南海地區季風建立時期對流的日夜變化

Abstract

南中國海夏季風的建立連接了南亞季風與東亞季風兩個系統，南海地區在季風建立期間環境有明顯的變化。研究季風建立期間南海地區的降雨型態與雲特性的變化將有助於瞭解對流雲雨跟環境條件之間的關係，以及亞洲季風轉換的詳細物理過程。。南海地區的降水型態在季風建立前後皆存在著日夜變化的特徵，海陸的不均衡加熱造成海陸風的產生，海上降雨的極值大多出現在上半天，島嶼及陸地上的降水極值大多出現在下半天。在季風建立過後，低層風場轉為西南風，與清晨的陸風在海面上更明顯輻合，進一步加強海上對流發展，且降雨分布的變化大致以北緯十二度為界，主要降水區由北緯十二度以南移往十二度以北。 為了能進一步了解對流的日夜變化在季風建立期間的變化，本研究分析了ERA-Int再分析資料組， TRMM 3B42 v7衛星降水推估資料組，以及結合CloudSat雲剖雷達、CALIPSO光達與MODIS光譜輻射儀的多儀器DARDAR-MASK雲區產品及2B-FLXHR-LIDAR輻射產品，針對南海區域降水日夜循環時空間分布，垂直雲量分布、雲種的與對流雲樹目及尺寸在季風建立前後進行統計。。在季風建立後，降水日夜變化幅度增加與降雨極值出現的時間改變，整個南海海上降水極值出現的時間呈現更一致化、同步的趨勢。雲量方面，北南海低層雲量在季風建立前後差異不大。季風建立前，高雲雲量在夜間明顯少於白天，季風建立後則明顯增加。南南海季風建立後的雲量變化明顯較降雨的變化幅度更大。以不同雲種來看，季風建立後北南海積雨雲雲量在日夜皆明顯增加，其伴隨的捲雲與砧狀雲的雲量也有成長。南南海垂直發展的積雨雲與塔狀積雲在季風建立後呈現減少的趨勢。在季風建立後，北南海對流雲個數與尺寸都有明顯成長，在南南海則減少。季風建立後北南海雲量增加、反射更多短波輻射進入太空，而南南海雲量減少使短波雲輻射效應降低。在季風建立過後，北南海高雲雲量增加所以大氣層頂外逸長波輻射減少，大氣層底由雲向地表放出的長波輻射在白天增加而在夜晚減少。南南海大氣層頂的外逸長波輻射增加被釋放到太空中，大氣層底由雲向地表放出的長波輻射主要在晚間減少。本研究結果顯示南海地區在夏季季風建立前後的對流結構發生改變，未來值得更深入探討相關的物理機制，包括濕度與垂直風風切等環境條件、局部海陸風環流與平均風場的交互作用、地形效應等扮演的角色。

The South China Sea (SCS) summer monsoon is the bridge between the south Asia and East Asia systems. The better understanding of the evolution of precipitation and cloud patterns during monsoon onset period can help to understand the relationships between convection and environment, as well as the physical mechanisms of the Asian monsoon transition. Precipitation over the SCS exhibits significant diurnal variation during both pre- and post-onset periods. The post-onset southwesterly winds and land-sea breeze enhances the convergence at the low level and generates the convection. To better understand the changes of diurnal variation of convection during monsoon onset period, this study analyzed multiple datasets including the reanalysis data, the TRMM precipitation estimates, and the DARDAR-MASK and 2B-FLXHR-LIDAR cloud products. Climatological statistics for analyzed periods were derived for (1) temporal and spatial distribution of precipitation diurnal cycle, (2) the vertical cloud frequency, cloud types, and size of convection clouds, and (3) the cloud radiative effects. The post-onset precipitation diurnal cycle (DC) exhibits enhanced amplitude and the peak time over ocean becomes more synchronized. After monsoon onset, over the northern SCS, the cloud frequency, number, and size of convective cloud shows a significant increase. The cloud radiative effect becomes stronger. Over the southern SCS, the change in cloud frequency is more significant than the change in rainfall. The number and size of convective cloud both decrease. The present results indicate a change in convection organization after the monsoon onset. Future study can be focused on the investigation of the mechanisms of such organization, including the roles of environmental moisture and vertical wind shear, the interactions between local land-sea breeze and mean wind, and effects of topography.