報告書番号

V04010

タイトル（和文）

森林におけるCO2吸収量評価法へのシンチロメータの適用

タイトル（英文）

Application of scintillometer to evaluate CO2 uptake in a forest

概要 （図表や脚注は「報告書全文」に掲載しております）

シンチロメータを適用したCO2フラックス測定手法を開発し，森林におけるシンチロメータの有効性を明らかにするために，森林上に設置したシンチロメータと従来法である渦相関法と比較した。(1)センサ間の消散率の差異は風向によって異なり，シンチロメータの測定パスに直交する風向の場合にシンチロメータによる消散率が渦相関センサによる消散率を大きく上回る傾向が見られた。これは，長い測定パスを持つシンチロメータがより広い領域を起源とする乱流信号の空間平均値を検出したためと考えられた。(2)渦相関センサによる気温，比湿およびCO2密度変動の測定値を参照することでシンチロメータ由来のスカラー消散率を算定する手法を開発した。その結果，シンチロメータはフラックスの過小評価問題を改善させる効果があり，より精度の高いCO2フラックス測定が可能であった。

概要 （英文）

In Kyoto mechanism, the amount of CO2 uptake by a forest can be counted as greenhouse gas reduction. Many studies on turbulent flux measurement over forests have been implemented to evaluate the forest CO2 uptake capacity precisely. However, the eddy-covariance method, which is the conventional flux measurement technique, has problems that tend to underestimate the CO2 flux. Area averaged flux measurement is attractive for evaluating amount of CO2 uptake in a forest. A scintillometer is a turbulence measuring instrument that consists of a laser emitting transmitter and a receiver. It offers advantages: it has a larger footprint because of its long measurement path and it is free from flow distortion resulting from the instrument itself. While flux measurements using scintillometer had been studied on bare soil or grassland, the applicability of scintillometer to forest sites had not been examined. We applied the displaced-beam small aperture scintillometer (DBSAS) to measure turbulent flux 10m above a deciduous forest canopy of 18m height, from June to November, 2002. Scalar fluxes, i.e., sensible heat, latent heat and CO2, were calculated based on the dissipation method by the combination of DBSAS and the eddy-covariance (EC) system. DBSAS tended to overestimate the dissipation rates of turbulent kinetic energy (TKE) and scalar fluctuations against the EC system when wind directions are perpendicular to the DBSAS' path. The footprint differences are attributable to the measurement path-length and the turbulent intensity. Increase of heat flux resulting from an increase of the dissipation rates improve the energy balance issue of heat flux. The balance of heat flux and net radiation is a kind of the index for appropriate flux measurement. The DBSAS can show spatial representative fluxes when the wind direction is perpendicular to the path.

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