黄土高原复杂地形上高质量湍流通量数据获取方法

利用兰州大学半干旱气候与环境观测站(Semi-Arid Climate and Environment Observatory of Lanzhou University,SACOL)湍流观测资料,分析了二次坐标旋转(double rotation,DR)、平面拟合(planar fit,PF)和分风区平面拟合(fetch planar fit,FPF)在复杂地形上的适用性,总结出一套适用于SACOL的总体湍流特征参数化方案.经过超声虚温订正、坐标旋转、空气密度脉动订正以及平稳性检验、总体湍流特征检验、总体质量分级处理,摩擦速度(u*)、感热通量、潜热通量、CO_2通量高质量数据所占比例分别为45%~62%、66%~68%、62%~65%、52%~54%.采用DR得到的高质量数据比例与采用PF相比,u*提高了17%,后三种通量略降低2%~3%.PF和FPF两种结果的差别主要体现在u*上,只考虑主导风向数据DR得到的u*质量仍最好.综合兼顾数据质量和计算工作量,在复杂地形上处理湍流观测资料的最优坐标旋转方法是DR.

Method of acquiring high-quality surface turbulent fluxes over the Loess Plateau

The eddy covariance (EC) technique measuring the turbulent exchanges of heat, moisture, CO₂ and momentum between surface and atmosphere has been used widely. However, the use of EC is based on some assumptions which do not exist in practice. So the results are not accurate without necessary corrections. Using data collected at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL), this work studies a method of acquiring high quality surface turbulent flux. In this approach, eddy covariance data processing includes eliminating despikes, coordinate rotations, sonic temperature correction, and WPL correction (correction for density fluctuations). Double rotation and planar fit are used to do coordinate rotations. The fluctuations of sonic temperature include the effect of humidity on the speed of sound, and should be converted into actual temperature. WPL correction was required to latent heat flux and CO₂ flux for the density effects due to heat and water vapor transfer on turbulent flux measurements. Quality control is performed by the stationarity test, integral turbulent characteristics (ITC) test and overall quality is controlled to permit selecting high quality data. The results show that sonic temperature correction decreases sensible heat flux by about 7.3% when using DR, but 5.9% when using PF. The stability can influence the results of sonic temperature correction. WPL increases latent heat flux by 7.4% and decreases CO₂ flux by 72.2%, respectively. Three coordinate rotations have great influence on momentum but little on scalar fluxes. The values of u_* obtained from DR and PF are decreased by 6% and 3%, respectively. Only considering the dominant wind direction, the wind direction has no relationship with the correction of u_* by DR which reduces u_* by 5%. As the turbulent exchange strengthened (u_*> 0.3 m·s^-1), PF and FPF gradually exhibit that the corrected values in southeast are bigger than the northwest area, PF in the southeast wind area increases u_* by 9.23%, yet decreases 3.86% in the northwest area and FPF in southeast wind area increases u_* by 10.09%, yet decreases 1.18% in the northwest area. The differences of the steady state tests between DR and PF are mainly in u_*. A parameterization scheme of DR and PF for SACOL provided in the ITC test works well. The overall quality shows that about 45%~62% of the total data is higher for u_*, 66%~68% for sensible heat flux, 62%~65% for latent flux and 52%~54% for CO₂ flux. The proportion of the high quality of u_* obtained by DR is 17% higher than PF, while that of the latter three kinds of fluxes obtained by PF is 2%~3% higher than DR. The difference between PF and FPF is mainly in u_*. Comparing the three coordinate rotations in the dominant wind direction, DR still obtains the best quality of u_*. The use of DR is recommended in the complex terrain for reducing calculation and improving the data quality.