利用地面與空載光達進行熱帶高空卷雲之研究; A Study of the High Altitude Tropical Cirrus Clouds using Ground and Space based Lidar

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题名:	利用地面與空載光達進行熱帶高空卷雲之研究;A Study of the High Altitude Tropical Cirrus Clouds using Ground and Space based Lidar
作者:	達斯;Subrata-Kumar Das
贡献者:	物理研究所
关键词:	東風氣流;CALIPSO;卷雲;光達;TEJ;CALIPSO;Lidar;Cirrus clouds
日期:	2011-03-01
上传时间:	2011-06-04 15:56:39 (UTC+8)
出版者:	國立中央大學
摘要:	卷雲在大氣輻射平衡中扮演相當重要的角色，並且卷雲也是氣候系統中一個不確定因子。這也是為何氣候模式在不同地理位置必須正確地參數化卷雲之物理性質。光達的時間和空間解析度已被證實對於量測熱帶卷雲和其結構是相當有用之工具之一。因此我們在中壢地區(24.58 oN, 121.10 oE, 167 m MSL)利用偏振光達對卷雲進行10年（1999-2009）之觀測。本研究以長期觀測資料綜合討論卷雲之特性包括卷雲形成與消散的動力和熱力學因素。我們藉由消偏振比值、lidar ratio、消光係數和光學厚度等參數將卷雲依其季節、幾何、光學和熱等特性表示。卷雲出現在夏季之頻率相較於冬季高，為了瞭解卷雲的幾何和光學特性與溫度之關連，我們利用板橋站之探空氣球每日兩次的溫度資料進行分析討論。也利用光達測量卷雲冰晶的降落速度並結合理論用以估算卷雲冰晶的有效大小。光達所推導的卷雲冰晶粒子之有效大小和光學厚度等與雲中心溫度進行分析探討。我們發現當溫度較低時出現較小的冰晶粒子和較小的光學厚度，這清楚說明雲-輻射間的交互作用。此外我們也想瞭解在對流層頂附近大尺度擾動對卷雲的形成和消散所造成之影響。這些波的擾動可調節對流層頂附近的溫度，並且在這非常低溫的區域也觀測到卷雲的沈降。此外，也利用CALIPSO, Aura MLS and NCEP/NCAR 資料研究熱帶對流層上層水汽的重新分佈在熱帶卷雲形成過程中所扮演的角色。研究結果說明對流層上層水汽由強對流區域到印度半島區域的重新分佈導致熱帶卷雲的形成。並且和亞洲夏季季風有關的熱帶東風氣流（tropical easterly jet）會造成上層水汽的水平傳輸。這些結果讓我們瞭解熱帶東風氣流在對流層上層水汽的重新分佈和熱帶卷雲的形成過程中所扮演的角色。 Cirrus clouds play a major role in the global radiation budget and consider as one of the uncertain components in the climate system. This calls for accurate parameterizations of cirrus clouds physical properties in different geographic locations, which are highly essential for climate modelling. The high temporal and spatial resolutions of lidar have proven to be valuable remote sensing tools to monitor the structure and dynamical processes of upper tropical cirrus clouds. At this juncture, by using a polarization diversity lidar (Nd:YAG, 532 nm), the cirrus clouds have been observed over Chung-Li (24.58 oN, 121.10 oE, 167 m MSL), Taiwan over a period from 1999 to 2009. This work synthesis the characteristics of cirrus clouds with a long term data base. It also focuses on the dynamical and the thermodynamical factors that control their formation and dissipation. Cirrus clouds were characterized in terms of the seasonal, geometrical, optical (viz., depolarization ratio, lidar ratio, extinction coefficient, and optical depth), and thermal properties. The occurrences of cirrus cloud are more in the summer and comparatively less in the winter. In order to characterize the temperature dependence of the optical and geometrical properties of cirrus clouds, we have utilized the temperature information from the radiosonde launched twice daily from the nearby lidar site (nearly 30 km). A theoretical estimation of an effective size of ice crystals in cirrus clouds using fall velocity derived from the lidar measurements has also been attempted. The lidar derived mean effective size of cirrus crystals are also parameterized in terms of the cloud mid-height temperature as well as optical depth. It is found that the size of ice crystals becomes small with the decrease in temperature. This will reflect in the decrease in optical depth, which clearly revealed the manifestation of cloud-radiation interaction. An attempt has been made for the first time to understand the impact of planetary-scale disturbances on the formation and disappearance of cirrus cloud in the vicinity of tropopause (VOT). These wave disturbances greatly modulate the temperature in the VOT and during the cold phase of anomalies, the descending cirrus clouds are observed. In addition, the role of redistribution of the tropical upper tropospheric humidity, which supports the formation of the tropical cirrus clouds, has been investigated by using the CALIPSO, Aura-MLS and NCEP/NCAR reanalysis data. Results show that the redistribution of upper tropospheric humidity from a highly convective zone to the Indian peninsular region leads to the formation of the tropical cirrus. Advection of upper layer humidity is caused by the tropical easterly jet (TEJ) associated with the Asian Summer Monsoon (ASM). Thus, the results bring out for the first time, the role of TEJ in redistribution of the upper tropospheric humidity and consequently, in the formation of tropical cirrus.
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