Low-level jets and above-canopy drainage as causes of turbulent exchange in the nocturnal boundary layer
Low-level jets and above-canopy drainage as causes of turbulent exchange in the nocturnal boundary layer
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DOI：
10.5194/bg-11-4507-2014
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发表时间：
2014-08
期刊：
影响因子：
4.9
通讯作者：
T. El-Madany;H. Duarte;D. Durden;Bastian Paas;M. J. Deventer;J. Juang;M. Leclerc;O. Klemm
中科院分区：
文献类型：
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作者：
T. El-Madany;H. Duarte;D. Durden;Bastian Paas;M. J. Deventer;J. Juang;M. Leclerc;O. Klemm
在台湾北方复杂地形下，连续17个晚上进行了声纳、涡度相关和塔廓线风速和二氧化碳的测量。该研究的范围是确定间歇性湍流事件的原因，并分析其重要性，在夜间的大气-生物圈交换量化涡度协方差测量。如果在一个测量地点经常发生不稳定性，则需要对这一过程进行量化，以便获得生态系统特征的可靠值，如净生态系统交换或净初级生产。14个事件的间歇性湍流被确定和分类为冠层以上排水流（ACDF）和低空急流（LLJ）根据风速最大值的高度。间歇性湍流持续时间为30和110分钟之间。接近年底的LLJ或ACDF事件，积极的垂直风速，并在某些情况下，上坡流发生，抵消了一般的流态在夜间。观测结果表明，LLJ和ACDF深入山谷中的冷空气池，由于密度差异，它们在那里经历了强大的浮力，导致上坡气流或向上的垂直风。湍流被认为是更强，更好地发展在LLJ和ACDF，涡度协方差数据呈现出更高的质量。垂直风速的频谱分析和垂直风速与水平风分量、温度和二氧化碳结合的时间序列的稳态测试特别表明了这一点。显着较高的通量的感热，潜热，和剪切应力发生在这些时期。在LLJ和ACDF期间，感热、潜热和CO2通量主要是单向的。例如，只有负的感热通量发生时，间歇性湍流。在LLJ和ACDF期间，潜热通量大多为正值，中值为34 W m −2，而在这些时期之外，中值为2 W m −2。总之，间歇性湍流周期表现出强烈的夜间能量和质量通量的影响。
Sodar (SOund Detection And Ranging), eddy-covariance, and tower profile measurements of wind speed and carbon dioxide were performed during 17 consecutive nights in complex terrain in northern Taiwan. The scope of the study was to identify the causes for intermittent turbulence events and to analyze their importance in nocturnal atmosphere–biosphere exchange as quantified with eddy-covariance measurements. If intermittency occurs frequently at a measurement site, then this process needs to be quantified in order to achieve reliable values for ecosystem characteristics such as net ecosystem exchange or net primary production. Fourteen events of intermittent turbulence were identified and classified into above-canopy drainage flows (ACDFs) and low-level jets (LLJs) according to the height of the wind speed maximum. Intermittent turbulence periods lasted between 30 and 110 min. Towards the end of LLJ or ACDF events, positive vertical wind velocities and, in some cases, upslope flows occurred, counteracting the general flow regime at nighttime. The observations suggest that the LLJs and ACDFs penetrate deep into the cold air pool in the valley, where they experience strong buoyancy due to density differences, resulting in either upslope flows or upward vertical winds. Turbulence was found to be stronger and better developed during LLJs and ACDFs, with eddy-covariance data presenting higher quality. This was particularly indicated by spectral analysis of the vertical wind velocity and the steady-state test for the time series of the vertical wind velocity in combination with the horizontal wind component, the temperature, and carbon dioxide. Significantly higher fluxes of sensible heat, latent heat, and shear stress occurred during these periods. During LLJs and ACDFs, fluxes of sensible heat, latent heat, and CO 2 were mostly one-directional. For example, exclusively negative sensible heat fluxes occurred while intermittent turbulence was present. Latent heat fluxes were mostly positive during LLJs and ACDFs, with a median value of 34 W m −2 , while outside these periods the median was 2 W m −2 . In conclusion, intermittent turbulence periods exhibit a strong impact on nocturnal energy and mass fluxes.