Exchange coefficients over agricultural land surface

農耕地と大気間における運動量や熱・水蒸気等の輸送過程を理解することは，作物生育に適した環境の保全や，周辺の微気候に与える影響を予測する上で大変重要である．本解説では農耕地に焦点を当て，大気陸面間で交換される運動量や熱・水蒸気等を定量的に評価するために欠かせない交換係数について，定義や特性，導出方法を含めて紹介する．交換係数の決定要素は，地表面の粗度とゼロ面変位，そして大気の安定度である．農耕地において，地表面の粗度とゼロ面変位は植生高と葉面積指数の関数として表すことが可能である．ただし，水面のように滑らかな地表面や微風時の交換係数の扱いには注意が必要である．Understanding the transport processes of momentum, heat and water vapor between the agricultural land surface and the atmosphere is highly important for controlling cultivation environments and predicting surrounding microclimate. I introduce the exchange coefficients of momentum, heat and water vapor between the agricultural land surface and the atmosphere, including their definitions, characteristics and derivation methods. The determinant factors of the exchange coefficients are roughness lengths, zero-plane displacement height, and stability of the atmosphere. In the case of agricultural land surface, the roughness lengths and the zero-plane displacement height can be expressed as a function of vegetation height and leaf area index. It must be noted that the exchange coefficients of smooth surface and/or under natural convective conditions have characteristics different from other conditions