Dynamic and Thermodynamic Characteristics of a Prefrontal Convective Rainband in Northern Taiwan Determined Form Tamex Data
Abstract
In this study, structural features of a convective rainband associated with the Mei-Yu front on 25 June 1987 over northwestern Taiwan are investigated. Dual-Doppler data were collected from the CP-4 and TOGA Doppler radars positioned along the northwest coast of Taiwan during the Taiwan Area Mesoscale Experiment (TAMEX). Fields of the system-relative wind and reflectivity were derived in a horizontal domain of 40 km by 36 km using an objective analysis scheme with 1 km grid spacing. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment. Results show that the low-level jet (LLJ) in the boundary layer provides large vertical shear ahead of the cold front. The depth of the cold air associated with the Mei-Yu front is rather shallow (1-2 km). To the south of the front, winds are from the southwest in the lower troposphere and veer with height in the middle and upper troposphere. The low-level convergence enhances lifting, resulting in a narrow band of convection on the warm side of the front. The rainband is composed of many cells which are accompanied by moderate convective updrafts (6-8 m s^{-1}) and weak downdrafts (2-4 m s^{-1}). Because the system traveled very slowly within the domain of interest, up to 200 mm of rainfall in 24 hours occurred on the west coast of Taiwan. The retrieved pressure and temperature fields are closely related to each other and are nearly in hydrostatic balance at most levels. The vertical component of horizontal momentum flux in a direction normal to the rainband is broadly countergradient, while the vertical momentum flux for the component parallel to the band is downgradient. The vertical transport of horizontal momentum during convection are largely responsible for the weakening of a LLJ and the growth of a MLJ as revealed by rawinsonde observations.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1992
- Bibcode:
- 1992PhDT.......159C
- Keywords:
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- CONVECTIVE RAINBAND;
- RAINBAND;
- Physics: Atmospheric Science