The Structure of a Subtropical Prefrontal Convective Rainband. Part I: Mesoscale Kinematic Structure Determined from Dual-Doppler Measurements

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In Part I of this study, structural features of a convective rainband associated with the Mei-Yu front on 25 June 1987 over northwestern Taiwan are investigated. The dynamic and thermodynamic structures and momentum budgets of the convective rainband will be reported in Part II. Dual-Doppler data were collected from 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 × 36 km using the objective analysis scheme with a 1-km grid spacing in all three directions. There were ten analysis levels in the vertical ranging from 0.8 to 9.8 km. 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 orientation of the jet is in the direction almost parallel to the cold front. The depth of the cold air associated with the Mei-Yu front is rather shallow (1–2 km), in agreement with other TAMEX case studies reported in the literature. 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 southwest monsoon flow at low levels transports high-θe, environmental air toward the wind-shift line. Behind the front, the northwesterly flow dominates, carrying much cooler air from northern China. The low-level convergence enhances lifting, resulting in a narrow band of convection on the warm side of the front. The length of the rainband analyzed is 5–10 km wide and 50 km long and is composed of many cells. Each cell is accompanied by the moderate convective updraft (6–8 m s−1) and weak downdraft (2–4 m s−1). The precipitation is deep but not intense on the warm side of the front. The maximum reflectivity within the rainband is less than 42 dBZ. The precipitation cores are elongated southeastward in the direction parallel to the environmental shear vector between the lower and middle layers. By contrast, the precipitation behind the front is weak and shallow, showing stratiform precipitation. Because the system traveled very slowly within the domain of interest, up to 200 mm of rainfall in 24 h occurred on the west coast of central Taiwan.