利用雲解析模式探討組織性對流對於極端降雨之影響

Abstract

本研究主要探討對流系統的組織結構以及極端降雨在不同大尺度環境下的反應為何，利用具有明確物理過程之三維雲解析模式VVM作為模擬工具，透過GATE(GARP Atlantic Tropical Experiment)探空觀測之大尺度環境外力來改變環境水氣的狀態，另外搭配垂直風切有無的設定進行一系列針對垂直風切作用於不同環境下的敏感性實驗，將模式輸出結果進行統計分析。 研究結果顯示藉由四邊連結分割法所診斷出的雲尺寸大小於垂直高度1.5~5km處受到風切影響最為顯著，突顯出風切作用的垂直範圍和雲尺寸變化的垂直範圍具有一定的相關性。乾濕環境的差異主導雲尺寸發展的大小，同時也影響垂直風切在對流組織作用的程度，雲尺寸在有無垂直風切下的差異於前10%以及1%中特別明顯，顯示前10%雲即有良好的組織結構存在，在1%的分析中，雲尺寸於最濕的環境中透過組織作用甚至能夠發展至8倍。另外雲尺寸頻率分佈也受到垂直風切作用而有所改變，其分布範圍變廣且小尺寸的雲數量所佔的比例增多。 降雨分析方面，前1%的極端降雨強度在垂直風切環境存在下有明顯的增強，此外第99.9百分位的極端降雨相對增加量值於最乾的環境(~60%)相較於最濕的環境(~20%)下更為顯著。為了去探討組織性對流與降雨的關聯性，我們結合核心雲尺寸以及地面降雨進行分析，結果顯示核心雲大小與降雨強度存在著正比的關係，即越大的雲具有越強的降雨。最後在單雲降雨強度的分析結果下則顯示模擬空間內前10%大的雲在有垂直風切下的強度較強，其結果在濕環境中更加明顯，此意味著在具有一定程度的垂直風切作用下，模擬空間中的前10%的核心雲可以視為組織性對流系統，並有利於整體前1%的極端降雨上的增強。 關鍵字: 對流、組織、風切、濕度、極端降雨

Our study focuses on how convective organizations and the associated precipitation extremes respond to various environments. Thus, we use a 3D cloud resolving model VVM with explicit physical process to examine the im-pacts. The model is imposed with sounding data of large-scale forcing observed in GATE (GARP Atlantic Tropical Experiment) to change the environmental moisture. Combining the vertical wind shear, we set a series of idealized sensitivity tests and do statistical analyses with model outputs. Results reveal that a dramatic increasing of cloud size diagnosed by the four-connected segmentation method is shown from 1.5~5km height under each environment, which indicates a strong correlation between wind shear and the cloud size. Environmental moisture is crucial to the cloud size and organization. The difference of size between w/wo wind shear is much larger in 10% and 1% analysis, so the first 10% clouds can be considered as ones with larger and more organized structures. In 1% analysis, the cloud size can be even 8 times larger under the moistest environment. Cloud size distribution is broadened and the ratio of small cloud size increases when vertical wind shear exits. The effect of wind shear on enhancing precipitation becomes significant in the 1% extreme. Besides, relative enhancement at the 99.9 percentile is especially greater in the driest environment (~60%) than the moistest one (~20%). To explore the relationship between cloud size and precipitation strength, we combine core cloud size with surface precipitation and show a proportional relation. Moreover, the precipitation strength of first 10% size-averaged shows a significant enhancement, especially in moistening cases, which implies the first 10% clouds can be considered as organized convections leading to the enhancement of 1% extreme precipitation. Key words: convection, organization, wind shear , moisture, extreme precipitation