Nighttime free convection characteristics within a plant canopy
Nighttime free convection characteristics within a plant canopy
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植物冠层内夜间自由对流特性
DOI：
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发表时间：
1994
期刊：
影响因子：
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通讯作者：
R. El
中科院分区：
文献类型：
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作者：
A. Jacobs;J. H. Boxel;R. El
在玉米行冠层内和上方进行了密集的测量活动，以调查该植被内的流动特性。注意寻找适当的尺度参数的冠层内风速和气温廓线在冠层稳定stabilization.During晴朗和平静的夜晚，冠层内的条件下，搜索器ecanopy状态有很大的不同。与白天相比，风速和温度廓线分别不与冠层以上摩擦速度u* 和定标温度T* 成比例。一个自由对流流动制度的产生，迫使土壤热通量在冠层的地板和冠层的顶部冷却。然而，风速和温度廓线似乎分别与自由对流速度尺度w* 和自由对流温度尺度Tf成比例。冠层内的自由对流状态与自由对流准则Gr> 16Re 2（u*）很好地吻合，其中Gr是格拉肖夫数，Re（u*）是雷诺数，这是一个经常用于技术流动问题的准则。在冠层内自由对流条件下，如果雷诺数是以自由对流速度尺度为基础的，则Grashof数Gr与雷诺数之间存在唯一的关系.在冠层内自由对流条件下，用相对简单的方差技术可以很好地估计冠层内的热量和水汽通量.在这些条件下，格拉肖夫数或瑞利数代表了林冠内湍流动能的量度。
An intensive measurement campaign within and above a maize row canopy was carried out to investigate flow characteristics within this vegetation. Attention was given to finding adequate scaling parameters of the within-canopy windspeed and air temperature profiles under above-canopy stable stratification.During clear and calm nights the within-canopy condition differs considerably from the abovecanopy state. In contrast to the daytime, the windspeed and temperature profiles do not scale with the above-canopy friction velocity,u*, and the scaling temperature,T*, respectively. A free convection flow regime is generated, forced by the soil heat flux at the canopy floor and by cooling at the top of the canopy. However, the windspeed and temperature profiles appear to scale well with the free convective velocity scale,w*, and the free convective temperature scale,Tf, respectively. The free convective state within the canopy agrees well with the free convection criterion Gr>16Re2(u*), where Gr is the Grashof number and Re(u*) the Reynolds number, a criterion often used in technical flow problems. Also it is shown that under within-canopy free convection, there is a unique relation between the Grashof number, Gr, and the Reynolds number if the latter is based on the free convective velocity scale.Under within-canopy free convective conditions, it appears that within the canopy the fluxes of heat and water vapour can be estimated well with the relatively simple variance technique. Under these conditions, the Grashof, or Rayleigh number, represents a measure for the kinetic energy of the turbulence within the canopy.