| 摘要: | 位渦與地面位溫在中緯度綜觀尺度系統的動力過程中扮演非常重要的角色。可是單一的等壓面或等熵面分析並無法把位渦與地面位溫的整體貢獻同時呈現出來，因此在研究綜觀尺度天氣系統時，我們通常仍需綜合不同的等壓面或等熵面分析才能對其動力過程有比較完整的瞭解。本研究利用位溫310K作為交界面將對流層大氣簡化為兩層系統。其主要目的是探討層位渦(Layered potential vorticity，LPV) 是否可以比等熵位渦(Isentropic potential vorticity，IPV)更清晰而簡潔的表現出中緯度綜觀尺度天氣系統的特徵與動力過程。 本研究分成兩個主要部份。第一部份是，經由比較2001年一月每天四筆的下對流層位渦、衛星雲圖、1000 hPa的重力位高度場、和TRMM的降水資料，來驗證層位渦是否可以適當的表現出中緯度綜觀尺度天氣系統的強度、位置、發展、移動、和降水的特徵。第二部份是利用上下對流層的局地位渦南北梯度來探討局地斜壓穩定度與綜觀尺度天氣系統的發展的關聯性。結果顯示，下對流層位渦與衛星雲圖、1000 hPa的重力位高度場、或降水都有相當好的一致性。位渦極大值與溫帶氣旋的中心、低壓槽、鋒面和雨帶都配合的很好而低位渦則與高壓脊有明顯的相關。另一方面，經由蒙地卡羅檢驗(Monte Carlo test)得知，局地斜壓穩定度與綜觀尺度天氣系統的強度在統計上具有顯著的相關。也就是說，在不穩定狀態下的系統強度會比穩定時來的強。 本研究的結果顯示，層位渦的確可以比等熵位渦更清晰而簡潔的表現出中緯度綜觀尺度天氣系統的特徵與動力過程。另一方面，我們也發現層位渦也與在熱帶地區的擾動有相當一致的表現。因此層位渦分析是一個研究綜觀尺度擾動的動力過程非常好的工具。 The potential vorticity (PV) on each isentropic surface and potential temperature at the lower boundary play important roles in dynamic process of midlatitude synoptic-scale systems. But single isobaric surface or isentropic surface can not represent both PV and potential temperature at the same time. We need understand the dynamic process by combining variant isobaric or isentropic surfaces when exploring the synoptic-scale system. In this paper, we use 310K isentropic surface as the interface to simplify the atmosphere into a two layer system. The main purpose is to explore whether the layered potential vorticity (LPV) can represent the characteristic and dynamic process of midlatitude synoptic-scale system more clearly and concisely than isentropic potential vorticity (IPV). This study is divided into two parts. The first part is, by comparing LPV of lower troposphere with satellite image, 1000 hPa geopotential height, and TRMM rainfall data, during January 2001, to examine whether LPV can show appropriately the characteristics of intensity, position, growth, movement and precipitation of midlatitude synoptic-scale system. The results clearly shows lower troposphere is consistent with satellite image, 1000 hPa geopotential height, and precipitation. The maximum of PV is well-matched with the extratropical cyclone center, low pressure trough, front and rain belt. Similarly, low PV has salient relation with high pressure ridge. The second part is by computing local meridional PV gradients of upper and lower troposphere to get stability, then comparing the pressure of weather systems in stable and unstable status to explore the connection between the local baroclinic instability and the growth of synoptic-scale system. From Monte Carlo test, local baroclinic instability has highly relationship with the intensity of synoptic-scale system. In other words, the system intensity in unstable status is stronger then stable status. In summary, this study indeed shows LPV can represent the characteristics and dynamic process of midlatitude synoptic-scale system more clearly and concisely than IPV. Besides, we also discover that LPV consistent with perturbation in tropical area pretty well. Thus, LPV is a very good tool in studying the dynamic process of system. |
