通用风速廓线模型在山区森林的适用性

Applicability of the generalized wind profile model over mountainous forests

基于莫宁-奥布霍夫相似理论的解析方程(风速廓线模型)自20世纪70年代以来一直被用于平坦地形上的风速预测,成熟且准确,但在非平坦地形适用性尚不清楚。其适用性标志着对非平坦地形近地层空气动力学参数(如零平面位移高度及粗糙度)估计的准确性,将决定基于涡度协方差法计算碳水及痕量气体通量中频率修正及其源区分析的准确性。因此,风速廓线模型在非平坦地形的适用性评价是确认通量模型是否需要改进的前提。本研究利用辽东山区清原森林站科尔塔群测量三维风速数据,并与辽河平原盘锦农田站同类数据相比较,评估了以莫宁-奥布霍夫相似理论为基础建立的通用风速廓线模型在非平坦地形上的适用性。结果表明:通用风速廓线模型无法准确预测清原森林站3座通量塔所在地的风速,不适用于复杂地形。在非生长季和生长季,清原森林站3座通量塔实际观测与模型预测风速的决定系数(R2)为0.12~0.30,观测数据与预测值的标准误超过2 m·s^(-1),模型风速约为实际观测风速的2.0倍,显著高估。然而,该模型可准确预测盘锦农田站平坦农田景观的水平风速,实际观测风速与模型预测之间的R2为0.90~0.93,观测数据与预测值之间的标准误仅为0.5 m·s^(-1)。经F检验指出,非平坦地形风速观测值与预测值的均方根误差远大于平坦地形,其中,地形是影响通用风速廓线模型适用性的主要因素,其次是季节性(无/有叶期)因素。风速廓线模型在非平坦地形上不适用,原因是零平面位移被低估,或零平面位移和粗糙度均被低估,进而使空气动力高度估计不准。

The analytical equation based on Monin-Obukhov(M-O)similarity theory(i.e.,wind profile equation)has been adopted since 1970s for using in the prediction of wind vertical profile over flat terrains,which is mature and accurate.However,its applicability over complex terrains remains unknown.This applicability signifies the accuracy of the estimations of aerodynamic parameters for the boundary layer of non-flat terrain,such as zero-displacement height(d)and aerodynamic roughness length(z0),which will determine the accuracy of frequency correction and source area analysis in calculating carbon,water,and trace gas fluxes based on vorticity covariance method.Therefore,the validation of wind profile model in non-flat terrain is the first step to test whether the flux model needs improvement.We measured three-dimensional wind speed data by using the Ker Towers(three towers in a watershed)at Qingyuan Forest CERN in the Mountainous Region of east Liaoning Province,and compared them with data from Panjin Agricultural Station in the Liaohe Plain,to evaluate the applicability of a generalized wind profile model based on the Monin-Obukhov similarity theory on non-flat terrain.The results showed that the generalized wind profile model could not predict wind speeds accurately of three flux towers separately located in different sites,indicating that wind profile model was not suitable for predicting wind speeds in complex terrains.In the leaf-off and leaf-on periods,the coefficient of determination(R2)between observed and predicted wind speeds ranged from 0.12 to 0.30.Compared to measured values,the standard error of the predicted wind speeds was high up to 2 m·s^(-1).The predicted wind speeds were high as twice as field-measured wind speed,indicating substantial overestimation.Nevertheless,this model correctly predicted wind speeds in flat agricultural landscape in Panjin Agricultural Station.The R2 between observed wind speeds and predicted wind speed ranged from 0.90 to 0.93.The standard error between observed and predicted values was only 0.5 m·s^(-1).Results of the F-test showed that the root-mean-square error of the observed and predicted wind speeds in each secondary forest complex terrain was much greater than that in flat agricultural landscape.Terrain was the primary factor affecting the applicability of wind profile model,followed by seasonality(leaf or leafless canopy).The wind profile model was not applicable to the boundary-layer flows over forest canopies in complex terrains,because the d was underestimated or both the d and z0 were underestimated,resulting in inaccurate estimation of aerodynamic height.