大氣穩定度對不同地表類型之可感熱通量擴尺度研究

Abstract

能量通量的平衡和閉合 (energy closure) 是研究地表類型時很重要的特徵和 參數。過去最常用於量測能量通量的方法為渦度相關法 (eddy-covariance method)， 其也被廣泛用於量測生態系的能量、物質之淨交換量。由於其理論假設和限制， 例如單點量測，使他仍有空間代表性不足等缺點。相對於渦度相關法，地面紊流 閃爍測量儀 (surface layer turbulent scintillometer) 應用莫寧-奧布荷夫相似理論 (Monin-Obukhov similarity theory)，偵測步徑內的大氣結構，並計算得步徑距離內 的平均可感熱通量 (sensible heat flux)，為一較大空間尺度的量測結果，此結果可 以作為地表參數、渦度相關法計算過程的參考與修正。本研究於溪頭實驗林中架 設閃爍測量儀，於夏季和冬季各量測一個月；另在關渡自然保護區架設，於冬季 量測三個月。由計算結果得知在不同樣區之間，閃爍測量儀皆較渦度相關法得較 低的可感熱通量值。大氣條件和不同尺度之渦流可能經兩種方法在空間和時間解 析度上的優勢以及平流效應對其量測造成影響。於溪頭樣區之量測顯示在大氣穩 定時，兩種方法之可感熱通量比值約為 0.35，而大氣不穩定時，冬季之量測顯示 閃 爍 測 量 儀 對 渦 度 相 關 法 之 可 感 熱 通 量 比 值 與 穩 定 度 參 數 之 對 數 呈 y=0.76x2-0.56x+1.76 之線性關係，而夏季時則呈 y=1.56x2-2.07x+1.81 的回歸情形。 關渡樣區之可感熱通量比值於大氣穩定時則呈現為 0.9 之定值關係，而大氣不穩定 時則與穩定度參數之對數呈 y=3.25e-0.044x-2.14 之關係。本研究結果比較渦度相關法 與較大空間尺度之閃爍測量儀之結果，期以中尺度現地量測作為單點量測與航測、 遙測等大尺度技術之間的連結，並作為大範圍生態模式的研究基礎。

Abstract Eddy-covariance (EC) system has been used to measure exchange of scalars, momentum and energy components between land surface and atmosphere in the past decades. But the limitations and assumptions of EC method such as being a point-measurement make EC method uncertain over complex terrain. To systematically quantify area-averaged energy fluxes, comparing between energy fluxes measured by EC method and surface-layer scintillometer (SLS) over hilly terrain and estuarial grass marsh is studied in this research. SLS system applied Monin-Obukhov Similarity Theory (MOST) to estimate sensible heat fluxes by linking variances and covariances of laser beam intensity within path-length between laser transmitter and receiver and SLS system was established at Xitou flux tower during summer and winter, and at Guandu Nature Park during winter. Owing to the measurement within path-length, SLS system provides better spatial representative than conventional tower-based EC results. In this study, surface parameters and properties such as friction velocity, Bowen ratio and surface roughness length are investigated to understand the features of turbulence formation in each site. Results suggest the under-estimated sensible heat flux by scintillometer and an explanation to the difference of measurement is established under various atmospheric conditions. In this study, it’s concluded that at Xitou, SLS measures about 0.35 times of sensible heat flux to that by EC under stable state and the correlations y=0.76x2-0.56x+1.76 and y=1.56x2-2.07x+1.81 could describe the relationship between logarithm of stability and H ratio under unstable state in winter and summer respectively. At Guandu, the H ratio is constantly 0.9 under stable state and in correlation y=3.25e-0.044x-2.14 between logarithm of stability and H ratio under unstable state. The relationship between results of EC and SLS could be a linkage among tower-based studies and high-scale methods such as remote sensing and be estimated to be a base to regional micrometeorological modeling.