從日變化尺度探討臺灣山區雲霧森林的水文氣候循環及其特殊性

雲霧森林在一天之中經常有雲霧形成，過去研究多著重於定量霧對通量的抑制以及從水收支的角度分析雲霧森林的水文循環。經常性的雲霧是雲霧森林水文氣候重要的日尺度特徵，然而霧和地表通量日變化的關係卻較少被討論。我們從棲蘭的觀測發現不對稱的潛熱通量日變化，其峰值較太陽輻射的峰值提早約兩個小時，而在不會起霧的一般森林(以蓮華池森林為例)卻沒有這個現象。因此本研究欲從日變化尺度討論不對稱的潛熱通量如何反映兩地水文氣候的不同，以及其成因。透過比較棲蘭與蓮華池的觀測資料發現，棲蘭的潛熱通量峰值提早，會造成溫度的日變化幅度較小，配合谷風平流加上當地森林蒸發散量所導致的水氣累積，有利於在下午形成霧。雲霧可作為冠層水的來源，同時，棲蘭豐沛的雨量與夜晚潮濕的環境，使冠層水不易在夜晚蒸發而可維持潮濕至隔天清晨。早晨時，太陽輻射將可觀的冠層水蒸發，造成棲蘭的潛熱通量具有峰值提早的特徵。本研究強調雲霧森林獨特的水文氣候特徵，亦即霧與潛熱通量的相互關聯。不對稱的潛熱通量、較小幅度的溫度日變化、經常性的午後雲霧以及可觀的冠層水，形塑棲蘭獨特的水文氣候循環。可觀的冠層水在棲蘭扮演著影響陸地與大氣交互作用的重要媒介，而降雨型態、溫度、長波輻射可能是控制冠層水量的重要因子。當氣候變遷造成冠層水減少，可能使起霧頻率減小，並影響雲霧森林的水文氣候。雖然雲霧減少是否有利於森林生態系的生長仍有待研究，但對於雨水豐沛的棲蘭而言，霧從能量方面影響植物生長的效應可能比從水量方面顯著。

關鍵字

潛熱通量；冠層水；蒸發；雲霧森林；霧；日變化

並列摘要

In Taiwan’s montane cloud-fog forest, frequent afternoon fog and canopy water are essential to regulate evapotranspiration, also known as latent heat (LH) flux. An asymmetric LH flux with the early peak at 9 a.m. is found in Chi-Lan (CL) montane cloud-fog forest, but this phenomenon cannot be seen in the non-cloud-fog forests (taken LienHuaChih (LHC) forest as an example) from flux tower datasets. Observational results show that the early peak of LH flux in CL may result in a slower increase in near-surface temperature. The small diurnal temperature range plus water vapor accumulation from valley wind and local evapotranspiration makes the air frequently saturated at about 3 p.m., thus favoring fog formation. Then, the canopy can intercept fog water in the afternoon. The wetness is allowed to sustain throughout the night due to high relative humidity, then evaporating the next morning. We further utilized the land surface model to demonstrate the critical role of canopy water in regulating LH flux. The sensitivity tests display that precipitation, temperature, and downward longwave radiation in the atmospheric forcing have positive impacts on the asymmetry of LH flux. In summary, the characteristics of the asymmetric LH flux, small diurnal temperature range, frequent fog occurrence, and sufficient canopy water comprise the unique hydro-climatological cycle in the montane cloud-fog forest

並列關鍵字

latent heat flux ； canopy water ； canopy evaporation ； cloud-fog forest ； fog ； diurnal analysis

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從日變化尺度探討臺灣山區雲霧森林的水文氣候循環及其特殊性

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