and humidity were well mixed in the convective boundary layer. The wind velocity was also well mixed, but there exists wind shear sometime. Under conditions with and without wind shear the turbulent kinetic energy budget was calculated. The possible reasons of wind shear formation in the convective boundary layer were discussed.
Convective Boundary Layer
Thermal wind
Convective storm detection
Log wind profile
Potential temperature
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As wind shear increases, the quasi-two-dimensional structure of flows becomes more significant in the convective boundary layer (CBL), indicating that wind shear plays an essential role in the variation of the field of atmospheric flow. Therefore, sensitive numerical experiments based on Large Eddy Simulation (LES) techniques were conducted to comprehensively investigate the effects of wind shear on the spatial variations in the velocity and potential temperature (θ) horizontal fields. Under the constant surface heat flux condition, the main findings are summarized. Firstly, in the CBL, the variances of the streamwise velocity (u), cross-stream velocity (v), and θ enhance as wind shear increases, whereas the variance of vertical velocity (w) is insensitive to wind shear. Secondly, in the CBL, with increasing wind shear, low-wavenumber Power Spectrum Densities (PSDs) of u, v, w, and θ increase significantly, suggesting that the increasing wind shear always enhances the large-scale motions of the atmosphere (i.e., low-wavenumber PSD). Therefore, it is more likely that some mesoscale weather processes will be triggered. Thirdly, generally, in the high-wavenumber range, with increasing wind shear, the PSDs of u, v, and θ increase slightly, whereas the PSD of w decreases slightly. This study provides a new perspective for understanding the role of wind shear in the spatial variations of the horizontal fields of meteorological elements under the same conditions of surface heat flux.
Convective Boundary Layer
Shear velocity
Wavenumber
Thermal wind
Log wind profile
Large-Eddy Simulation
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Combined LiDAR/cup anemometer observations performed in the summer of 2006 of wind speed profiles up to 161 m have been analyzed within an open sea sector at the Horns Rev offshore wind farm. The influence of atmospheric stability on the surface layer wind shear is studied by using a bulk formulation of the Richardson number to derive the Obukhov length from 10 minutes mean temperature and wind speed measurements. The influence of the boundary layer height on the wind speed profile gives a strong over-prediction of the wind speed in stable atmospheric conditions. A length scale model is suggested where the boundary layer height is taken into account. The resulting wind profile agrees well compared to the combined LiDAR/mast profiles in and beyond surface layer.
Log wind profile
Maximum sustained wind
Anemometer
Thermal wind
Atmospheric instability
Surface layer
Mast (botany)
Wind Stress
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