Impact of coherent eddies on airborne measurements of vertical turbulent fluxes
Impact of coherent eddies on airborne measurements of vertical turbulent fluxes
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相干涡流对垂直湍流通量机载测量的影响
DOI：
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发表时间：
2007
期刊：
影响因子：
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通讯作者：
F. Saïd
中科院分区：
文献类型：
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作者：
M. Lothon;F. Couvreux;S. Donier;F. Guichard;P. Lacarrére;D. Lenschow;J. Noilhan;F. Saïd
在湿季和旱季之间的过渡时期在尼日尔进行的水文-大气试点试验(HAPEX)-萨赫勒期间，两架法国飞机探测了萨赫勒边界层，以测量感热通量和潜热通量。对尼亚美地区的测量经常揭示出几公里尺度的有组织结构，这些结构与热侵入和干侵入有关。我们使用一天的飞机测量通量和当天的中尺度模式数值模拟来研究这些相干结构的影响。高水平分辨率（250米）的数值模拟包含了从清晨的条纹到中午的细胞结构。该模拟显示的分布、方差和偏度与观测值相似。特别是，数值模拟显示干侵入可以深入大气边界层（ABL），在某些情况下甚至到达地表，这与观测到的高度负偏斜的水汽波动一致。干侵入体和在几公里尺度上组织的热流给出了偏斜的通量统计，并可能在测量的通量中引入很大的误差。我们使用数值模拟：(i)评估有组织结构对总通量的贡献，以及（ii）估计有组织结构对飞机一维采样（与二维数值模拟估计相反）产生的系统和随机误差的影响。我们发现有组织的结构对总分解通量有显著的贡献。当卷筒发生时，当支腿长度约为ABL深度的30倍时，当一维采样方向与卷筒主轴方向相同时，一维采样通量有时比相应的二维通量低20%，而当支腿方向与卷筒横向时，系统误差要低得多。在细胞的情况下，由于对含能量尺度的采样不佳，使用与方向无关的一维方法仍然可以观察到约10%的低估。
During the Hydrological-Atmospheric Pilot Experiment (HAPEX)-Sahel, which took place in Niger in the transitional period between the wet and dry seasons, two French aircraft probed the Sahelian boundary layer to measure sensible and latent heat fluxes. The measurements over the Niamey area often revealed organised structures of a few km scale that were associated with both thermals and dry intrusions. We study the impact of these coherent structures using a single day’s aircraft-measured fluxes and a numerical simulation of that day with a mesoscale model. The numerical simulation at high horizontal resolution (250 m) contains structures that evolve from streaks in the early morning to cells by noon. This simulation shows distribution, variance and skewness similar to the observations. In particular, the numerical simulation shows dry intrusions that can penetrate deeply into the atmospheric boundary layer (ABL), and even reach the surface in some cases, which is in accordance with the observed highly negatively skewed water vapour fluctuations. Dry intrusions and thermals organised at a few km scale give skewed flux statistics and can introduce large errors in measured fluxes. We use the numerical simulation to: (i) evaluate the contribution of the organised structures to the total flux, and (ii) estimate the impact of the organised structures on the systematic and random errors resulting from the 1D sampling of the aircraft as opposed to the 2D numerical simulation estimate. We find a significant contribution by the organised structures to the total resolved fluxes. When rolls occur, and for a leg length of about 30 times the ABL depth, the 1D sampled flux is shown to be sometimes 20% lower than the corresponding 2D flux when the 1D sampling direction is the same as the main axis of the rolls, whereas the systematic error is much lower when the direction of the leg is transverse to the rolls. In the case of cells, an underestimate of around 10% can still be observed with the 1D approach independent of direction, due to poor sampling of the energy-containing scales.