Buoyancy and the sensible heat flux budget within dense canopies
Buoyancy and the sensible heat flux budget within dense canopies
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DOI：
10.1007/s10546-005-4736-1
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发表时间：
2006-01-01
影响因子：
4.3
通讯作者：
Giostra, U
中科院分区：
文献类型：
--
作者：
Cava, D;Katul, GG;Giostra, U
与大气表面层湍流不同，湍流热通量（F（T））和冠层内平均气温垂直梯度之间的线性关系受到许多因素的影响，包括热源和热汇的局部变化以及大尺度涡动，其特征通常与喷射-扫掠循环有关。此外，大气稳定性如何改变这种关系仍然知之甚少，特别是在稳定的冠层流。到目前为止，还没有明确的模式存在F（T）的平均气温梯度，浮力，和统计特性的喷射扫描周期内的树冠体积。使用三阶累积量展开法（CEM）和热通量预算方程，一个“诊断”的分析关系，连接喷射和扫描和显热通量的大范围的大气稳定度类的推导。封闭模型的假设，涉及标量耗散率与感热通量，并在连接喷射和扫描与三重标量速度相关性的CEM的有效性，进行了测试的混合硬木林在Lavarone，意大利。结果表明，当热源（S（T））和F（T）具有相同的符号（即冠层加热和感热通量为正）时，扫描占主导地位的感热通量。相反，如果S（T）和F（T）的符号相反，标准梯度扩散闭合模型预测，喷射必须占主导地位的感热通量。
In contrast to atmospheric surface-layer (ASL) turbulence, a linear relationship between turbulent heat fluxes (F(T)) and vertical gradients of mean air temperature within canopies is frustrated by numerous factors, including local variation in heat sources and sinks and large-scale eddy motion whose signature is often linked with the ejection-sweep cycle. Furthermore, how atmospheric stability modifies such a relationship remains poorly understood, especially in stable canopy flows. To date, no explicit model exists for relating F(T) to the mean air temperature gradient, buoyancy, and the statistical properties of the ejection-sweep cycle within the canopy volume. Using third-order cumulant expansion methods (CEM) and the heat flux budget equation, a "diagnostic" analytical relationship that links ejections and sweeps and the sensible heat flux for a wide range of atmospheric stability classes is derived. Closure model assumptions that relate scalar dissipation rates with sensible heat flux, and the validity of CEM in linking ejections and sweeps with the triple scalar-velocity correlations, were tested for a mixed hardwood forest in Lavarone, Italy. We showed that when the heat sources (S(T)) and F(T) have the same sign (i.e. the canopy is heating and sensible heat flux is positive), sweeps dominate the sensible heat flux. Conversely, if S(T) and F(T) are opposite in sign, standard gradient-diffusion closure model predict that ejections must dominate the sensible heat flux.