臺灣梅雨季鋒面對流渦旋個案尺度交互作用之模擬與診斷研究

Abstract

本研究探討梅雨鋒面與其伴隨之中尺度過程，包含低層噴流、中尺度渦旋以及深對流等多重尺度交互作用下，各尺度在影響渦度貢獻上所扮演的角色。選擇兩個梅雨鋒面個案作為研究，個案一為2003年6月6至7日自華南和南海北部移入臺灣南部近海的四個中尺度對流系統（MCSs），MCSs強度持續增強且向東移，為中南部地區帶來豪（大）雨事件。個案二為2014年5月19至20日受梅雨滯留鋒面影響，於華南附近形成一中尺度對流系統沿鋒前分布排列與發展，並逐漸向東移動至臺灣。使用CReSS模式模擬兩個個案，結果顯示梅雨鋒面及其中尺度對流系統模式結果皆有不錯的掌握，無論地面梅雨鋒面的位置、風場以及中尺度對流系統與觀測空間尺度相符，雖在時間尺度上有30～60分鐘的落後，使24小時累積雨量分布有所差異，但其強度一致。在中尺度渦旋之區域做垂直渦度收支分析結果顯示，在中尺度渦旋最顯著時，個案一局地渦度趨勢項正貢獻為低層扭轉項、中低層渦度輻合及中高層垂直平流項，顯示低層強垂直風切與輻合是渦度增加的原因；個案二北部區域局地渦度趨勢項正貢獻為低層渦度輻散項、中低層扭轉項以及水平平流項，顯示低層輻合與中層強垂直風切是渦度增加的原因之一；南部區域則為整層的渦度輻散項、垂直平流項與水平平流項，顯示低層輻合與垂直上升運動是渦度增加的原因。利用帶通濾波法將兩個案的數值模擬結果做大尺度、中尺度與對流尺度的分離，結果顯示該方法能有效保留個案中各尺度的特徵。尺度分離後渦度收支各項分析顯示，個案一正渦度貢獻為渦度輻散項與扭轉項，各項皆以對流尺度最為重要，中尺度為輔。個案二北部區域正渦度貢獻為渦度輻散項與扭轉項，各項以中尺度加乘對流尺度為主。尤其在中尺度渦旋發展期，渦度輻合項與水平平流項中的對流尺度其值能與中尺度相當，可見深對流胞在空間分布上比例雖少，但提供的正渦度卻不可忽視；南部區域分析顯示，正渦度貢獻為渦度輻散項與渦度垂直平流項。渦度輻散項以對流尺度加乘大尺度；渦度垂直平流項為大尺度加乘中尺度，顯示大尺度環境已有相當程度的背景渦度值，深對流的潛熱釋加強低層輻合與垂直上升運動，可將渦度回饋至大尺度。This study explores the role of each scale in influencing the contribution of vorticity under the interaction of the Meiyu front and its accompanying mesoscale processes, including low-level jet, mesoscale convective vortex (MCV), and deep convection. Case 1 was four mesoscale convective systems (MCSs) that moved from South China and the northern South China Sea to the southern coast of Taiwan from June 6 to 7, 2003. The MCSs continued to intensify and moved eastward, bringing heavy rain to the central and southern Taiwan. Case 2 is affected by the lingering of Meiyu front from May 19 to 20, 2014. MCSs were formed near South China, arranged and developed along the front, and gradually moved eastward to Taiwan. Using the CReSS model to simulate two cases, show that the results of the Meiyu front and the MCSs are well controlled. Regardless of the location of the ground Meiyu front, development and movement of MCS and wind field configuration, they are consistent with the observed spatial scale. Although there is a lag of 30 to 60 minutes on the time scale, which makes the different distribution of daily accumulated rainfall, its intensity is the same.The results of vertical vorticity budget analysis in the region of the MCV show that the positive contributions of the local vorticity trendey term in case 1 are the low level tilting term, the middle-low level vorticity divergence and the middle-high level vertical advection term. It shows that the strong vertical wind shear and convergence in the lower level are the reasons for the increase of vorticity. The positive contribution of the local vorticity trendey term in the northern region of case 2 is the low-level vorticity divergence term, the middle-low level tilting term, and the horizontal advection term, indicating that the low-level convergence and the mid-level strong vertical wind shear for the increase in vorticity. In the southern region display the vorticity divergence term, vertical advection term and horizontal advection term of the whole layer, indicating that the convergence and vertical upward motion of the lower layer are the reasons for the increase of vorticity.The numerical simulation results of the two cases were separated into large-scale, mesoscale and convective scales using the band-pass filtering method. The analysis of vorticity budget after scale separation shows that the positive vorticity contribution of Case 1 is the vorticity divergence term and tilting term, and the convective scale is the most important, secondary by mesoscale. The positive vorticity contribution in the northern region of Case 2 is the vorticity divergence term and the tilting term, and the terms are dominated by the mesoscale synergistic convection scale. Especially in the development period of MCV, the convective scale in the vorticity divergence term and the horizontal advection term can be equivalent to the mesoscale. It can be seen that although the proportion of deep convection cells in the spatial distribution is small, the positive vorticity provided by them cannot be ignored. The analysis of the southern region shows that the positive vorticity contribution is the vorticity divergence term and the vorticity vertical advection term. The vorticity divergence term is the convective scale synergistic the large scale, and the vorticity vertical advection term is the large scale synergistic the mesoscale, which shows that the large-scale environment has a considerable degree of background vorticity value, and the latent heat release of deep convection enhances the low-level convergence and vertical upward movement, which can feedback the vorticity to the large-scale.