Generation of free convection due to changes of the local circulation system
Generation of free convection due to changes of the local circulation system
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DOI：
10.5194/acp-9-8587-2009
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发表时间：
2009-11
影响因子：
6.3
通讯作者：
R. Eigenmann;S. Metzger;T. Foken
中科院分区：
文献类型：
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作者：
R. Eigenmann;S. Metzger;T. Foken
抽象的。利用2007年COPS（Convective and Orographically-induced Precipitation Study）野外活动的涡度相关和Sodar/RASS实验测量数据，研究了德国西南部黑森林Kinzig山谷近地面自由对流条件（FCC）的产生。测量的高质量的湍流通量数据显示，FCC的启动在地面附近的情况下，中等到高的浮力通量和同时发生的风速下降。最小风速-通过Sodar测量可观察到山谷大气的整个垂直延伸-是热诱导山谷风系统的结果，该系统在早上将风向从下风向改变为上风向。然后，浮力在近地面湍流动能的产生中超过剪切占主导地位。当从奥布霍夫长度以上开始的自由对流水平下降到低于音速风速计的水平时，通过稳定性参数（测量高度与奥布霍夫长度之比）检测到这些情况。利用小波变换对流化床反应器中湍流运动尺度的分析表明，流化床反应器中存在大尺度湍流结构。关于整个COPS测量期间，约50%的天发生在早晨的FCC。这些天的潜热和感热的地面通量增强。
Abstract. Eddy-covariance and Sodar/RASS experimental measurement data of the COPS (Convective and Orographically-induced Precipitation Study) field campaign 2007 are used to investigate the generation of near-ground free convection conditions (FCCs) in the Kinzig valley, Black Forest, Southwest Germany. The measured high-quality turbulent flux data revealed that FCCs are initiated near the ground in situations where moderate to high buoyancy fluxes and a simultaneously occurring drop of the wind speed were present. The minimum in wind speed – observable by the Sodar measurements through the whole vertical extension of the valley atmosphere – is the consequence of a thermally-induced valley wind system, which changes its wind direction from down to up-valley winds in the morning hours. Buoyancy then dominates over shear within the production of turbulence kinetic energy near the ground. These situations are detected by the stability parameter (ratio of the measurement height to the Obukhov length) when the level of free convection, which starts above the Obukhov length, drops below that of the sonic anemometer. An analysis of the scales of turbulent motions during FCCs using wavelet transform shows the occurrence of large-scale turbulence structures. Regarding the entire COPS measurement period, FCCs in the morning hours occur on about 50% of all days. Enhanced surface fluxes of latent and sensible heat are found on these days.