scholarly journals Generation of free convection due to changes of the local circulation system

2009 ◽  
Vol 9 (21) ◽  
pp. 8587-8600 ◽  
Author(s):  
R. Eigenmann ◽  
S. Metzger ◽  
T. Foken
Keyword(s):  
Wind Speed ◽  
Free Convection ◽  
Large Scale ◽  
Sonic Anemometer ◽  
Measurement Data ◽  
Surface Fluxes ◽  
Circulation System ◽  
Local Circulation ◽  
Valley Wind ◽  
Obukhov Length

Abstract. Eddy-covariance and Sodar/RASS experimental measurement data of the COPS (Convective and Orographically-induced Precipitation Study) field campaign 2007 are used to investigate the generation of near-ground free convection conditions (FCCs) in the Kinzig valley, Black Forest, Southwest Germany. The measured high-quality turbulent flux data revealed that FCCs are initiated near the ground in situations where moderate to high buoyancy fluxes and a simultaneously occurring drop of the wind speed were present. The minimum in wind speed – observable by the Sodar measurements through the whole vertical extension of the valley atmosphere – is the consequence of a thermally-induced valley wind system, which changes its wind direction from down to up-valley winds in the morning hours. Buoyancy then dominates over shear within the production of turbulence kinetic energy near the ground. These situations are detected by the stability parameter (ratio of the measurement height to the Obukhov length) when the level of free convection, which starts above the Obukhov length, drops below that of the sonic anemometer. An analysis of the scales of turbulent motions during FCCs using wavelet transform shows the occurrence of large-scale turbulence structures. Regarding the entire COPS measurement period, FCCs in the morning hours occur on about 50% of all days. Enhanced surface fluxes of latent and sensible heat are found on these days.

scholarly journals Generation of free convection due to changes of the local circulation system

2009 ◽  
Vol 9 (3) ◽  
pp. 11367-11411 ◽  
Author(s):  
R. Eigenmann ◽  
S. Metzger ◽  
T. Foken
Keyword(s):  
Wind Speed ◽  
Free Convection ◽  
Measurement Data ◽  
Circulation System ◽  
Local Circulation ◽  
Valley Wind ◽  
Thermally Induced ◽  
Obukhov Length ◽  
Vertical Extension ◽  
Parameter Ratio

Abstract. Eddy-covariance and Sodar/RASS experimental measurement data of the COPS (Convective and Orographically-induced Precipitation Study) field campaign 2007 are used to investigate the generation of near-ground free convection events in the Kinzig valley, Black Forest, Southwest Germany. The measured high-quality turbulent flux data revealed free convection to be induced in situations where high buoyancy fluxes and a simultaneously occurring wind speed collapse were present. The minimum in wind speed – observable by the Sodar measurements through the whole vertical extension of the valley atmosphere – is the consequence of a thermally-induced valley wind system, which changes its wind direction from down to up-valley winds in the morning hours. Buoyant forces then dominate over shear forces within turbulence production. These situations are detected by the stability parameter (ratio of the measurement height to the Obukhov length) calculated from directly measured turbulent fluxes. An analysis of the scales of turbulent motions during the free convection event using wavelet transform confirms the large-eddy scale character of the detected plume-like coherent structures. Regarding the entire COPS measurement period, free convection events (FCEs) in the morning hours occur on about 50% of all days.

Coherence of Fluctuating Wind Speed and Along-Wind Force Power Spectrum of Tall Building

2014 ◽  
Vol 1025-1026 ◽  
pp. 922-925
Author(s):  
Yong Chul Kim ◽  
Sung Won Yoon
Keyword(s):  
Wind Speed ◽  
Power Spectrum ◽  
Large Scale ◽  
Measurement Data ◽  
Power Spectra ◽  
Tall Building ◽  
Theoretical Formula ◽  
Mathematical Treatment ◽  
Wind Force ◽  
Fluctuating Wind

In evaluating wind load effects on large-scale structures, correlations in the frequency domain (i.e., coherences) and power spectra of fluctuating wind speed should be evaluated in advance. Most existing formulas for coherence are expressed as exponential functions based on field measurement data for ease of mathematical treatment. However, these simple mathematical expressions have many limitations. In the present study, after examining the existing coherence formulas, a semi-theoretical formula was proposed, and the corresponding along-wind force power spectrum of a tall building with a square cross-section was numerically calculated. A comparison showed that both the coherence and along-wind power spectrum were in good agreement with those of actual wind tunnel data.

Fallacies of the Enthalpy Transfer Coefficient over the Ocean in High Winds

2011 ◽  
Vol 68 (7) ◽  
pp. 1435-1445 ◽  
Author(s):  
Edgar L Andreas
Keyword(s):  
Wind Speed ◽  
Latent Heat ◽  
Large Scale ◽  
Scaling Laws ◽  
Surface Fluxes ◽  
Neutral Stability ◽  
Transfer Coefficients ◽  
Wide Range ◽  
Interfacial Transfer ◽  
High Winds

Abstract Mesoscale and large-scale atmospheric models use a bulk surface flux algorithm to compute the turbulent flux boundary conditions at the bottom of the atmosphere from modeled mean meteorological quantities such as wind speed, temperature, and humidity. This study, on the other hand, uses a state-of-the-art bulk air–sea flux algorithm in stand-alone mode to compute the surface fluxes of momentum, sensible and latent heat, and enthalpy for a wide range of typical (though randomly generated) meteorological conditions over the open ocean. The flux algorithm treats both interfacial transfer (controlled by molecular processes right at the air–sea interface) and transfer mediated by sea spray. Because these two transfer routes obey different scaling laws, neutral-stability, 10-m transfer coefficients for enthalpy CKN10, latent heat CEN10, and sensible heat CHN10 are quite varied when calculated from the artificial flux data under the assumption of only interfacial transfer—the assumption in almost all analyses of measured air–sea fluxes. That variability increases with wind speed because of increasing spray-mediated transfer and also depends on surface temperature and atmospheric stratification. The analysis thereby reveals as fallacious several assumptions that are common in air–sea interaction research—especially in high winds. For instance, CKN10, CEN10, and CHN10 are not constants; they are not even single-valued functions of wind speed, nor must they increase monotonically with wind speed if spray-mediated transfer is important. Moreover, the ratio CKN10/CDN10, where CDN10 is the neutral-stability, 10-m drag coefficient, does not need to be greater than 0.75 at all wind speeds, as many have inferred from Emanuel’s seminal paper in this journal. Data from the literature and from the Coupled Boundary Layers and Air–Sea Transfer (CBLAST) hurricane experiment tend to corroborate these results.

scholarly journals Turbulent surface fluxes and air–ice coupling in the Baltic Air–Sea–Ice Study (BASIS)

2001 ◽  
Vol 33 ◽  
pp. 237-242 ◽  
Author(s):  
Jouko Launiainen ◽  
Bin Cheng ◽  
Juha Uotila ◽  
Timo Vihma
Keyword(s):  
Wind Speed ◽  
Sea Ice ◽  
Drag Coefficient ◽  
Gradient Method ◽  
Sonic Anemometer ◽  
Mean Value ◽  
Surface Fluxes ◽  
Eddy Flux ◽  
Aerodynamic Roughness ◽  
The Baltic

AbstractTurbulent surface fluxes were studied using observations taken over sea ice in the Baltic Sea in March 1998. The fluxes of momentum and sensible heat were measured by a sonic anemometer and compared with fluxes derived from wind velocity and air-temperature profiles. The neutral 10 m drag coefficient showed no apparent dependence on wind speed (in the range 2–20 m s–1), resulting in a mean value of 1.0 × 10–3 for smooth snow-covered ice and 1.5 × 10−3 for deformed ice. The overall mean value was 1.28 × 10–3. The roughness length for temperature revealed a greater apparent dependence on wind speed and was slightly larger than the aerodynamic roughness for low wind speeds, and vice versa for moderate and high winds. We give an empirical expression that predicts how the scalar roughness depends on the aerodynamic roughness (drag coefficient) and wind speed. Agreement of the gradient-method results with the eddy-flux results supports the validity of the Monin-Obukhov similarity theory. Fluxes modelled by a coupled air-ice-sea model compared well with the eddy-flux and gradient methods. Surface temperature estimates by the three methods also agreed well. Tests and sensitivity analysis emphasize the need for especially accurate sensor calibration and strict information about the sensor heights for the gradient method.

scholarly journals Monin–Obukhov Similarity Functions for the Structure Parameters of Temperature and Humidity in the Unstable Surface Layer: Results from High-Resolution Large-Eddy Simulations

2014 ◽  
Vol 71 (2) ◽  
pp. 716-733 ◽  
Author(s):  
Björn Maronga
Keyword(s):  
Surface Layer ◽  
Free Convection ◽  
Measurement Data ◽  
Surface Fluxes ◽  
Large Eddy Simulations ◽  
Structure Parameters ◽  
Similarity Functions ◽  
Dry Air ◽  
Large Eddy ◽  
Temperature And Humidity

Abstract Large-eddy simulations (LESs) of free-convective to near-neutral boundary layers are used to investigate the surface-layer turbulence. The article focuses on the Monin–Obukhov similarity theory (MOST) relationships that relate the structure parameters of temperature and humidity to the surface fluxes of sensible and latent heat, respectively. Moreover, the applicability of local free convection (LFC) similarity scaling is studied. The LES data suggest that the MOST function for is universal. It is shown to be within the range of the functions proposed from measurement data. It is found that follows MOST if entrainment of dry air from the free atmosphere is sufficiently small. In this case the similarity functions for and are identical. If entrainment is significant, dissimilarity between the transport of sensible heat and moisture is observed and no longer follows MOST. In the free-convection limit the LFC similarity functions should collapse to universal constants. The LES data suggest values around 2.7, which is in agreement with the value proposed in the literature. As for MOST, the LFC similarity constant for becomes nonuniversal if entrainment of dry air is significant. It is shown that LFC scaling is applicable even if shear production of turbulence is moderately high.

scholarly journals Stepwise Onset of Monsoon Weather Observed in the Nepal Himalaya

2008 ◽  
Vol 136 (7) ◽  
pp. 2507-2522 ◽  
Author(s):  
K. Ueno ◽  
K. Toyotsu ◽  
L. Bertolani ◽  
G. Tartari
Keyword(s):  
Large Scale ◽  
Time Lag ◽  
Local Circulation ◽  
Valley Wind ◽  
Nepal Himalaya ◽  
The North ◽  
Weather Changes ◽  
Mountain Weather ◽  
Onset Of Monsoon

Abstract Mountain weather changes in the Nepal Himalaya were intensively examined during the 2003 monsoon onset using in situ datasets, observed by multiple automatic weather stations (AWS) distributed across the Coordinated Enhanced Observing Period (CEOP) Himalaya reference site at locations with significant differences in altitude. Onset of monsoon rainfall characterized as nighttime precipitation was initiated simultaneously at all stations with the occurrence and migration of a monsoon depression in the north of the Bay of Bengal. Satellite infrared images detected evident suppression of diurnal cloud development after the onset. About two weeks prior to the onset, the mountain weather abruptly changed such that the daytime valley wind at lower elevations weakened associated with morning clouds and the nighttime southerly wind began at upper elevations. The timing corresponded with the weakening of the westerly wind over the Himalaya due to the northward shift of the upper subtropical jet stream. The time lag between the precipitation onset and the change in the mountain weather was confirmed by 9 yr of in situ AWS data. The mechanism of nighttime southerly winds at high elevations is also discussed in relation to large-scale monsoon flow and local circulation.

Surface wind speed probability distribution in the Southeast Pacific of Marine Stratus and Stratocumulus regions

2009 ◽  
Vol 1 (4) ◽  
Author(s):  
Yanping He
Keyword(s):  
Wind Speed ◽  
Probability Distribution ◽  
Large Scale ◽  
Surface Wind ◽  
Ocean Surface ◽  
Surface Wind Speed ◽  
Surface Fluxes ◽  
Offshore Wind ◽  
Marine Stratus ◽  
Southeast Pacific

AbstractProbability distributions of surface wind speeds (SWS) near coastal regions are needed for applications such as estimating offshore wind power and ocean surface fluxes and for offshore wind risk assessments. Ocean surface wind speed probability distribution (PDF) is characterized using three-year QuikSCAT and AIRS satellite observations in the southeast Pacific of marine stratus and stratocumulus (MSC) regions. Seasonal variation is removed from wind statistics. It was found that the observed SWS standard deviation has a linear positive relationship with its mean SWS; while the SWS skewness decreases with mean SWS in regimes of strong winds and increases with mean SWS in regimes of weak winds. A simple 1D conceptual model is developed near the Peruvian region, which successfully reproduces the observed relationship between higher moments of SWS and its mean value. The model based physical picture among ocean surface winds, SST, and marine boundary clouds are supported by three-year QuikSCAT surface wind observations and fifteen-year ERA40 re-analysis data. Model sensitive tests suggest that large-scale divergence, and strengths of momentum and cloud fluctuations have significant effects on the ocean SWS-PDF in marine stratus and stratocumulus regions.

Soil/Tire Interaction Analysis Using FEM and FVM

2005 ◽  
Vol 33 (1) ◽  
pp. 38-62 ◽  
Author(s):  
S. Oida ◽  
E. Seta ◽  
H. Heguri ◽  
K. Kato
Keyword(s):  
Large Scale ◽  
Interaction Analysis ◽  
Yield Criterion ◽  
Surrounding Medium ◽  
Flow Analysis ◽  
Measurement Data ◽  
Traction Force ◽  
Elastoplastic Material ◽  
The Road ◽  
Soil Flow

Abstract Vehicles, such as an agricultural tractor, construction vehicle, mobile machinery, and 4-wheel drive vehicle, are often operated on unpaved ground. In many cases, the ground is deformable; therefore, the deformation should be taken into consideration in order to assess the off-the-road performance of a tire. Recent progress in computational mechanics enabled us to simulate the large scale coupling problem, in which the deformation of tire structure and of surrounding medium can be interactively considered. Using this technology, hydroplaning phenomena and tire traction on snow have been predicted. In this paper, the simulation methodology of tire/soil coupling problems is developed for pneumatic tires of arbitrary tread patterns. The Finite Element Method (FEM) and the Finite Volume Method (FVM) are used for structural and for soil-flow analysis, respectively. The soil is modeled as an elastoplastic material with a specified yield criterion and a nonlinear elasticity. The material constants are referred to measurement data, so that the cone penetration resistance and the shear resistance are represented. Finally, the traction force of the tire in a cultivated field is predicted, and a good correlation with experiments is obtained.

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