從不同海拔高地之向下太陽輻射通量觀測估計雲輻射驅動力

The Estimation of Cloud Radiative Forcing from Different Level DSI Observations at Highland

周立人(Lee-Jen Chou)

指導教授 ： 林博雄

國立臺灣大學/理學院/大氣科學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02688

摘要

雲與氣膠對於向下太陽輻射通量(downward solar irradiance, DSI)的衰減，是氣候系統能量收支的重要環節。本研究利用同一區域內(45 km × 15 km)不同海拔高度的地面DSI觀測資料、雲量和雲狀觀測，仔細篩選出晴空條件個案及有雲條件個案，用以分析和估計該地區地面短波雲輻射驅動力(surface solar cloud radiative forcing, SSCRF) 的月際與年際變化。我們首先探討中央氣象局所屬日月潭、阿里山、玉山測站雲量資料之可用性評估，結果顯示雲量呈現隨海拔增加而減少、夏季大於冬季、以及雲量逐月變化三測站類似等合理結果。此外，我們也對月平均雲量與日照率的相關性加以分析，結果顯示負相關係數均達 -0.97以上，三測站的線性趨勢亦相當接近，顯示雲量主觀觀測與日照率儀器觀測特性一致。在輻射資料檢核方面，DSI與雲量的線性迴歸方程式斜率，用以代表單位雲量對DSI的影響，結果呈現冬季斜率較小，夏季斜率較大。
本研究進一步利用阿里山測站資料，經資料檢定、篩選及比對驗證後，得到逐月晴空條件個案共25個，最後建立全年各月份晴空條件之日累積DSI基準值，以做為計算SSCRF的基礎。資料分析顯示SSCRF平均值月際變化在 -64.7 Wm-2 ~ -170.4 Wm-2之間，月際之間單位雲量的SSCRF值則介於 -16.9 Wm-2 ~ -28.6 Wm-2，年際平均的SSCRF逐月變化有季節趨勢，呈現夏季大於冬季；相對地，單位雲量的SSCRF數值之月際變化較為穩定；這些數據與Chou and Zhao (1997)和Gautier and Landsfeld (1997）的研究結果相近。我們透過已驗證之晴空個案，做為該太陽日條件下的逐時DSI標準，針對相同太陽日條件下，估計出逐時SSCRF值的變化，並利用雲量、雲種資料，分析出不同雲種對SSCRF值的影響。在過濾雲量、雲重疊效應與環境遮蔽等變因後，得到不同雲種之單位雲量SSCRF值49例。依據中央氣象局測報作業規範之雲屬分類，分析得到積雲個案SSCRF值介於 -20.6 Wm-2 ~ -118.1 Wm-2之間，層積雲個案SSCRF值介於-22.2 Wm-2 ~ -75.3 Wm-2之間，層雲個案SSCRF值介於 -28.1 Wm-2 ~ -94.7 Wm-2之間。
此外，本研究另從台大實驗林所屬微氣象站具有完整DSI資料期間，通過驗證條件的晴空個案共有11個，計算出中午時段DSI垂直梯度共66筆，結果顯示DSI隨高度每1000公尺遞增約7.5 ％，與阿爾卑斯山區的密集觀測結果7-10 ％相近(鄭，1995) 。以2005年3月6日大氣乾淨程度良好個案而言，百公尺DSI垂直梯度為 2.65 Wm-2 ，是2003年11月30日個案 7.71 Wm-2 的1/3，此結果與Iziomon and Mayer (2002)針對德國西南部山區的觀測研究結果相近。

關鍵字

太陽輻射通量 ； 雲輻射驅動力 ； 雲量

並列摘要

The decline of downward solar irradiance (DSI) by Cloud and aerosol is one of the important processes on earth climate radiation budget. In this study, we used the measurements of shortwave DSI and cloud (amounts and types) in a highland area with 15 km × 45 km area and 700 m to 2700 m altitude variation at Taiwan, to estimate the monthly and yearly SSCRF through the DSI deviation between clear-sky cases and cloudy-sky cases. The observed cloud amounts at Son-moon-Lake, Ali-shan, Yu-shan weather stations in our research area were studied first. We found there existed several reasonable phenomena including (1) the inverse trend of cloud amounts and weather stations’ altitude, (2) more cloud amounts in summer than in winter, and (3) similar monthly cloud amounts variations among these stations. Furthermore, we also analyzed the correlation between month-average cloud amounts (from observer) and sunshine duration ratio record (from instrument). It showed that the coefficient of negative correlation R is over -0.97 and the linear regressions at these stations are similar to each other. The linear regression slope, which is explained as DSI decline effect of per cloud amount, was smaller in winter than in summer.
In order to build up the baseline of SSCRF (surface solar cloud radiative forcing) computation in this study, twenty-five DSI cases under whole daily clear-sky at Ali-shan station were selected and processed. Then we got the monthly mean SSCRF with a range of -64.7 Wm-2 to -170.4 Wm-2, and the monthly mean SSCRF per cloud amount with a range of -64.7 Wm-2 to -170.4 Wm-2. These results are compatible to the works done by Chou and Zhao (1997) and Gautier and Landsfeld(1997）.
For discussing the hourly DSI variation with cloud amount and cloud types, forty-nine cases of the cloudy-sky days which had the same Julian days to the SSCRF baseline cases were diagnosed. The factors of cloud multi-layer overlapping and environment shade were excluded carefully, and we found the SSCRF of cumulus is about -20.6 Wm-2 to -118.1 Wm-2, the SSCRF of stratocumulus is about -22.2 Wm-2 to -75.3 Wm-2 , and SSCRF of stratus has the magnitude between -28.1 Wm-2 to -94.7 Wm-2.
This study also chose 11 clear-sky cases from the complete DSI operated by NTU experimental forest, to compute the vertical gradient of DSI at noon time. From sixty-six cases, they showed that the 7.5 ％ vertical decrease per 1000 m on DSI agreed with the result 7-10 ％ from Alpine intense observations (Zheng, 1995). The most clean and clear-sky case on March 6, 2005 provided the lowest 2.65 Wm-2 per 100 m, and this amount is close to the observation result at Germany west-southern highland area by Iziomon and Mayer (2002).

並列關鍵字

solar irradiance ； cloud radiative forcing ； cloud amounts

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