Mechanism of the energy balance non-closure associated with eddy covariance fluxes

涡度相关法测感热、潜热通量偏低是世界范围内通量观测塔站普遍存在的问题。从阔叶林站点在川越（日本）的数据集的分析表明，显着的不足往往会发生在大净辐射和低风速的条件下。数值流体动力学模拟运行这种条件下表明，一个单点涡协方差测量不捕捉通量的低频分量与准静态对流的空间尺度一样大的边界层深度，从而低估了总通量。观测到的温度和水蒸气的低频波动之间的差异支持这一想法，因为它是一致的模拟结果表明，在边界层的顶部夹带的热和干燥的空气达到通量测量的高度（附近的边界层的底部）由于大尺度对流，并导致低频的异相波动。

Undermeasure of the sensible and latent heat fluxes by the eddy covariance method is a world-wide common problem among the flux-monitoring tower sites. Analysis of a dataset from a broadleaf forest site in Kawagoe (Japan) revealed that significant undermeasure tended to occur under the condition of large net radiation and low wind speed. Numerical fluid dynamics simulations run for such conditions indicated that a single-point eddy-covariance measurement does not capture the flux's low-frequency components associated with quasi-stationary convections of spatial scale as large as the boundary layer depth, and thus underestimates the total flux. Observed dissimilarity between temperature's and water-vapor's low-frequency fluctuations supports this idea because it is consistent with the simulation results showing that hot and dry air entrained at the top of the boundary layer reaches the height of flux measurements (near the bottom of the boundary layer) owing to the large-scale convection and causes out-of-phase fluctuations of low frequencies.