LES study of the energy imbalance problem with Eddy covariance fluxes
LES study of the energy imbalance problem with Eddy covariance fluxes
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DOI：
10.1023/b:boun.0000007225.45548.7a
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发表时间：
2004-03-01
影响因子：
4.3
通讯作者：
Watanabe, T
中科院分区：
文献类型：
--
作者：
Kanda, M;Inagaki, A;Watanabe, T
热通量的空间代表性的单塔测量的基础上，和所谓的能量不平衡问题的机制，研究通过数值实验，使用大涡模拟（LES）。LES实验是为白天的大气边界层加热在一个平坦的表面，作为一个最好的情况下，完全免费的传感器误差和现场条件的不确定性。不平衡被定义为网格点处的“湍流”热通量与水平平均的“总”热通量的偏差。本研究的理论和数值结果都表明了单塔测量的局限性和水平分布观测网络的必要性。基于点测量的时间平均“湍流”通量系统地低估了“总”通量（负不平衡）。这是由于湍流有组织结构（TOS）的存在，其时间尺度比热羽流的时间尺度长得多所造成的本地平流效应。TOS模式的时间和空间变化导致速度和温度数据的低频趋势，导致通量估计的大分散。讨论了地转风速、平均时间、观测高度、计算区域大小和分辨率对塔测通量的影响。最后，有人建议，在表面加热弱的不均匀性可能会减少通量估计的负偏差。
The spatial representativeness of heat fluxes on the basis of single-tower measurements, and the mechanism of the so-called energy imbalance problem, are investigated through numerical experiments using large-eddy simulation (LES). LES experiments are done for the daytime atmospheric boundary layer heated over a flat surface, as a best-case scenario completely free of sensor errors and the uncertainties of field conditions. Imbalance is defined as the deviation of the 'turbulent' heat flux at a grid point from the horizontally averaged 'total' heat flux. Both the theoretical and numerical results of the present study suggest the limitation of single-tower measurements and the necessity of horizontally-distributed observation networks.The temporally averaged 'turbulent' flux based on a point measurement systematically underestimates the 'total' flux (negative imbalance). This is attributed to local advection effects caused by the existence of turbulent organized structures (TOS), whose time scale is much longer than that of thermal plumes. The temporal and spatial change of TOS patterns causes low-frequency trends in the velocity and temperature data resulting in large scatter of the flux estimates. The influences of geostrophic wind speed, averaging time, observation height, computational domain size and resolution on tower-measured fluxes are also discussed. Finally, it is suggested that a weak inhomogenity in surface heating may reduce the negative bias of flux estimates.