Development of Estimation Method for Wind Load on Forest Trees

在日本长野县Minami-Minowa村信州大学实验林对日本柏树树冠周围进行了气象观测。我们通过风速廓线和动量通量观测测量了树木的风荷载。结果表明：1)动量通量在林冠高度上是恒定的，在林冠上部被吸收；2)风速峰值出现在林干空间；因此，我们不能忽略树冠下的动量通量。3)森林在相同高度的动量通量是恒定的。利用这些结果和风速廓线，研究了风阻系数与林冠叶面积指数的关系。风速越大，系数越小，冠层的临界值为0.2。由于低估了林冠下动量通量的影响，林下动量通量值小于云杉和杂杉。考虑到以上结果，我们提出了由粗糙度、位移高度、树高和LAI估算阻力系数的方程。我们对三种类型的森林进行了测试，结果在实际水平上是一致的。接下来，我们用树高和LAI来简化这个方程。估计这个系数几乎是足够的。

Meteorological observation around the Japanese cypress canopy was carried out in the experimental forest of Shinshu University, Minami-Minowa village in Nagano prefecture, Japan. We measured wind load on trees from wind speed profile and momentum flux observation. The results are :1)The momentum flux is constant for height over the forest canopy and is absorbed in the upper part of the canopy.2)Peak of wind speed was observed in the trunk space of the forest. Therefore, we cannot neglect the momentum flux beneath the canopy.3)The momentum flux is constant at the same height for the forest.Using these results and wind speed profiles, the relationship between drag coefficient and LAI of the forest canopy was investigated. Larger wind speed makes the coefficient smaller and the critical value is 0.2 for the canopy. The value is smaller than that for Picea abis and Tsuga heterophylla because we underestimated momentum flux under the canopy. Considering the result above, we proposed the equation to estimate the drag coefficient from roughness, displacement height, tree height and LAI.We tested this equation for three types of forests and the results were agreeing in the practical levels. Next, we simplified this equation with tree height and LAI.It is almost sufficient to estimate the coefficient.