梅雨鋒面對流雨帶雙都卜勒雷達分析

Dual-Doppler Radar Analysis of a Convective (Mei-Yu) Frontal Rainband

周仲島(Ben Jong-Dao Jou) ； 洪景山(Jing-Shang Hong) ； 鄧秀明(Shiung-Ming Deng)

《大氣科學》 18卷3期 (1990/09) Pp. 239-264

摘要

本文利用民航局中正機場和NCAR CP-4雙都卜勒雷達合成三度空間風場，分析1987年6月25日一個緩慢移動之梅雨鋒面對流雨帶的降水回波與運動場結構，並探討鋒面與其所伴隨的線性對流系統間相互維持的關係。雙都卜勒雷達合成風場結果顯示鋒面在最低2公里有強的氣旋式風切，風切線向鋒後冷空氣一方傾斜，同時鋒面上並發展有週期性的氣旋式擾動。三度空間風場結構顯示梅雨鋒面對流雨帶具有相當的二維性。低層鋒面扮演介面舉升的角色，配合中對流層存在的對流不穩定層可使經由近地面鋒面舉升之空氣塊在此獲得浮力而自由對流到15公里。垂直運動在5公里和8公里高度各有一個上升運動極大值，可突顯出機械舉升和自由對流在此所扮演的角色。伴隨鋒面的對流雨帶騎跨在鋒的前緣，環境的低層風切和鋒後的冷空氣相互平衡，可提供低層動力上有力的輻合。而中對流層存在的不穩定層則是對流持續與發展所不可或缺的能量來源。另一方面，高層風切使得降水回波向鋒前傾斜，在熱力上和動力上都將產生不利雨帶發展的因子。因此，本個案中雨帶的演化可視為是環境之高低層風切，鋒面和對流三者交互作用的結果。

關鍵字

梅雨鋒面 ； 對流雨帶 ； 雙都卜勒雷達分析

並列摘要

In the study, kinematic and precipitation structures of a convective rainband associated with slow-moving Mei-Yu front which passed over northern Taiwan area on 24-25 June 1987 in TAMEX IOP13 were investigated by using Doppler radar data collected from CP-4 (NCAR) and CAA (Civil Aeronautic Administration). Dual-Doppler radar data analysis technique developed by NCAR (CEDRIC software) were used for the analysis. Detail procedures of the data decoding, editing, correction, and analysis for CAA and CP-4 Doppler radars were also given for reference.The major findings of this research are:(1) Strong cyclonic wind shear was found at lowest 2 km on the surface front. The axis of maximum cyclonic vorticity tilted to the cold side of the surface front with increasing altitudes. The intensity of the vorticity can reach 10^(-3) s^(-1).(2) Pronounced wave-type vorticity disturbances possibly generated by the shear instability were found along the surface frontal interface with wavelength approximately of 10 km.(3) Three-dimensional wind field structure with the convective rainband to the west end of the front revealed a significant two-dimensional characteristics. Southwesterly flow with moderate intensity collided with weak westerly flow at the frontal surface provided the lifting mechanisim at the low level. The potentially unstable layer in the middle troposphere due to cold air interaction from the northwest provided excess energy for further development.(4)The reflectivity pattern at upper levels tilted forward. This forward tilt of precipitation is possibly due to the strong wind shear in the upper level. This forward tilt of precipitation, dynamically and thermodynamically, provided unfavorable conditions for rainband development.The Results suggested that the maintainence of the convective frontal rainband depends closely on the interaction among the upper and lower level wind shears, the lifting force of surface front. and the mid-tropospheric instability.

並列關鍵字

TAMEX ； Mei-Yu front ； convective rainband ； Dual-Doppler radar analysis