Lin-Glendening02 - 15 AUGUST 2002 LEN AND GLENDENING 2479...

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15 A UGUST 2002 2479 LEN AND GLENDENING Large Eddy Simulation of an Inhomogeneous Atmospheric Boundary Layer under Neutral Conditions C HING-LONG LIN Department of Mechanical and Industrial Engineering and IIHR Hydroscience and Engineering, The University of Iowa, Iowa City, Iowa J OHN W. GLENDENING Naval Research Laboratory, Monterey, California (Manuscript received 28 October 2001, in final form 25 February 2002) ABSTRACT Flow structures in an inhomogeneous neutrally stratified atmospheric boundary layer flow, obtained from large eddy simulation, are analyzed and compared with homogeneous case counterparts. The inhomogeneity is imposed in the streamwise direction by using two different surface roughness heights z o , each covering a streamwise distance of 4.8 km, to produce internal boundary layers. Adjustments of the mean velocity profiles are primarily confined to the lowest 140 m of the overlying boundary layer, with parcels being accelerated and decelerated over the respective smooth and rough surfaces. For large fetches downwind of the surface roughness discon- tinuities, the mean velocity profiles close to the surface are approximately logarithmic, but fitting to Monin– Obukhov similarity profiles using the z o and stress of the underlying surface requires that the von Ka ´rma ´n constant be k 5 0.4 over the smooth surface and k 5 0.37 over the rough surface. Much of this difference is attributed to velocity accelerations created by locally induced pressure gradient forces within the boundary layer, requiring k to be adjusted when horizontally homogeneous similarity expressions are utilized. Quadrant analysis indicates that ejection and sweep intensities differ from those of homogeneous surface cases but the occurrence frequencies are similar. Budget analysis of momentum flux indicates that the shear production and pressure destruction terms roughly balance, which is consistent with previous homogeneous surface findings. Flow vi- sualization and conditional sampling demonstrate that these two terms are physically associated with ejections, sweeps, and vortical structures. 1. Introduction Flow structures under neutral conditions in the sur- face layer of the atmospheric boundary layer (ABL) have been studied experimentally and numerically due to their importance in turbulent transport of momentum, heat, and moisture. Observationally, gust microfronts resulting from the collision of high- and low-speed mo- mentum fluxes were identified by Mahrt (1991), ejection and sweep events in the near-neutral atmospheric sur- face layer were investigated by Bergstro ¨m and Ho ¨gs- tro ¨m (1989) and Ho ¨gstro ¨m and Bergstro ¨m (1996), and streaky structures were revealed by Weckwerth et al. (1997) using high-resolution Doppler lidar in the Lidars in Flat Terrain (LIFT) project.
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This note was uploaded on 11/07/2011 for the course EAS 8803 taught by Professor Staff during the Spring '08 term at Georgia Institute of Technology.

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Lin-Glendening02 - 15 AUGUST 2002 LEN AND GLENDENING 2479...

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