台灣北部地區五～七月雷達回波特徵與局部環流探討

On the Climatological Characteristics of Radar Reflectivities and Local Circulations over Northern Taiwan in May — July

陳熙揚(Shi-Yang Chen) ； 陳泰然(George Tai-Jen Chen) ； 林麗芬(Ly-Fen Lin)

《大氣科學》 28卷2期 (2000/06) Pp. 73-90

摘要

本文利用1989～1996年5～7月民航局中正機場都卜勒雷達回波資料與地面風場資料，分析台灣北部橫跨不同地形高度之四個剖面上的平均回波強度與不同回波強度出現頻率之時空分布特徵，並探討雷達回波顯示之對流時空分布與局部環流的關係。
結果顯示，5～7月各處平均回波強度有隨季節增強之趨勢，且具有顯著之日夜變化，不同月份與不同剖面之日夜分布，亦有明顯差異，特別是竹北以東剖面，5月平均回波強度中午前後之最低値，於6月轉為相對高値。由局部環流日夜分布顯示，此現象可能與5月新竹沿海0800～0900 LST之氣旋式渦旋有關。氣旋式渦旋伴隨的輻合作用可能激發午前對流發展，被消耗的部分對流可用位能需時間來恢復，故5月中午前後對流出現頻率較小。此外，除7月之竹北外，各剖面均有午後最大值，且隨季節演進逐漸成為顯著之主高峰，顯示大氣環境條件與局部強迫作用，在午後對流降水扮演重要的角色亦隨著季節演變。
強對流頻率高峰分布與風場局部環流分析顯示，竹北與以東6～7月之對流在山坡先行發展後，對流之下衝流可能沿山坡而下與海風產生輻合，故平地之對流出現時間較山坡為晚。台北盆地與其鄰近地區之午後對流，6月與7月均在山坡先行發展，隨後在盆地內發展。此種現象與海風、上坡風在盆地附近形成之氣旋式渦旋出現位置、時間一致，顯然此氣旋式渦旋對該地區午後對流之形成應扮演重要之角色。

關鍵字

回波強度 ； 回波頻率 ； 日夜變化 ； 局部環流 ； 雙高峰 ； 海風 ； 上坡風 ； 氣旋式渦旋

並列摘要

The reflectivity data observed by the CKS Airport Doppler radar of the Civil Aeronautics Administration and the surface wind observations of the Central Weather Bureau over northern Taiwan in the period of May-July 1989-1996 were used to study the climatological characteristics of temporal and spatial distributions of mean echo intensity and echo frequency along four cross sections of various altitudes. In addition, the relationship between characteristics of radar echoes and local circulations was investigated.
Results showed that the mean echo intensity exhibited an obvious diurnal variation and an increasing tendency from May to July. A remarkable difference in diurnal variation was also observed among different months as well as different cross sections. It is interesting to note that a relative minimum in mean echo intensity near noon in May turned to be a maximum in June along cross section to the east of Chu-Bei. Local circulations showed that a cyclonic vortex existed along coast of Hsin-Chu in the period of 0800-0900 LST in May. The cyclonic vortex might trigger the convection before noon and used up part of CAPE. It took time to recover the CAPE in the atmosphere environment. As a result, the convection frequency near noon became a minimum. Furthermore, there was an obvious maximum of mean echo intensity in the afternoon along each cross section except Chu-Bei in July. The afternoon maximum value became the primary peak as season proceeded. This suggests that the local environmental conditions and local forcing played an important role in the formation of afternoon convection.
To the east of Chu-Bei, convection first developed on the mountain slope. The downslope flow from convective downdraft and the sea breeze might provide the convergence over Chu-Bei and its eastern region. Consequently, the convection over plain area developed later than that over the mountain slope. Over the Taipei basin and its vicinities, the afternoon convection developed earlier on the mountain slope than over the basin areas in both June and July. This was consistent with the development of cyclonic vortex which probably formed due to the sea breeze and upslope wind in the vicinities of the Taipei basin. Apparently, the cyclonic vortex played an important role on triggering the afternoon convection over these areas.

並列關鍵字

Echo intensity ； Echo frequency ； Diurnal variation ； Local circulation ； Double peak ； Sea breeze ； Upslope wind ； Cyclonic vortex