| Summary: | 博士 === 國立臺灣大學 === 大氣科學系 === 86 === The paper investigate the convective behavior by using the
dual Doppler radar
observations and the two dimensional nonhydrostatic cloud model. There are
three major part in the paper. The Part I describes the the frontal rainband
by using the observation data and addresses the interested questions
for further studies.
The momentum budget analysis have been studied by using the dual-Doppler
radar wind data and the retrieved perturbation pressure field in part II.
The results show that the pressure gradient force in mid-troposphere, which
results from the interaction of the convective updraft and the vertical wind
shear, plays important roles in the evolution of the long-lifed convective
cell and leads to the tilting of the convection toward the downshear side.
The convection transports the along-line component of momentum down the mean
gradient and the cross-line component upgradient. The downshear tilt of
the convective system is a necessary condition which transports the cross-line
momentum upgradient. Finally, we try to interpret what the interaction of the
convective updraft and the vertical shear is by the momentum
transport thinking.
The horizontal acceleration induced by the vertical momentum flux convergence
tends to generate a high pressure at downstream side and low pressure
at upstream side. The pressure dipole can be considered as the inertia of
the flow field and the associated pressure gradient force tends to offset
the external forcing which is due to the momentum transport. The
inertia depends on the aspect ratio of the external forcing, e.g. , the
pressure gradient force will be larger when the scale along the
forcing is larger than the scale cross the forcing.
In the case, the net force of the momentum flux convergence and the
pressure gradient force will be smaller and tends to keep the initial status
of the flow field. The thinking can be also extended to the
interpretation of the buoyancy induced pressure gradient force.
Part III systematically analyze the charactristics of the downshear tilt
convection by the idealized two dimensional nohydrostatic cloud model
and compare with the observations. By the way, the simulation results also
show that using the CAPE (Convective Available Potential Energy) to present
the convective instability of the atmosphere may casuse
improper interpretation.
The vertical profile of the CAPE is a more realistic idea. By the way, the
eimulataion results also suggest the low level jet will moisterize the
boundary layer and then modify the vertical profile of CAPE, thus favor
the development of the convection.
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