屏東平原海風環流之SPOL雷達觀測特徵

Observations of Sea Breeze Circulation by SPOL over the Pingtung Plain

周仲島(Ben Jong-Dao JOU) ； 修榮光(Radiant Rong-Guang HSIU)

《大氣科學》 43卷1期 (2015/03) Pp. 47-67

摘要

2008年6月20日在太平洋副熱帶高壓影響下，屏東平原午後發展出一多胞雷暴，帶來顯著局部降雨和落雷。當天除了有3小時探空和額外地面測站資料外，NCAR SPOL雷達對於此現象也有密集觀測資料，記錄了大氣邊界層發展、海風環流移入、以及雷暴激發和演變等過程。本文針對高解析度地面測站和SPOL雷達觀測之海風環流結構進行分析並和密集探空進行比對，探討屏東平原海風環流的演變特徵，並針對SPOL觀測之各項雙偏極化參數進行分析討論。利用高時間解析度地面測站資料分析海風鋒面在屏東平原的每小時位置。海風環流前緣有明顯鋒面特徵，包括溫度下降、水氣上升、風速增大、風向變化、以及極大陣性等。屏東平原海風鋒面移動速度大約每小時7-9公里（平均移速2.2m/s），接近山區速度有加快情形。海風環流在鋒前地面平均風速約僅有1m/s，在鋒後平均風速可以達4-5m/s。海風環流接觸到地形後不論是地面測站或是SPOL雷達都無法有效觀測它的存在。利用SPOL觀測資料，本文特別探討海風環流結構隨時間演變的一些特性。在鋒前、鋒上、和鋒後由於大氣穩定度差異，造成SPOL觀測風場結果有很大不同。鋒前以垂直混合流體為主，鋒後以成層流體為主，鋒上則介於兩者之間。鋒上回波可達5-15dBZ，在水平形成細線回波，可以作為海風環流前緣的指標，其垂直剖面形狀類似重力流。鋒前近地面1公里由原本平均為離岸流（東風）轉變成向岸流（西風）約需一個半小時（由9:00到10:30am），之後西風層有明顯抬高情形，但是風速都不大，顯示對流混合邊界層發展特性。鋒後則明顯為兩層流體型式，低層為西風，高層為東風。低層西風厚度由原本600公尺增厚至1000公尺，速度明顯增強。東風層抬高趨勢更為顯著，由原本1-2公里抬高至1.5-3公里，顯示較高層環流也受到海風進入的影響。海風環流在激發雷暴前，大氣邊界層內主要雷達回波反射物屬於生物（昆蟲或鳥類），並非天氣降雨回波，也是我們通稱的晴空回波。本文同時也探討利用SPOL觀測之雙偏極化參數處理晴空回波的方法與限制。

關鍵字

西南氣流實驗 ； 雙偏極化參數 ； 晴空回波 ； 海風環流

並列摘要

On 20 June 2008, under the influence of Western Pacific subtropical high, a multi-cellular thunderstorm developed over Pingtung plain and brought pronounced local heavy rainfall and lightning. In this study, atmospheric boundary layer evolution and sea breeze circulation were studied using data collected during intensive field observational program SoWMEX/TiMREX. Three-hourly soundings were used to reveal the evolution of convective boundary layer over the Pingtung plain. The mesoscale structure and evolution of the sea breeze circulation was investigated and documented using high-temporal surface observations and polarimetric observables obtained by NCAR SPOL. The location and timing of sea breeze front (SBF) in the Pingtung plain is identified by using high-temporal surface station observations. The leading edge of sea breeze circulation has shown pronounced frontal-like structure, including temperature drop, humidity and wind speed increase, and wind direction change. The corresponding SPOL observations are analyzed accordingly. It is demonstrated the clear-air echo signal of SPOL can be very useful to identify the location and movement of the sea breeze circulation with proper clutter filtering. The averaged moving speed of SBF is 2.2m/s, the inflow from ocean can reach 4-5 m/s at height 500m and the return flow can reach 6 m/s at height 1.5km. It becomes difficult to identify SBF after the front moved into the mountain area. The wind field derived from the clear-air echo is consistent with the sounding observations and vertical structure of the sea breeze circulation is described. The flow structures at SBF, pre-SBF and post-SBF show significant differences possibly due to different atmospheric stability conditions. At pre-SBF, the flow was turbulent with pronounced vertical mixing and at post-SBF, the flow revealed two-layer stratified fluid characteristics. The depth of SB circulation increased from 600m to 1000m and the return flow increased from 1-2km to 1.5-3km. Application of polarimetric observables for signal identification and the limitations of using clear-air echo are also discussed in the paper.

並列關鍵字

SoWMEX/TiMREX ； SPOL ； Polarimetric observables ； Clear-air echo ； Sea breeze circulation ； Sea breeze front