| Summary: | 博士 === 國立中央大學 === 大氣科學學系 === 105 === The four-dimensional Variational Doppler Radar Analysis System (VDRAS) developed at the National Center for Atmospheric Research (NCAR) is significantly improved by
implementing a terrain-resolving scheme to its forward model and adjoint based on the Ghost Cell Immersed Boundary Method (GCIBM), which allows the topographic effects to be considered without the necessity to re-build the model on a terrain-following coordinate system. The new system, called IBM_VDRAS, is able to perform forward forecast and backward adjointmodel integration over non-flat lower boundaries, ranging from mountains with smooth slopes to buildings with sharp surfaces. To evaluate the performance of the forward model over complex terrain, idealized numerical experiments of a two-dimensional linear mountain wave and three-dimensional lee side vortices are first conducted, followed by a comparison with a simulation by the Weather Research and Forecasting (WRF) model. In addition, the building-scale simulation demonstrates its flexibility in resolving terrain as well. An Observation Simulation System Experiment (OSSE) is also conducted with the assimilation of simulated radar data to examine the ability of IBM_VDRAS in analyzing orographically forced moist convection. It is shown that the IBM_VDRAS can retrieve terrain-influenced three-dimensional
meteorological fields including winds, thermodynamic and microphysical parameters with reasonable accuracy.
This new system is utilized to study precipitation process in a real case of pre-frontal rainbands passed over southern Taiwan on June 14 of 2008, collected by IOP 8 of the 2008
Southwest Monsoon Experiment (SoWMEX). Prior to physical discussion, the multivariate analysis is verified by various types of measurements including mesonet stations, wind profiler, radiosonde and Doppler radar radial velocity. It shows that IBM_VDRAS has robust performance in retrieving the meteorological states. Further examinations on the frequent updated analyses demonstrate the importance of quasi-steady convergence line near southwestern coast of Taiwan.
The major mechanisms leading to the formation and maintenance of this convergence line is decomposed into pure topographic, pure evaporation cooling, nonlinear interactive effect, and is further explored through a series of sensitivity experiments. Pure topographic effect is found to
be the dominant factor in modulating the convection development. However, the contributions from nonlinear interaction and pure evaporation, although with weaker magnitudes, cannot be neglected. A schematic diagram is formulated to explain how a local circulation system is built and maintained to support the above mentioned quasi-stationary coastal convergence line, which is responsible for a long-lasting severe precipitation event. It is clear that the interaction among mountain blockage, evaporation cooling effect, and the development of an enhanced widespread high pressure zone over the southwest plain area, are the key factors to modulate the evolution of the convective system.
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