• 作者： 蔡清彥; 王時鼎
• 作者服務機構： 國立臺灣大學大氣科學系; 中央氣象局預報中心
• 中文摘要： 本文首先分析強寒潮合成個案，在冷鋒過境臺灣北部前三天至冷鋒過境後一天期間，在30°～50°N及50°～70°N 兩個緯帶上之 500 mb 波動動能的時間變化情形。我們發現在此期間動能變化最顯著的是波數 3 及 6 。波數 3 的動能先從冷鋒過境前約兩天半開始增加，而約從冷鋒過境前一天半至冷鋒過境當時有相對最大值，然後隨時聞遞減。此波數 3 之動能變化幅度在50°～70°N緯帶者較在30°～50°N 者為大。至於波數 6 的動能，則約從冷鋒過境前一天半開始，隨時間顯著增加，其增加幅度在30°～50°N緯帶上最為明顯。另外，波數 1 動能的時間變化大致與波數3者反相位，但變化幅度較小。 本文另外選擇兩個強寒潮個案，計算寒潮發生前後各緯度圈上之波動動能方程式，分析造成波動動能變化的原因。個案A所造成臺灣地區氣溫下降情形，以及波數3和6之動能變化特徵，均與合成個案者相近，是強寒潮的典型個案。我們發現波數3在50°～70°N之初始成長，主要是先由非線性交互作用提供動能所造成的；而在其振幅變大後，波動則大量吸收可用位能，以維持其動能或促成進一步的成長；等到非線性交互作用大量損耗動能後才開始衰減。至於在寒潮冷鋒通過臺灣北部前一天半至冷鋒過境後一天期間，波數6在30°～50°N的消長機制則亦是由非線性交互作用控制其初始成長和動能之減弱；而波動振幅變大時，則由位動能轉換作用維持其相對最大動能值。 個案B所造成臺灣地區氣溫持續下降的期間，則遠較一般寒潮個案為長，例如臺北日最低溫在此個案持續下降七天之久（下降12.8°C）而在一般強寒潮個案則持續兩至三天（平均下降7.2°C）。我們發現波數3及6在30°～60°N仍是，在寒潮前後，具有較顯著之動能變化者。在寒潮冷鋒通過臺灣北部前，此個案波數3及6之動能變化及其物理機制亦均與個案A者相近。但此個案波數3及6，透過非線性交互作用提供動能，而持續成長或維持動能的時問較個案A者長甚多。
• 英文摘要： In this study, we first analyzed the compositetime variations of 500 mb wave kinetic energiesrelated to severe cold surges in Taiwan. Theanalysis was done for 30°N-50°N and 50°N-70°Nlatitude belts in the period of four days includingthree days before the passage of a cold frontthrought northern Taiwan and one day after itspassing. We found that waves of wave numbers3 and 6 have significant time variations duringthis period. The kinetic energy of wavenumber3 in 50°N-70°N latitude belt starts to grow attwo and half days before the passage of a coldfront through northern Taiwan. It then main-tains its maximum value from one and half daysbefore the frontal passage until the frontalpassing time. It starts decreasing afterward.The kinetic energy of wavenumber 3 in 30°N-50°N latitude belt has a similar characteristicsbut smaller amplitude change in time variation.On the other hand, the kinetic energy ofwavenumber 6 in 30°N-50°N starts to grow atone and half days before the passage of a coldfront through northern Taiwan. In addition, thetime variation of the kinetic energy of wavenum-ber 1 is out of phase to that of wavenumber 3.However, its amplitude change is rather small. We then chose two cases of severe coldsurges in Taiwan. For the computation of wavekinetic energy equation and the study of mech-anism responsible for the wave kinetic energychanges. Case A is a typical case. Its charac-teristics in temperature drops and kinetic energychanges of wavenumbers 3 and 6 are similar tothose of the composite cold-surge case. Wefound that the wave of wavenumber 3 in 50°N-70°N grows initially by receiving kinetic energythrough nonliner interaction mechanism, andthen keeps its further growth or maintains itskinetic energy through conversion of availablepotential energy to kinetic energy, finally decaysby loosing energy through nonlinear interac-tions. The initial growth and final decay ofthe wave of wavenumber 6 are also controlledby the nonlinear interaction mechanism. Inaddition, the conversion of available potentialenergy to kinetic energy also maintains thekinetic energy of wavenumber 6 after its reach-ing certain finite amplitude. Case B represents a case of a special longlasting cold surge. For example, the dailyminimum temperature in Taipei decreased con-tinuously for seven days (12.8°C), which is muchlonger than two or three days (7.2°C) for anormal cold surge case. We also found thatwaves of wavenumbers 3 and 6 in 30°N-60°Nare the major time varying components in thiscase. Bofore the passage of the cold frontthrough northern Taiwan, the time variationsand their controlling mechanism of kinetic ener-gies of wavenumbers 3 and 6 are similar tothose of case A. However, the time duration offurther growth or maintenance of finite ampli-tudes of wavenumbers 3 and 6 last much longerthan those in case A. They are also mainlycontributed by the nonlinear interaction mechan-tsm.
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