Deviation Of Vertical Velocity Fluctuations Research Articles

Avaliação dos parâmetros de turbulência em fluxos de metano para uma área de arroz irrigado no sul do Brasil

Methane (CH4) is one of the main greenhouse gases. An important source of CH4 emission is flooded rice paddy. In this paper, the Eddy Covariance methodology was used to estimate the surface fluxes of CH4 in a flooded rice paddy in the Southern Brazil (lat: -30.2771 N; long: -53.1479 W; altitude - 40.5 m). In the event of low turbulence, this method can generate unrealistic flux data. In this situation, it is recommended remove the flux of the dataset. In general, the friction velocity (u*) is used to remove CH4 flux in low turbulence situations. However, u* is a flux and the use of the standard deviation of vertical velocity fluctuations (σw) can be an alternative. The objective of this work is to analyze the behavior of CH4 flux with friction velocity (u*), with the standard deviation of vertical velocity fluctuations (σw), and with atmospheric and soil variables in a flooded rice paddy.

Observed winds, turbulence, and dispersion in built-up downtown areas of Oklahoma City and Manhattan

Wind and tracer data from the Oklahoma City Joint Urban 2003 (JU2003) and the Manhattan Madison Square Garden 2005 (MSG05) urban field experiments are being analyzed to aid in understanding air flow and dispersion near street-level in built-up downtown areas. The mean winds are separately calculated for groups of anemometers having similar exposures such as “near street level” and “on building top”. Several general results are found, such as the scalar wind speed at street level is about 1/3 of that at building top. Turbulent standard deviations of wind speed components and temperature, and vertical fluxes of momentum and sensible heat, are calculated from sonic anemometers near street level at 20 locations in JU2003 and five locations in MSG05, and from two rooftop locations in MSG05. The turbulence observations are consistent with observations in the literature at other cities, although the JU2003 and MSG05 data are unique in that many data are available near street level. For example, it is found that the local (i.e., at the measuring height) \(\sigma_{w}/u_{\ast}\) averages about 1.5 and the local \(u_{\ast}/u\) averages about 0.25 in the two cities, where \(\sigma_{w}\) is the standard deviation of vertical velocity fluctuations, \(u_{\ast}\) is the friction velocity, and u is the wind speed. The ratio of temperature fluctuations to temperature scale, \(\sigma_{T}/T_{\ast}\) , averages about −3 in both cities, consistent with similarity theory for slightly unstable conditions, where \(\sigma_{T}\) is the standard deviation of temperature fluctuations, and \(T_{\ast}\) is the temperature scale. The calculated Obukhov length, L, is also consistent with slightly unstable conditions near street level, even at night during JU2003. The SF6 tracer concentration observations from JU2003 are analyzed. Values of \(uC_{max}/Q\) for the continuous releases are calculated for each release and arc distance, where \(C_{max}\) is the 30-min average arc maximum concentration, Q is the continuous source emission rate, and u is the spatial-averaged wind speed in the downtown area. The basic characteristics of the JU2003 plot of averaged \(uC_{max}/Q\) agree reasonably well with similar plots for other urban experiments in Salt Lake City and London (i.e., at \(x < 1000 m, C_{max}/Q = Ax^{-2})\) . A is found to be about 3 during the day and about 10 during the night.

A large-eddy simulation of a line source in a convective atmospheric boundary layer—II. Dynamics of a buoyant line source

We consider a large-eddy simulation of a buoyant line source in a convective boundary layer. The motion of the line source as a function of time is influenced by two contributions: internal buoyancy (plume rise) and ambient turbulence. The advantage of large-eddy modelling with respect to laboratory and atmospheric experiments is that the simulations allow us to distinguish between these two contributions. For the part due to internal buoyancy we formulate an integral model for plume rise. A new feature in this model is the fact that plume looping, which is characteristic for plume dispersion in a convective boundary layer, promotes entrainment and therefore reduces plume rise. The contribution by ambient turbulence to plume motion is modelled in terms of the standard deviation of vertical velocity fluctuations. The results of these models are consistent with our large-eddy simulations.

Similarity scaling applied to sodar observations of the convective boundary layer above an irregular hill

Nine profiles of the temperature structure parameter C T 2 and the standard deviation of vertical velocity fluctuations (Σw) in the convective boundary layer (CBL) were obtained with a monostatic Doppler sodar during the second intensive field campaign of the First ISLSCP Field Experiment in 1987. The results were analyzed by using local similarity theory. Local similarity curves depend on four parameters: the height of the mixed layer (z i ), the depth of the interfacial layer (δ), and the temperature fluxes at the top of the mixed layer (Q i ) and the surface (Qo). Values of these parameters were inferred from sodar data by using the similarity curve for C T 2 and observations at three points in its profile. The effects of entrainment processes on the profiles of C T 2 and Σwnear the top of the CBL appeared to be described well by local similarity theory. Inferred estimates of surface temperature flux, however, were underestimated in comparison to fluxes measured by eddy correlation. The measured values of Σwappeared to be slightly smaller than estimates based on available parmeterizations. These discrepancies might have been caused by experimental error or, more likely, by the distortion of turbulence structure above the site by flow over the nonuniform terrain at the observation site.