赤道印度洋準雙日週期對流擾動：DYNAMO實驗觀測

Abstract

本研究分析2011年十月Dynamics of the Madden-Julian Oscillation （DYNAMO）現地觀測實驗當中，在赤道中印度洋所觀測到發生週期約莫為兩天（準雙日）的西行對流擾動。本研究運用DYNAMO實驗期間的探空觀測、衛星與雷達資料，分析七組準雙日對流擾動個案。 合成分析與波譜分析結果顯示：1、準雙日對流擾動包含波速為10–12 m/s的西行單日週期對流擾動。該對流擾動的強度在中印度洋Gan Island附近、緯向空間尺度約為1000公里的範圍被調整為準雙日週期。2、準雙日對流擾動中的對流活動變化具有典型的中尺度對流生命期特徵，包含顯著的“淺對流－深對流－層狀雲”變化。3、準雙日對流擾動的發生週期，受中尺度對流系統發展的時間尺度與邊界層恢復時間決定，長時間的邊界層恢復將單日週期對流擾動調整為準雙日週期。4、觀測證據顯示在一些準雙日對流擾動個案當中，包含東西向相對移行的雲系，此結果類似Yamada等過去透過印度洋觀測資料所提出的概念模型。 結合以上分析結果、並根據過去在西太平洋與印度洋的觀測，本研究提出一項新的概念模型，從赤道印度洋MJO期間的風切環境及中尺度對流過程的角度，解釋DYNAMO實驗期間所觀測到的準雙日對流擾動發生機制及其特殊發生週期。

This study examines the westward-propagating convective disturbances with quasi-2-day intervals of occurrence identified over Gan Island in the central Indian Ocean from mid to late October 2011 during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign. Atmospheric sounding, satellite, and radar data are used to develop a composite of seven such disturbances. Composites and spectral analyses reveal that: (1) the quasi-2-day convective events comprise westward-propagating diurnal convective disturbances with phase speeds of 10–12 m/s whose amplitudes are modulated on a quasi-2-day time scale on a zonal scale of ~1000 km near the longitudes of Gan; (2) the cloud life cycle of quasi-2-day convective disturbances shows a distinct pattern of tropical cloud population evolution—from shallow-to-deep-to-stratiform convection; (3) the time scales of mesoscale convective system development and boundary layer modulation play essential roles in determining the periodicity of the quasi-2-day convective events; and (4) in some of the quasi-2-day events there is evidence of counter-propagating (westward and eastward) cloud systems along the lines proposed by Yamada et al. Based on these findings, an interpretation is proposed for the mechanisms for the quasi-2-day disturbances observed during DYNAMO that combines concepts from prior studies of this phenomenon over the western Pacific and the Indian Ocean.