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|Title: ||Numerical simulation of a squall line over Hong Kong|
|Other Titles: ||Xianggang biao xian shu zhi mo ni|
|Authors: ||Tsui, Chi Yan (徐摯仁)|
|Department: ||Department of Physics and Materials Science|
|Degree: ||Master of Philosophy|
|Issue Date: ||2008|
|Publisher: ||City University of Hong Kong|
|Subjects: ||Squall lines -- Mathematical models -- China -- Hong Kong.|
Squall lines -- Computer simulation -- China -- Hong Kong.
|Notes: ||xviii, 106 leaves : ill. (some col.) 30 cm.|
Thesis (M.Phil.)--City University of Hong Kong, 2008.
Includes bibliographical references (leaves 102-106)
CityU Call Number: QC880.4.S65 T78 2008
|Abstract: ||The objective of this study is to investigate the squall line that swept through Hong
Kong on 9th May 2005 through simulation using the Regional Atmospheric Model
System (RAMS). The investigation includes the genesis, lifetime and propagation of
the squall line system.
Two numerical models, the PSU/NCAR mesoscale model (MM5) and RAMS were
used for trial propose and the latter was selected in this study. Three types of data are
used for trial as well including MM5-exported, Operational Regional Spectral Model
(ORSM)-exported and National Centers for Environmental Prediction (NCEP) data.
NCEP data are introduced to the model as initial and boundary conditions. The squall
line system is simulated by 3 nested grids with horizontal resolutions of 10, 4 and 1
km. Automatics Weather Station and Meteorological Information Comprehensive
Analysis and Processing System (MICAPS) data are also used for data assimilation
in the finest grid.
The lifetime of the squall system is about 50 min from 0410UTC to 0500UTC. By
visualizing the locations of the well-defined gust front during its mature stage just
before the dissipating stage, it is estimated that its propagation speed is about 11.0 m
s-1 from 308˚ (southeastward). The speed is similar to previous studies. However, the
horizontal dimensions of the squall system in this study are much smaller than those
that often exist in meso-alpha scale.
The simulated kinematic and thermodynamic structures also agree well with those
found in previous studies. In the kinematics aspects, a confluent line in the wind flow
pattern, which corresponds to the gust front, can be seen just behind a linear-oriented
convergence zone. An inflow entering the system ahead of the gust front contributes
to the convergence zone while the rear outflow behind the front is associated with the
divergence zone that results in the cold pool developing underneath the leading edge.
A characteristic convective tower can also be visualized by plotting cloud mixing
ratio through a selected vertical cross-section normal to the front. In the
thermodynamic aspects, the pattern of equivalent potential temperature (θe) across
the front resembles that described in previous conceptual models. The low θe region
is located ahead of the gust front aloft while lying on the surface underneath the
convective cumulus, coinciding with the cool pool. The low θe value of the cold pool
can be explained by evaporation of precipitation at low levels, which leads to
downdraft of air and brings the cold air down, spreading outward on the ground
The simulated results are also validated using available observations. It found that
the model is able to simulate the evolution of the squall system, with only about a
10-minute delay with respect to the actual system. However, the model tends to
underestimate the wind speed in the vicinity of the gust front. The wind pattern in the
pre-squall environment is similar to the observed one but the post-squall environment
is not well simulated. It is believed that this disagreement leads to an angle between
the deduced and the simulated gust front, which become more significant with at the
later time. Overall, the evolution of the squall line system from mature to dissipating
stage can be described in this simulation.|
|Online Catalog Link: ||http://lib.cityu.edu.hk/record=b2268783|
|Appears in Collections:||AP - Master of Philosophy |
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