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H. N. Lee

Abstract

Advective transport using the flexible and stable semi-Lagrangian scheme coupled with the highly accurate spectral interpolation in a limited domain is demonstrated. The spectral method for solving nonperiodic boundary problems is based on a technique of decomposing a solution into a polynomial and a periodic Fourier residual. The fast Fourier transform is applied only to a periodic residual. A fifth-order polynomial is used and a method to obtain it is described. The numerical tests of the advective transport of a mass in a nonuniform deformational and a uniform rotational flow, as well as the plume transport from a continuous source, are also presented. As will be shown in the paper, the technique proved to be efficient and accurate, providing excellent results.

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H. N. Lee

Abstract

A semi-Lagrangian model was developed based on the atmospheric transport-diffusion equation, employing the semi-Lagrangian technique incorporated with the spectral method. The numerical algorithms of the semi-Lagrangian technique and the spectral method for solving the transport-diffusion equation, subjected to boundary conditions, are described. The model simulated the transport and diffusion of pollutants from an elevated point source. The accuracy of the algorithms was revealed by numerical tests compared with analytical solutions in cases of simple profiles of the mean wind and the eddy diffusivity. To demonstrate the feasibility of the algorithms for studying air pollution in the atmospheric boundary layer, the model, using realistic profiles of the mean wind and the eddy diffusivity, simulated the distributions of pollutant concentrations under various atmospheric stabilities. As will be shown, the algorithms are capable of producing excellent results, making them attractive and promising for regional studies of pollutant transport and diffusion in the atmosphere.

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H. N. Lee

Abstract

Simple and efficient formulas for calculating surface fluxes are presented for the nondimensional profile functional forms of wind and temperature in the atmospheric surface layer. Four formulations representing nondimensional profile functional forms are examined. New approximate solutions for the relationship between z/L and the bulk Richardson number are proposed. These new solutions are an improvement over the existing approximate solutions and give much better agreement with the numerical solutions obtained by iteration. The approach can be extended to other similar nondimensional profile functional forms.

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H. N. Lee and J. K. Shi

Abstract

A pseudospectral method and its numerical solution algorithm for application to boundary layer problems in the atmosphere are presented. The method introduces the evaluation of a polynomial function when the solution is expressed as the sum of a periodic function and a polynomial function. The periodic function is then treated by Fourier expansion. In the paper, the accuracy of method has been demonstrated. Numerical results for a system of time dependent equations, modeling the atmospheric planetary boundary layer flow and nocturnal flow over terrain are encouraging. The method offers a promising alternative to finite-difference techniques.

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H. N. Lee and S. K. Kao

Abstract

A dynamic turbulent boundary-layer model in the neutral atmosphere is constructed, using a dynamic turbulent equation of the eddy viscosity coefficient for momentum derived from the relationship among the turbulent dissipation rate, the turbulent kinetic energy and the eddy viscosity coefficient, with aid of the turbulent second-order closure scheme. A finite-element technique was used for the numerical integration. In preliminary results, the behavior of the neutral planetary boundary layer agrees well with the available data and with the existing elaborate turbulent models, using a finite-difference scheme. The proposed dynamic formulation of the eddy viscosity coefficient for momentum is particularly attractive and can provide a viable alternative approach to study atmospheric turbulence, diffusion and air pollution.

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S. K. Kao and H. N. Lee

Abstract

An analysis of the linear and nonlinear interactions of atmospheric motion in the wavenumber-frequency domain indicates that the kinetic energy of the large-scale moving waves is essentially maintained by the nonlinear interactions and the pressure force. In middle latitudes where an eastward mean zonal flow prevails, the supply of kinetic energy to eastward moving waves through the nonlinear interactions is greater than the extraction of kinetic energy through the pressure force, whereas the supply of kinetic energy to westward moving waves through the pressure force is greater than the extraction of kinetic energy through the nonlinear interactions. Near the equator where a weak westward mean zonal Row occurs, the non-linear interactions generally extract kinetic energy from the eastward moving waves, but supply kinetic energy to the westward moving waves; the pressure force, however, supplies kinetic energy to both eastward and westward moving waves.

The primary contribution of the nonlinear interactions to the energy transfer in wavenumber-frequency domain is essentially through the interactions of the slowly moving waves, the stationary long waves and the zonal mean flow. The interactions between the stationary long waves and waves moving in the same (opposite) direction of the mean zonal flow generally extract (supply) kinetic energy from (to) the moving waves, whereas the interactions between the mean zonal flow and waves moving in the same (opposite) direction of the zonal flow generally supply (extract) kinetic energy to (from) the moving waves.

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H. N. Lee and R. J. Larsen

Abstract

Vertical profiles of 222Rn concentrations measured from 3 to 6 September 1995 in the northeastern United States, using a new radon instrument designed for aircraft measurements, are presented. A vertical diffusion model was employed to simulate the distributions of 222Rn concentrations by using time-dependent profiles of vertical eddy diffusivity in the lower atmosphere. To determine these profiles, O’Brien’s simple formulation was applied using the diurnal changes of the boundary layer height and surface fluxes. The model-calculated profiles of the 222Rn concentrations were then compared with the aircraft measurements of 222Rn.

Model simulations were also calculated using different values of the vertical eddy diffusivity above the boundary layer. Using the value of k z above the boundary layer equal to 10% of its maximum boundary layer value in the model resulted in the best vertical profiles of the calculated 222Rn compared with the measurements within the boundary layer.

From aircraft measurement data of the naturally occurring radionuclide 222Rn, the vertical mixing processes in the lower atmosphere were studied. These data will be useful for model validations.

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W. S. Kau, H. N. Lee, and S. K. Kao

Abstract

Statistical models for surface-wind predictions at a mountain and a valley station near Anderson Creek, California, have been constructed. It is found that the surface wind speed depends primarily on the slope wind, cross-isobaric angle, surface thermal stability and geostrophic wind. The correlations between the calculated and observed surface wind speeds are found to be high for all time periods of the day and night.

Because the variability of wind direction, which is greatly affected by topography, geostrophic wind and turbulent motion, is generally larger than that of the surface wind speed, statistical models for wind direction are more complicated than those for the wind speed. It is found that wind direction depends primarily on the geostrophic wind direction, aspect angle of the topography, up-canyon direction and cross-isobaric angle in the boundary layer.

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Dana K. Savidge, Jackson O. Blanton, Thomas N. Lee, and Robert H. Evans

Abstract

A 4.3-month-long dataset from moorings on the continental shelf off South Carolina during 1986 showed unusual midrecord shifts in bottom pressure, temperature, stratification, and alongshelf currents. The Gulf Stream moved farther offshore during the second half of the time period. This offshore position appears to have facilitated Gulf Stream influence on shelf waters all across the continental shelf.

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Hoffman H. N. Cheung, Wen Zhou, Sai-ming Lee, and Hang-wai Tong

Abstract

During the past decade (2004/05–2013/14), the number of cold days in Hong Kong (N CD), as a proxy of the temperature of southern China, appeared to have increased from the historical minimum, in contrast to a remarkable decline in the entire postwar period. This is related to the recent apparent changes in the large-scale circulation upstream and downstream of the East Asian winter monsoon (EAWM) region: the increase in Ural blocking (UB) that enhances cold advection from the polar region and reinforces the Siberian high and the decrease in a western Pacific (WP)-like index that corresponds to increasing meridional gradient of geopotential height over the EAWM region. Overall, UB and WP account for 26.4% of the interannual (≤8 yr) variance and 83.7% of the decadal (>8 yr) variance of N CD for the period 1948/49–2013/14, indicating that further study could lead to improvement in the prediction of N CD.

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