Surface Turbulent Flux Measurements over the Loess Plateau for a Semi-Arid Climate Change Study

In order to provide high quality data for climate change studies, the data quality of turbulent flux measurements at the station of SACOL （Semi-Arid Climate ＆ Environment Observatory of Lanzhou University）, which is located on a semi-arid grassland over the Loess Plateau in China, has been analyzed in detail. The effects of different procedures of the flux corrections on CO2, momentum, and latent and sensible heat fluxes were assessed. The result showed that coordinate rotation has a great influence on the momentum flux but little on scalar fluxes. For coordinate rotation using the planar fit method, different regression planes should be determined for different wind direction sectors due to the heterogeneous nature of the ground surface. Sonic temperature correction decreased the sensible heat flux by about 9%, while WPL correction （correction for density fluctuations） increased the latent heat flux by about 10%. WPL correction is also particularly important for CO2 fluxes. Other procedures of flux corrections, such as the time delay correction and frequency response correction, do not significantly influence the turbulent fluxes. Furthermore, quality tests on stationarity and turbulence development conditions were discussed. Parameterizations of integral turbulent characteristics （ITC） were tested and a specific parameterization scheme was provided for SACOL. The ITC test on turbulence development conditions was suggested to be applied only for the vertical velocity. The combined results of the quality tests showed that about 62%-65% of the total data were of high quality for the latent heat flux and CO2 flux, and as much as about 76% for the sensible heat flux. For the momentum flux, however, only about 35% of the data were of high quality.

A Re-examination of Density Effects in Eddy Covariance Measurements of CO_2 Fluxes

Corrections of density effects resulting from air-parcel expansion/compression are important in interpreting eddy covariance fluxes of water vapor and CO2 when open-path systems are used. To account for these effects, mean vertical velocity and perturbation of the density of dry air are two critical parameters in treating those physical processes responsible for density variations. Based on various underlying assumptions, different studies have obtained different formulas for the mean vertical velocity and perturbation of the density of dry air, leading to a number of approaches to correct density effects. In this study, we re-examine physical processes related to different assumptions that are made to formulate the density effects. Specifically, we re-examine the assumptions of a zero dry air flux and a zero moist air flux in the surface layer, used for treating density variations, and their implications for correcting density effects. It is found that physical processes in relation to the assumption of a zero dry air flux account for the influence of dry air expansion/compression on density variations. Meanwhile, physical processes in relation to the assumption of a zero moist air flux account for the influence of moist air expansion/compression on density variations. In this study, we also re-examine mixing ratio issues. Our results indicate that the assumption of a zero dry air flux favors the use of the mixing ratio relative to dry air, while the assumption of a zero moist air flux favors the use of the mixing ratio relative to the total moist air. Additionally, we compare different formula for the mean vertical velocity, generated by air-parcel expansion/compression, and for density effect corrections using eddy covariance data measured over three boreal ecosystems.

Simulation of Viscous Flows Around A Moving Airfoil by Field Velocity Method with Viscous Flux Correction

Based on the field velocity method,a novel approach for simulating unsteady pitching and plunging motion of an airfoil is presented in this paper.Responses to pitching and plunging motions of the airfoil are simulated under different conditions.The obtained results are compared with those of moving grid method and good agreement is achieved.In the conventional field velocity method,the Euler solver is usually used to simulate the movement of the airfoil.However,when viscous effect is considered,unsteady Navier-Stokes equations have to be solved and the viscous flux correction must be taken into account.In this work,the viscous flux correction is introduced into the conventional field velocity method when non-uniform grid speed distribution is occurred.Numerical experiments for the flow around NACA0012 airfoil showed that the proposed approach can well simulate viscous moving boundary flow problems.

ANTI-DIFFUSIVE FINITE DIFFERENCE WENO METHODS FOR SHALLOW WATER WITH TRANSPORT OF POLLUTANT

In this paper we further explore and apply our recent anti-diffusive flux corrected high order finite difference WENO schemes for conservation laws [18] to compute the SaintVenant system of shallow water equations with pollutant propagation, which is described by a transport equation. The motivation is that the high order anti-diffusive WENO scheme for conservation laws produces sharp resolution of contact discontinuities while keeping high order accuracy for the approximation in the smooth region of the solution. The application of the anti-diffusive high order WENO scheme to the Saint-Venant system of shallow water equations with transport of pollutant achieves high resolution

A Priori Subcell Limiting Approach for the FR/CPR Method on Unstructured Meshes

A priori subcell limiting approach is developed for high-order flux reconstruction/correction procedure via reconstruction(FR/CPR)methods on twodimensional unstructured quadrilateralmeshes.Firstly,a modified indicator based on modal energy coefficients is proposed to detect troubled cells,where discontinuities exist.Then,troubled cells are decomposed into nonuniform subcells and each subcell has one solution point.A second-order finite difference shock-capturing scheme based on nonuniform nonlinear weighted(NNW)interpolation is constructed to perform the calculation on troubled cells while smooth cells are calculated by the CPR method.Numerical investigations show that the proposed subcell limiting strategy on unstructured quadrilateral meshes is robust in shock-capturing.