Helicopter-Borne Flux Measurements in the Nocturnal Boundary Layer Over Land – a Case Study
Helicopter-Borne Flux Measurements in the Nocturnal Boundary Layer Over Land – a Case Study
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陆地上空夜间边界层的直升机机载通量测量——案例研究
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
R. Roth
中科院分区：
文献类型：
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作者：
J. Bange;R. Roth
本文介绍了用新的机载湍流测量系统HELIPOD对德国北部夜间边界层湍流通量的测量，详细地分析和检验了仪器的系统误差，并与当地相似理论和以往的实验进行了比较，以证实小垂直湍流通量的存在。所研究的夜间边界层为分析测量系统的质量提供了良好的条件。在这方面，对一个强地面反演的详细研究揭示了小的湍流通量，其光谱最大值在10米波长或更短的波长处，嵌入间歇性湍流中。为了验证这些通量，将测量结果与过去实验的既定结果进行了比较。应用局部相似理论计算了湍流量的无量纲方差，与其它观测结果吻合较好。由于切变和分层变化显着的水平飞行腿由于全球不稳定性，一种方法被开发来确定垂直梯度的水平飞行模式，通过使用测量高度的小波动。有了这些局部确定的梯度，梯度输运理论就变得适用，并且在孤立的强湍流爆发内估计了热量和动量的湍流扩散率、理查森数和通量理查森数。在这些爆发的通量理查森数被发现在0.1和0.2之间。梯度Richardson数和湍流Prandtl数之间的函数关系与过去的实验和大涡模拟的观测结果吻合得很好。分层的垂直湍流交换的影响，已经描述了使用莫宁-奥布霍夫相似性的表层，类似地观察到在非常稳定的分层的整体流时，局部缩放。
This case study introduces measurements of turbulent fluxes in a nocturnal boundary layer in North Germany with the new helicopter-borne turbulence measurement system HELIPOD, a detailed data analysis and examination in regard of systematic errors of the instrument, and some comparison with local similarity theory and experiments of the past, in order to confirm the occurrence of small vertical turbulent fluxes. The examined nocturnal boundary layer offered excellent conditions to analyse the quality of the measurement system. In this connection, a detailed look at a strong ground-based inversion disclosed small turbulent fluxes with a spectral maximum at ten metres wavelength or less, embedded in intermittent turbulence. For verification of these fluxes, the measurements were compared with well established results from past experiments. Local similarity theory was applied to calculate dimensionless variances of the turbulent quantities, which were found in good agreement with other observations. Since shear and stratification varied significantly on the horizontal flight legs due to global intermittency, a method was developed to determine vertical gradients on a horizontal flight pattern, by use of small fluctuations of the measurement height. With these locally determined gradients, gradient transport theory became applicable and the turbulent diffusivities for heat and momentum, the Richardson number, and the flux Richardson number were estimated within isolated strong turbulent outbursts. Within these outbursts the flux Richardson number was found between 0.1 and 0.2. The functional relationship between the gradient Richardson number and the turbulent Prandtl number agreed well with observations in past experiments and large eddy simulation. The impact of the stratification on the vertical turbulent exchange, as already described for the surface layer using Monin–Obukhov similarity, was analogously observed in the very stably stratified bulk flow when local scaling was applied.