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

涡动相关法对感热通量和潜热通量的测量不足是世界范围内通量监测塔场普遍存在的问题。对日本川越地区阔叶林立地数据的分析表明，在净辐射大、风速低的条件下，往往会出现明显的测量不足。在这种条件下进行的数值流体动力学模拟表明，单点涡旋协方差测量不能捕获与边界层深度等空间尺度准平稳对流相关的通量低频分量，因此低估了总通量。观测到的温度和水蒸气低频波动的不一致性支持了这一观点，因为这与模拟结果一致，即在边界层顶部携带的干热空气由于大规模对流而达到通量测量高度（接近边界层底部），并导致低频的非相位波动。

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.