坐标旋转订正对农田林网水热通量测算精度的影响

农田防护林的特殊结构使附近农田地表大气运动形成了特有的动力学和热力学效应,影响着周边物质和能量交换过程。坐标旋转订正方法的选择成为提高农田防护林水热通量测算精度的核心问题,选取2012年3月9日至5月7日共60 d的防护林网内农田的涡动相关观测数据,对相关湍流特征及能量闭合率等关键参数进行坐标旋转订正并进行比较,结果表明:(1)在整个试验过程中,采用平面拟合法进行坐标旋转时,当订正参数的计算周期小于15 d时,x-z平面内旋转角度有一定的波动,而在计算周期大于15 d后,则基本维持在-2.8°左右。采用二次旋转法订正的x-z平面内旋转角度受原始水平风向的影响始终很大;(2)2种坐标旋转方法对于三维风速的处理结果基本一致,在风速较大的情况下,需要考虑坐标旋转方法的选择对结果的影响。二次旋转方法订正后的垂直风速全部为零,平面拟合方法订正后的垂直风速接近于0;(3)平面拟合订正后摩擦风速基本与订正前一致,而二次旋转方法订正后的结果增大了将近5%;(4)平面拟合方法对显热通量及潜热通量的订正误差较小,二次旋转方法则分别将原始测得值提高了5%左右;(5)防护林网通量的能量闭合率经过平面拟合订正后提高了17%,而经过二次旋转法订正后则提高了30%。总之,坐标旋转方法可以对观测设备倾斜引起的误差进行订正,订正后的结果比订正前更精确。二次坐标旋转方法因为可以订正较短时间段内的单组数据,比较适用于下垫面情况变化较快时的观测;平面拟合方法适用于观测系统及其周围环境在整个测量过程中相对稳定时的观测。二次旋转方法对防护林网水热通量订正更好。

Effects of Coordinate Rotation on Precision of Heat and Latent Fluxes Estimating in Farmland Shelterbelts

The nearby farmland surface atmospheric motion forms the unique kinetic and thermodynamic effects by the special structure of shelterbelt. The unique effects contribute to the surrounding material and energy exchange process, so the selection of coordinate rotation method is the key to improve the accuracy of shelterbelts flux. This paper focuses on the effect of coordinate system on the eddy fluxes. Based on the data observed over the farmland shelterbelts in North China from March 9 to May 7 in 2012, the effects of two coordinate system transforming methods (Double Rotation-DR, and Planar Fit-PF) on the turbulent fluxes were analyzed with the key parameters of the turbulence characteristics and energy closure rate. It showed that: (1) The planar fit method utilizing many runs data to get the rotation angle of the x-z plane was from -2.5° to -3.0° when the calculation period was less than 15 days, and the angle almost kept invariant when the calculation period was more than 15 days. The rotation angle of the y-z plane was 0.15° during the experiments. The rotation angle of x-z plane with double rotation method was mainly determined by original horizontal wind. The change of the rotation angle was relatively large. (2) The composed velocities corrected by the two methods were almost the same; but the vertical velocities corrected by the two methods were different, and the vertical velocity corrected by double rotation methods was zero and that corrected by planar fit method was close to zero but unequal to zero. When the wind speed was large, it needed to consider the impact of selecting coordinate rotation method of the results. (3) It reflected the true vertical velocity after correction by planar fit method. The friction velocity corrected by the planar fit method was almost as the same as the corresponding result without correction individually, and those derived from double rotation method were 105% of the corresponding result without correction. The relative differences of friction velocities corrected by the two methods were investigated. The result showed that the stress correction error caused by double rotation method increased with the decreasing of velocity because of the mean velocity and the lateral stress sampling errors; (4) The heat and latent fluxes corrected by double rotation method were 106% and 104% of the corresponding result without correction individually, and those derived from Planar fit method were almost as the same as the corresponding result without correction. (5) The energy closure rate of shelterbelt corrected by the planar fit method was improved by 17% compared with the corresponding results without correction, and that derived from double rotation method was improved by 30% compared with the corresponding result without correction. In a word, the errors and uncertainties caused by the tilting observation device could be corrected with the effects of two coordinate system transforming methods. The double rotation method was suitable for the observation period with situation of underlying changed rapidly, and the planar fit method was suitable for observation period that the observing system and its surrounding environment was relatively stable throughout the measurement process.