The high frequency of neutral stability occurrences as determined by two practical atmospheric stability classification methods for an urban area

1989 ◽  
Vol 48 (4) ◽  
pp. 423-429
Author(s):  
Whei-May Lee ◽  
Yen-Lun Lo
Keyword(s):  
Urban Area ◽  
High Frequency ◽  
Atmospheric Stability ◽  
Neutral Stability ◽  
Classification Methods ◽  
Stability Classification

scholarly journals Evaluation of the Power-Law Wind-Speed Extrapolation Method with Atmospheric Stability Classification Methods for Flows over Different Terrain Types

2018 ◽  
Vol 8 (9) ◽  
pp. 1429 ◽  
Author(s):  
Chang Xu ◽  
Chenyan Hao ◽  
Linmin Li ◽  
Xingxing Han ◽  
Feifei Xue ◽  
...  
Keyword(s):  
Wind Speed ◽  
Power Law ◽  
Atmospheric Stability ◽  
Extrapolation Method ◽  
Speed Ratio ◽  
Stability Conditions ◽  
Classification Methods ◽  
Average Value ◽  
Different Types ◽  
Stability Classification

The atmospheric stability and ground topography play an important role in shaping wind-speed profiles. However, the commonly used power-law wind-speed extrapolation method is usually applied, ignoring atmospheric stability effects. In the present work, a new power-law wind-speed extrapolation method based on atmospheric stability classification is proposed and evaluated for flows over different types of terrain. The method uses the wind shear exponent estimated in different stability conditions rather than its average value in all stability conditions. Four stability classification methods, namely the Richardson Gradient (RG) method, the Wind Direction Standard Deviation (WDSD) method, the Wind Speed Ratio (WSR) method and the Monin–Obukhov (MO) method are applied in the wind speed extrapolation method for three different types of terrain. Tapplicability is analyzed by comparing the errors between the measured data and the calculated results at the hub height. It is indicated that the WSR classification method is effective for all the terrains investigated while the WDSD method is more suitable in plain areas. Moreover, the RG and MO methods perform better in complex terrains than the other methods, if two-level temperature data are available.

scholarly journals Comparison of the Causes of High-Frequency Heavy and Light Snowfall on Interannual Timescales over Northeast China

Atmosphere ◽  
2020 ◽  
Vol 11 (9) ◽  
pp. 936
Author(s):  
Lushan Wang ◽  
Ke Fan ◽  
Zhiqing Xu
Keyword(s):  
Water Vapor ◽  
High Frequency ◽  
Northeast China ◽  
Kuroshio Extension ◽  
Atmospheric Stability ◽  
Northern Eurasia ◽  
Heavy Snowfall ◽  
Air Temperatures ◽  
The Waves ◽  
Water Vapor Budget

This study investigates and compares the reasons for high-frequency heavy and light snowfall in winter on interannual timescales over northeast China (NEC) during 1961–2017. Results indicate that the frequency and its variability are strong over southeastern NEC for heavy snowfall but over northern NEC for light snowfall. Analysis of the annual cycle shows that the maximum frequency of heavy snowfall occurs in November and March due to more warm–wet air masses and increased atmospheric instability, and that of light snowfall occurs in December–January due to drier conditions and increased atmospheric stability. The frequency of heavy snowfall exhibits an increasing trend which partly results from the warming trend in NEC, while that of light snowfall shows a decreasing trend. High-frequency heavy snowfall is associated with a positive North Atlantic Oscillation (NAO), warmer regional air temperatures, an increased water vapor budget associated with an anomalous anticyclone occupying the Kuril Islands, and relatively unstable atmospheric layers. High-frequency light snowfall is associated with a strengthened East Asian winter monsoon, colder regional air temperatures, a decreased water vapor budget, and relatively stable atmospheric layers. High-frequency heavy and light snowfall are both related to eastward-propagating quasi-stationary waves over Eurasia, but with different features. The waves of the former are located in midlatitude Eurasia and related to the positive phase of the NAO. The waves of the latter exhibit two pathways, located in midlatitude and northern Eurasia, respectively. The northern one can be partially attributed to a weak polar vortex. In addition, higher sea surface temperatures of the Kuroshio Extension may contribute to high-frequency heavy snowfall.

The atmospheric turbulence characteristics in a diurnal cycle

2020 ◽  
Vol 34 (14n16) ◽  
pp. 2040109
Author(s):  
Yi-Lei Song ◽  
Lin-Lin Tian ◽  
Ning Zhao
Keyword(s):  
Wind Speed ◽  
Diurnal Cycle ◽  
Wind Shear ◽  
Atmospheric Stability ◽  
Atmospheric Flow ◽  
Eddy Simulation ◽  
Mean Wind Speed ◽  
Engineering Models ◽  
Wind Characteristics ◽  
Stability Classification

During a whole-day period, profiles of mean wind speed, wind shear and turbulence level shows great variability due to continuously varying atmospheric stability. Clearly understanding the spatial and temporal behaviors of the atmospheric wind flow is of great importance for science purposes. Large-eddy simulation (LES) technique is employed here to reproduce the evolution of atmospheric flow during a diurnal cycle. With the obtained LES results, wind characteristics in terms of wind speed, wind shear, turbulence intensity and turbulent kinetic energy can be examined referring to the stability classification. Besides, wind profiles obtained using currently available engineering models are also included for comparison. Disparities between the model predictions and the LES results illustrate that the standard engineering models cannot well capture the wind characteristics driven by the varying atmospheric stability solely, and a further improvement in models is highly needed.

The Change of Aerodynamic Parameters over Beijing Urban Area during 1991-2018

2020 ◽  
Author(s):  
Xiaoman Liu
Keyword(s):  
Wind Speed ◽  
Urban Area ◽  
Wind Direction ◽  
Wind Field ◽  
Atmospheric Stability ◽  
Meteorological Condition ◽  
Field Structure ◽  
Average Height ◽  
Growth Trend ◽  
Aerodynamic Parameters

<p>       Higher and denser building groups are the most concentrated reflection of urbanization on the underlying surface reconstruction. With the continuous city expanding, urban wind field structure was changed, also the aerodynamic parameters dependent on. Based on observational data (slow-response) collected at 15 levels on Beijing 325m meteorological tower from 1991-2018, time and vertical trends of atmospheric stability, wind direction, wind speed, aerodynamic parameters were analyzed. Through Sen's slope, Mann-Kendall trend test and mutation analysis, we believe that urbanization has made a significant influence on local meteorological condition, and all the above variables mutated around the year of 1999. Before 1999, the proportion of neutral and unstable conditions declined with a trend of -0.63% and -2.0% per year respectively, and increased with a trend of +0.08% and +0.06% per year after 1999. As for wind direction, the dominant wind direction below 47m turned from southwest/northwest before 1999 to southeast after 1999, while above 47m remain unchanged as southeast, reflecting that the action range of urban impact is clearly distinguished from that of atmospheric background field. In terms of wind speed, the annual mean value trended to decrease at -0.0019m/s per year, and vertical wind speed trended to increased with height (per meter) at m/s per year, which reflected the continuous enhancement of attenuation effect of complex underlying on the near-ground wind speed. Furthermore, we found that although there was indeed a weaken tendency for wind speed in Beijing urban areas, but near neutral wind speed maintained a growth trend under 140m during 1999-2018. It was possible the deal with urban wake effect, wind field structure mutation or turbulence effect. Aerodynamic parameters  and d have undergone significant changes during the peak stage of urbanization, and tended to develop steadily with a 7-years fluctuations trend after that. In the past 28 years, d has increased from 1.34m in 1991 to 26.19m in 2018, while  has decreased from 2.75m to 1.02m. This is due to the fact that the increase of buildings average height is the result of roughness superposition. If the 7-year fluctuations trend continues, d of Beijing urban area will soon enter the next uplift period, during which the wind speed may increase slightly under nearly neutral conditions, and the cleaning effect on the pollution may be gradually enhanced.</p><p> </p>

The Onset of Double-Diffusive Convection in a Two-Layer System With a Viscoelastic Fluid-Saturated Porous Medium Under High-Frequency Vibration

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
Moli Zhao ◽  
Huan Zhao ◽  
Shaowei Wang ◽  
Chen Yin
Keyword(s):  
Viscoelastic Fluid ◽  
High Frequency ◽  
Physical Parameters ◽  
Neutral Stability ◽  
Double Diffusive Convection ◽  
Layer System ◽  
High Frequency Vibration ◽  
Diffusive Convection ◽  
Wave Numbers ◽  
Double Diffusive

Abstract The effect of high frequency vibration in the gravity field on the double-diffusive convection in a two-layer system with a viscoelastic fluid-saturated porous layer is studied. The averaging method is employed to split the unknown functions into a periodic rapidly varying part and a slower mean part. Then, the governing equation of perturbations is numerically solved by the Chebyshev tau method and QZ decomposition method. The influence of physical parameters on the stability of system is investigated. It is shown that the neutral stability curves are bimodal under high frequency vibration. The parameter of the high frequency vibration mainly stabilizes the pure fluid layer for greater wave numbers and has a weak impact on the whole system for smaller wave numbers.

Evaluation of the AERMOD dispersion model as a function of atmospheric stability for an urban area

2006 ◽  
Vol 25 (2) ◽  
pp. 141-151 ◽  
Author(s):  
Ashok Kumar ◽  
Shobhit Dixit ◽  
Charanya Varadarajan ◽  
Abhilash Vijayan ◽  
Anand Masuraha
Keyword(s):  
Urban Area ◽  
Dispersion Model ◽  
Atmospheric Stability

scholarly journals Consequences of Urban Stability Conditions for Computational Fluid Dynamics Simulations of Urban Dispersion

2007 ◽  
Vol 46 (7) ◽  
pp. 1080-1097 ◽  
Author(s):  
Julie K. Lundquist ◽  
Stevens T. Chan
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Urban Area ◽  
Eddy Viscosity ◽  
Neutral Stability ◽  
Wake Region ◽  
Turbulence Parameterization ◽  
Computational Fluid Dynamics Simulations ◽  
Dynamics Simulations ◽  
Shear Production

Abstract The validity of omitting stability considerations when simulating transport and dispersion in the urban environment is explored using observations from the Joint Urban 2003 field experiment and computational fluid dynamics simulations of that experiment. Four releases of sulfur hexafluoride, during two daytime and two nighttime intensive observing periods (IOPs), are simulated using the building-resolving computational fluid dynamics model called the Finite Element Model in 3-Dimensions and Massively Parallelized (FEM3MP) to solve the Reynolds-averaged Navier–Stokes equations with two options of turbulence parameterizations. One option omits stability effects but has a superior turbulence parameterization using a nonlinear eddy viscosity (NEV) approach, and the other considers buoyancy effects with a simple linear eddy viscosity approach for turbulence parameterization. Model performance metrics are calculated by comparison with observed winds and tracer data in the downtown area and with observed winds and turbulence kinetic energy (TKE) profiles at a location immediately downwind of the central business district in the area labeled as the urban shadow. Model predictions of winds, concentrations, profiles of wind speed, wind direction, and friction velocity are generally consistent with and compare reasonably well to the field observations. Simulations using the NEV turbulence parameterization generally exhibit better agreement with observations. To explore further the assumption of a neutrally stable atmosphere within the urban area, TKE budget profiles slightly downwind of the urban wake region in the urban shadow are examined. Dissipation and shear production are the largest terms that may be calculated directly. The advection of TKE is calculated as a residual; as would be expected downwind of an urban area, the advection of TKE produced within the urban area is a very large term. Buoyancy effects may be neglected in favor of advection, shear production, and dissipation. For three of the IOPs, buoyancy production may be neglected entirely; for one IOP, buoyancy production contributes approximately 25% of the total TKE at this location. For both nighttime releases, the contribution of buoyancy to the total TKE budget is always negligible though positive. Results from the simulations provide estimates of the average TKE values in the upwind, downtown, downtown shadow, and urban wake zones of the computational domain. These values suggest that building-induced turbulence can cause the average turbulence intensity in the urban area to increase by as much as 7 times average upwind values, explaining the minimal role of buoyant forcing in the downtown region. The downtown shadow exhibits an exponential decay in average TKE, whereas the distant downwind wake region approaches the average upwind values. For long-duration releases in downtown and downtown shadow areas, the assumption of neutral stability is valid because building-induced turbulence dominates the budget. However, farther downwind in the urban wake region, which is found to be approximately 1500 m beyond the perimeter of downtown Oklahoma City, Oklahoma, the levels of building-induced turbulence greatly subside, and therefore the assumption of neutral stability is less valid.

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