臺灣高山地區雪季微氣象觀測與模擬

Micro-scale Meteorological Observation and Simulation During Snow Season at Taiwan

張譯心(Yi-Shin Jang)

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(2012年)

https://doi.org/10.6342/NTU.2012.02846

摘要

地表積雪是全球環流模式針對氣候模擬的過程中，相當重要的參數。先前的研究指出地表積雪是一層相當有效的阻隔層，減緩了陸氣之間的熱通量傳遞，並延遲了降水進入地表逕流和土壤的時間，可將積雪視作高山地區的「延遲性降水」；除此之外，融雪過程對潛熱的吸收使地表溫度長時間維持在0℃；因此，雪對於高山生態系是一個相當重要的環境因子。
臺灣雖位處於副熱帶地區，但在三千公尺以上的高山，到了冬季只要獲得充足水氣就有機會發生降雪，甚至累積可觀的積雪。本文利用「雪山生態系整合計畫」於雪山東稜沿線所架設的兩座微型氣象站(2010年至今)，分別位於海拔3,600公尺地表裸露的雪山圈谷和海拔3,400公尺受到臺灣冷杉遮蔽的黑森林，並藉由雪深尺界定出2012年1月至3月重要降雪事件的時段。由觀測結果可看出積雪層的確減緩了陸地和大氣之間熱量的交互作用，抑制了地表及土壤溫度的變化，且暫時阻斷了土壤水分的來源，使土壤水含量逐漸降低。積雪融化時，草溫維持在融點0℃、融雪注入土壤使水含量上升、陸氣交互作用恢復、地溫和草溫開始隨氣溫振盪變化。
降雪會造成可感熱通量驟降，受到森林遮蔽的影響，雪山黑森林站輻射收支和積雪深度皆未達圈谷的二分之一，雖然主要驅動地表積雪融化的淨日輻射通量較低，但受到森林的保溫作用，使融雪的過程較圈谷顯著且持續，潛熱通量逐漸增加。而位處不同緯度的雪山、合歡山主峰微氣象站(2011年12月至2012年5月)和玉山北峰氣象觀測站(1960年至今)，降雪事件雖然同步發生，但隨著緯度上升，積雪深度逐漸增加。
本文更透過遙測(Remote Sensing)資料了瞭解全臺灣的降雪分布；選用2010年和2011年各一個時段的衛星遙測資料，分別利用MODIS衛星光譜資料的Snow Cover Channel及SPOT衛星影像所計算的出的Normalized Difference Water Index (NDWI)，來和雪山圈谷相同時間的音波雪深計的測量結果進行校驗。在這兩個時段中，SPOT衛星影像資料計算出NDWI所推估的降雪面積，不論是春季雪山圈谷的殘雪，或是雪季時遍布聖稜線的積雪，表現都相當好；反之，MODIS衛星光譜資料的Snow Cover Channel受到森林遮蔽的影響，並沒有辦法得到正確的降雪分布。
最後，本文利用一維的陸氣交互作用模式－Simplified Simple Biosphere Model(SSiB)進行土壤水含量及溫度的模擬。藉由輸入地表以上的氣象觀測資料，以及設定地表和土壤條件的初始場，進而評估預判土壤水含量和土壤溫度的可行性。雖然在考慮了一層地表積雪厚的模擬結果和觀測的相關性提高了，但相關係數仍然偏低。未來工作可著重應用本研究所累積的觀測資料於其它陸氣模式(如：NOAH)的校驗。

關鍵字

雪季 ； 微氣象 ； 降雪量 ； 雪山圈谷 ； 雪尺

並列摘要

Snow/ice cover is one of the significant factors in global circulation models on climate simulation. Previous studies have revealed that snow cover is an effective insulator of the soil thermal column and is also the main source of soil water content after melting. In the high latitude or altitude places, the process of summer snow melting provides latent heat flux toward surface and result in prolonged period during which daily mean temperature maintain 0℃. The water equivalent of snow in the high latitude or altitude place is about 25% of annual precipitation there.
In Taiwan, there is less chance to snow, but on the mountain regions with altitude over 3,000m. From September of 2009, we have measured air/grass/soil temperature, soil water content, wind and precipitation at Syueshan Cirque (bare open space at 3600m, SP1) and Black Forest (canopy cover space at 3,400m, SP2). The measurement shows that the snow cover substantially damped temperature variability in winter and hold the source of soil water content on the ground.
During snow falling, the temperature, sensible heat flux (H) and water content decrease sharply. During snow melting, the net radiation is the most component of snow melting heat and the latent heat also increase significantly. Especially, the net radiation at SP1 is twice more than SP2 Site. Therefore, the ratio of snow melting heat at these two sites is up to 3. In addition, the variation of daily average air temperature, net radiation and snow depth at SP2 are smaller than SP1 site.
To know the area of snow, we collected two cases from SPOT satellite and MODIS satellite image. After comparing with observation data, the area of snow defined by NDWI(Normalized Difference Water Index) calculated by SPOT is better than the snow cover of MODIS.
The snow depth data of three weather stations over 3,000m (Hehuanshan, Syueshan and Yushan) in the wintertime of 2012 were collected and compared, the highest depth is in Syueshan. But, there were snow falling signal in every station at the same time. Confirming the different measuring methods is one of our ongoing works.
And we collect the long wave radiation at Hehuanshan for the input parameterization of SSiB (Simplified Simple Biosphere) model. The model simulation will be tested to compare our field measurement at Syueshan Cirque. With one-dimension land-biosphere-atmosphere model, the temperature inter-annual variation can be represented. However, in the preliminarily test, the air temperature is underestimated and the soil wetness and heat flux are not consistent to our measurements. Confirming parameterization skills and initial data setting in SSiB keeps going. Besides, the observation data collected in this research can be compared with the simulation of other land-air interaction model (ex: NOAH, etc.) in the future.

並列關鍵字

snow season ； micro meteorology ； snowfall ； Syueshan Cirque ； snow stick

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