THESIS
2013
xv, 150 pages : illustrations (chiefly color), maps (chiefly color) ; 30 cm
Abstract
This study employed the Weather Research and Forecasting (WRF) model to analyze variations
in the PBL structures over a subtropical region (Hong Kong) and a mid-high latitude region
(southeast England), predicted by different PBL schemes over a range of stability conditions.
Evaluated were two nonlocal mixing PBL schemes, YSU and ACM2, and three local mixing
PBL schemes, MYJ, Boulac and MYNN2. The PBL height predictions exhibit large intrascheme
variance ( 20%). ACM2 diagnoses the PBL height thermodynamically using the bulk
Richardson number method, which leads to a good agreement with the lidar data. The modeled
vertical profiles, such as wind speed, exhibit large spreads across the PBL schemes. The
turbulent kinetic energy (TKE) predicted by MYJ were found to be too small and...[
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This study employed the Weather Research and Forecasting (WRF) model to analyze variations
in the PBL structures over a subtropical region (Hong Kong) and a mid-high latitude region
(southeast England), predicted by different PBL schemes over a range of stability conditions.
Evaluated were two nonlocal mixing PBL schemes, YSU and ACM2, and three local mixing
PBL schemes, MYJ, Boulac and MYNN2. The PBL height predictions exhibit large intrascheme
variance (> 20%). ACM2 diagnoses the PBL height thermodynamically using the bulk
Richardson number method, which leads to a good agreement with the lidar data. The modeled
vertical profiles, such as wind speed, exhibit large spreads across the PBL schemes. The
turbulent kinetic energy (TKE) predicted by MYJ were found to be too small and show much
less diurnal variation as compared with observations over London. MYNN2 produces better
TKE predictions at low levels (0-200 m AGL) than MYJ, but its turbulent length scale increases
with height in the upper part of the strongly convective PBL. The local PBL schemes
considerably underestimate the entrainment heat fluxes for convective cases.
This study also compares the predictions for momentum mixing from two nonlocal closure PBL
models, i.e., the YSU and the modified Frech and Mahrt (FMX) PBL models, and from one local
closure PBL model (YSU-local), using the WRF model. The YSU model, in which the nonlocal momentum flux is aligned with the surface wind shear, often predicts larger vertical wind shear
than YSU-local in the interior of the convective PBL. In contrast, the FMX model produces more
well-mixed wind profiles that agree better with observations and LES results than the YSU and
YSU-local models, especially when the sign of the bulk wind shear in the convective PBL is
opposite to that of the surface wind shear.
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