scholarly journals The aerodynamic drag of the earth’s surface and the value of von Karman’s constant in the lower atmosphere

1947 ◽  
Vol 188 (1013) ◽  
pp. 208-222 ◽  
Keyword(s):  
Velocity Profile ◽  
Aerodynamic Drag ◽  
Atmospheric Stability ◽  
Lower Atmosphere ◽  
Mixing Length ◽  
Horizontal Plate ◽  
Simultaneous Measurements ◽  
Atmospheric Stratification ◽  
Stable Conditions ◽  
Good Agreement

The drag which the earth’s surface exerts on the wind has been measured directly by observing the deflexion of a horizontal plate, floating in oil and under torsional control, and forming part of the earth’s surface. Simultaneous measurements of the velocity profile above the surface have enabled von Karman’s constant for the lower atmosphere to be deduced. The value obtained under moderately unstable atmospheric stratification is 0.46, which is considered to be in good agreement with Nikuradse’s value of 0.40, obtained from the flow of water through pipes under isothermal conditions. Evidence is also adduced for a considerable variation of von Karman’s constant with atmospheric stability. The observations of drag and velocity profile have also been used to determine the variation of mixing length with height. In unstable conditions the mixing length increases more rapidly than the height, while in stable conditions the increase is likely to be less rapid than the height.

Connections between the Ozmidov scale and mean velocity profile in stably stratified atmospheric surface layers

2016 ◽  
Vol 797 ◽  
Author(s):  
Dan Li ◽  
Scott T. Salesky ◽  
Tirtha Banerjee
Keyword(s):  
Velocity Profile ◽  
Atmospheric Stability ◽  
Length Scale ◽  
Surface Layers ◽  
Mean Velocity ◽  
Stable Conditions ◽  
Stringent Constraint ◽  
Ozmidov Scale ◽  
The Mean ◽  
Good Agreement

The mean velocity profile (MVP) in thermally stratified atmospheric surface layers (ASLs) deviates from the classic logarithmic form. A theoretical framework was recently proposed (Katulet al.Phys. Rev. Lett., vol. 107, 2011, 268502) to link the MVP to the spectrum of turbulence and was found to successfully predict the MVP for unstable stratification. However, the theory failed to reproduce the MVP in stable conditions (Saleskyet al.Phys. Fluids, vol. 25, 2013, 105101), especially when${\it\zeta}>0.2$(where${\it\zeta}$is the atmospheric stability parameter). In the present study, it is demonstrated that this shortcoming is due to the failure to identify the appropriate length scale that characterizes the size of momentum transporting eddies in the stable ASL. Beyond${\it\zeta}\approx 0.2$(near where the original theory fails), the Ozmidov length scale becomes smaller than the distance from the wall$z$and hence is a more stringent constraint for characterizing the size of turbulent eddies. An expression is derived to connect the Ozmidov length scale to the normalized MVP (${\it\phi}_{m}$), allowing${\it\phi}_{m}$to be solved numerically. It is found that the revised theory produces a prediction of${\it\phi}_{m}$in good agreement with the widely used empirical Businger–Dyer relation and two experimental datasets in the stable ASL. The results here demonstrate that the behaviour of${\it\phi}_{m}$in the stable ASL is closely linked to the size of momentum transporting eddies, which can be characterized by the Ozmidov scale under mildly to moderately stable conditions ($0.2<{\it\zeta}<1-2$).

The aerodynamic drag of grassland

1950 ◽  
Vol 202 (1068) ◽  
pp. 143-153 ◽  
Keyword(s):  
Aerodynamic Drag ◽  
Atmospheric Stability ◽  
Roughness Parameter ◽  
Concrete Surface ◽  
Lower Atmosphere ◽  
Wind Velocities ◽  
Drag Plate ◽  
Plane Displacement ◽  
Preliminary Application ◽  
Plate Apparatus

An improvised drag-plate apparatus, on the principle of that used by Sheppard (1947) on a concrete surface, but suitably modified in design, has been used for exploratory measurements of the aerodynamic drag of grassland. The grass cover was variable (1 to 15 cm. in height), and measurements were made at a number of positions (not simultaneously) in order to obtain an approximate representative value of the drag over a considerable area. Wind velocities were in the region of 500 cm./sec. and, judged in terms of the Richardson number, effectively adiabatic conditions of flow prevailed. The drag (r 0 ) and the simultaneous vertical distribution of wind velocity (u z ) up to a height of 2 m. were found to be expressible in terms of the law well established in the laboratory, i.e. with k (von Kármán’s constant) = 0.37, d (the zero plane displacement) = 8 cm. and z 0 (the roughness parameter) = 0.66 cm. For reasons which are discussed the drag measurements are regarded as approximate, and the close agreement of the numerical value of k with the laboratory value of 0.4 is probably fortuitous. However, the general consistency achieved in this preliminary application suggests that the technique could be profitably developed for a critical investigation of the relation between drag and wind profile and its dependence on atmospheric stability. In a brief discussion of previous work some evidence is now provided for the validity of the Reynolds formulation of the turbulent shearing stress. Attention is drawn to the application of the present results in treatments of the diffusion of matter in the lower atmosphere.

scholarly journals Turbulence and diffusion in the lower atmosphere

1946 ◽  
Vol 186 (1004) ◽  
pp. 20-35 ◽  
Keyword(s):  
Water Vapour ◽  
Atmospheric Turbulence ◽  
Classical Theory ◽  
Quantitative Approach ◽  
Lower Atmosphere ◽  
Mixing Length ◽  
Self Consistent ◽  
Consistent Treatment ◽  
And Diffusion ◽  
Turbulence And Diffusion

The only existing theory of atmospheric turbulence which is capable of giving a quantitative approach to the complex problems of diffusion in the lower atmosphere is the classical theory in which it is generally assumed that the effect of eddies in the atmosphere is completely analogous to that of molecules in a gas apart from a difference of scale. This assumption, which later evidence has shown to be incorrect, is not essential to the theory, and in the present paper is replaced by the assumption that the mixing length of an eddy increases with both height above and nature of the earth’s surface . With this assumption a self-consistent treatment of diffusion is developed which is able to account quantitatively for such meteorological phenomena as the distribution of water vapour over land and sea (including evaporation from the oceans) and the diffusion of smoke near the ground. The treatment is mainly confined to diffusion in an adiabatic atmosphere.

scholarly journals Wind Estimation in the Lower Atmosphere Using Multirotor Aircraft

2017 ◽  
Vol 34 (5) ◽  
pp. 1183-1191 ◽  
Author(s):  
Ross T. Palomaki ◽  
Nathan T. Rose ◽  
Michael van den Bossche ◽  
Thomas J. Sherman ◽  
Stephan F. J. De Wekker
Keyword(s):  
Wind Speed ◽  
Sonic Anemometer ◽  
Unmanned Aircraft ◽  
Lower Atmosphere ◽  
Promising Alternative ◽  
Atmospheric Structure ◽  
Structure And Dynamics ◽  
Sonic Anemometers ◽  
Vertical Profiling ◽  
Good Agreement

AbstractUnmanned aerial vehicles are increasingly used to study atmospheric structure and dynamics. While much emphasis has been on the development of fixed-wing unmanned aircraft for atmospheric investigations, the use of multirotor aircraft is relatively unexplored, especially for capturing atmospheric winds. The purpose of this article is to demonstrate the efficacy of estimating wind speed and direction with 1) a direct approach using a sonic anemometer mounted on top of a hexacopter and 2) an indirect approach using attitude data from a quadcopter. The data are collected by the multirotor aircraft hovering 10 m above ground adjacent to one or more sonic anemometers. Wind speed and direction show good agreement with sonic anemometer measurements in the initial experiments. Typical errors in wind speed and direction are smaller than 0.5 and 30°, respectively. Multirotor aircraft provide a promising alternative to traditional platforms for vertical profiling in the atmospheric boundary layer, especially in conditions where a tethered balloon system is typically deployed.

scholarly journals Wind turbine power production and annual energy production depend on atmospheric stability and turbulence

2016 ◽  
Author(s):  
Clara M. St. Martin ◽  
Julie K. Lundquist ◽  
Andrew Clifton ◽  
Gregory S. Poulos ◽  
Scott J. Schreck
Keyword(s):  
Energy Production ◽  
Atmospheric Stability ◽  
Power Production ◽  
Department Of Energy ◽  
Power Performance ◽  
Stability Parameters ◽  
Wind Speeds ◽  
Stable Conditions ◽  
Wind Measurements ◽  
Power Curves

Abstract. Using detailed upwind and nacelle-based measurements from a General Electric [GE] 1.5 sle model with a 77 m rotor diameter, we calculated power curves and annual energy production (AEP) and explored their sensitivity to different atmospheric parameters. This work provides guidelines for the use of stability and turbulence filters in segregating power curves to gain a clearer picture of the power performance of a turbine. The wind measurements upwind of the turbine include anemometers mounted on a 135 m meteorological tower and lidar vertical profiles. We calculated power curves for different regimes based on turbulence parameters such as turbulence intensity (TI) and turbulence kinetic energy (TKE), as well as atmospheric stability parameters such as Bulk Richardson number (RB). AEP was also calculated with and without these atmospheric filters and differences between these calculations are highlighted in this article. The power curves for different TI and TKE regimes revealed that, at the U.S. Department of Energy (DOE) National Wind Technology Center (NWTC) at the National Renewable Energy Laboratory (NREL), increased TI and TKE undermined power production at wind speeds near rated, but increased power production at lower wind speeds. Similarly, power curves for different RB regimes revealed that periods of stable conditions produced more power at wind speeds near rated and periods of unstable conditions produced more power at lower wind speeds. AEP results suggest that calculations done without filtering for these atmospheric regimes may be overestimating the AEP. Because of statistically significant differences between power curves and AEP calculated with these turbulence and stability filters for this turbine at this site, we suggest implementing an additional step in analyzing power performance data to take atmospheric stability and turbulence across the rotor disk into account.

scholarly journals A Case Study of Clear-Air Turbulence at Upper Levels

2021 ◽  
Author(s):  
Léo Rogel ◽  
Didier Ricard ◽  
Eric Bazile ◽  
Irina Sandu
Keyword(s):  
Turbulent Flow ◽  
Partial Information ◽  
Weather Prediction ◽  
Turbulent Fluxes ◽  
Small Scale ◽  
Mixing Length ◽  
Vertical Resolution ◽  
Stable Conditions ◽  
Study Results

&lt;p&gt;Because of the technical difficulties of achieving measurements at high altitudes, it is not clear how well turbulent phenomena are represented in the upper levels of current Numerical Weather Prediction (NWP) operational models.&lt;br&gt;Indeed, turbulence in strongly stable conditions near the tropopause is known to be particularly difficult to correctly parameterize. The constraining buoyancy forces on the vertical lead to anisotropic turbulence, potentially inhibiting turbulent production in NWP models.&lt;br&gt;Partial information for high altitude turbulence events is nonetheless available in the form of in-situ measurements from aircrafts. However, it only allows for qualitative comparisons with model outputs.&lt;br&gt;This study focuses on a turbulent episode induced by a winter upper-level jet above east Belgium on January 27, 2018, for which in-situ EDR (Eddy Dissipation Rate) reports indicate moderate-or-greater turbulence levels. Numerical simulations are performed with the M&amp;#233;t&amp;#233;o-France operational model AROME, and with the mesoscale research model MesoNH (Laero/CNRM), at the same horizontal grid resolution (1.3km). These two models also use the eddy-diffusivity turbulence scheme of Cuxart et al (2000), a 1.5 order closure scheme based on a prognostic Turbulent Kinetic Energy (TKE) evolution equation, with a diagnostic computation of the mixing length.&lt;br&gt;TKE budgets, as well as stability indices and gradient-based quantities (Richardson number, vertical wind shear) are computed from the model outputs, and qualitative comparison with in-situ data is presented. Time evolution of the turbulent event over Belgium is well captured by both models, agreeing with EDR data.&lt;br&gt;Several sensitivity tests on the vertical resolution, on the mixing length formulation and on the parameters of the TKE equation are then performed. Most notably, the use of an increased vertical resolution near the tropopause greatly enhances the turbulent fluxes in both operational and research models. Secondly, comparison of various expressions of the mixing length shows that the Bougeault and Lacarrere (1989) formulation produces the higher amount of subgrid TKE and turbulent mixing. A decreased turbulent dissipation parameter also significantly increases the amount of subgrid TKE. On the contrary, the use of a 3D turbulence scheme appears to have very limited impacts on the turbulent flow at this kilometer-scale horizontal resolution.&lt;br&gt;On a second part of this study, results from ongoing Large Eddy Simulations (LES) will be presented. These simulations aim at representing small-scale features of the turbulent flow. They will be used as a reference for the computation of turbulent fluxes at kilometer-scale resolution using a coarse-graining method, allowing for a comparison with the parameterized fluxes from the turbulence scheme. In particular, the dissipation term of the TKE equation will be examined. These results are expected to give insight on the leading turbulent mechanisms for which the current turbulence parameterization can be improved in stable conditions.&lt;/p&gt;

Planetary aerosol electrification: Lessons learned from a terrestrial analogy for Venus

2021 ◽  
Author(s):  
Amethyst Johnson ◽  
Karen Aplin
Keyword(s):  
Atmospheric Stability ◽  
Ionic Mobility ◽  
Mean Free Path ◽  
Lessons Learned ◽  
Lower Atmosphere ◽  
Aerosol Layer ◽  
Electrical Measurements ◽  
Near Surface ◽  
Charged Aerosol ◽  
The Mean

&lt;p&gt;Planetary atmospheric electrification has the potential to damage spacecraft, yet for planets with thick, deep atmospheres such as Venus, the level of electrification remains open to interpretation. Partly due to the difficulty of access and potential hostility to spacecraft, there are limited in-situ observations of deep atmospheres, making terrestrial analogies attractive. One proposed explanation of the observations of near-surface electrification on Venus from sensors on Venera 13 &amp; 14 is a haze of charged aerosol. As the Sahara is an environment with lofted dust that is potentially similar to Venus in terms of atmospheric stability, a simple model was developed estimating a mean aerosol charge based on typical Saharan haze aerosol distributions. Spacecraft surface area and descent speeds were used to estimate the accumulated charge and discharge current measured by the Venera missions, but this model underestimated Venera's electrical measurements by three orders of magnitude. This suggests that an aerosol layer alone cannot explain the charge apparently present in the lower atmosphere of Venus. The simple terrestrial analogy employed may not have been suitable due to the modified pressure and temperature profile affecting the mean free path, ionic mobility and consequently the mean charge. Discrepancies in atmospheric stability and wind patterns must also be evaluated, as the effect of terrestrial wind on aerosol distributions may not be directly applicable to other planets. More detailed calculations of ion-aerosol attachment and re-evaluation of the terrestrial analogy may be able to resolve some these issues, but it looks likely that additional significant sources of charge are required to explain the Venera observations. Triboelectric charging of lofted surface material could exceed charging observed in terrestrial situations, or some unknown atmospheric or non-atmospheric source of charge could have contributed to the Venera electrical measurements.&amp;#160;&lt;/p&gt;

scholarly journals Evolution of a pyrocumulonimbus event associated with an extreme wildfire in Tasmania, Australia

2020 ◽  
Vol 20 (5) ◽  
pp. 1497-1511 ◽  
Author(s):  
Mercy N. Ndalila ◽  
Grant J. Williamson ◽  
Paul Fox-Hughes ◽  
Jason Sharples ◽  
David M. J. S. Bowman
Keyword(s):  
Weather Forecasting ◽  
Fire Severity ◽  
Atmospheric Stability ◽  
Lower Atmosphere ◽  
Fire Weather ◽  
Natural Ecosystems ◽  
Near Surface ◽  
South Eastern ◽  
Crown Defoliation ◽  
Extreme Fire

Abstract. Extreme fires have substantial adverse effects on society and natural ecosystems. Such events can be associated with the intense coupling of fire behaviour with the atmosphere, resulting in extreme fire characteristics such as pyrocumulonimbus cloud (pyroCb) development. Concern that anthropogenic climate change is increasing the occurrence of pyroCbs globally is driving more focused research into these meteorological phenomena. Using 6 min scans from a nearby weather radar, we describe the development of a pyroCb during the afternoon of 4 January 2013 above the Forcett–Dunalley fire in south-eastern Tasmania. We relate storm development to (1) near-surface weather using the McArthur forest fire danger index (FFDI) and the C-Haines index, the latter of which is a measure of the vertical atmospheric stability and dryness, both derived from gridded weather reanalysis for Tasmania (BARRA-TA); and (2) a chronosequence of fire severity derived from remote sensing. We show that the pyroCb rapidly developed over a 24 min period on the afternoon of 4 January, with the cloud top reaching a height of 15 km. The pyroCb was associated with a highly unstable lower atmosphere (C-Haines value of 10–11) and severe–marginally extreme (FFDI 60–75) near-surface fire weather, and it formed over an area of forest that was severely burned (total crown defoliation). We use spatial patterns of elevated fire weather in Tasmania and fire weather during major runs of large wildfires in Tasmania for the period from 2007 to 2016 to geographically and historically contextualise this pyroCb event. Although the Forcett–Dunalley fire is the only known record of a pyroCb in Tasmania, our results show that eastern and south-eastern Tasmania are prone to the conjunction of high FFDI and C-Haines values that have been associated with pyroCb development. Our findings have implications for fire weather forecasting and wildfire management, and they highlight the vulnerability of south-east Tasmania to extreme fire events.

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