湍流通量计算方法和误差的比较研究

A comparison of the different methods for estimating turbulent fluxes and their errors

近地层的风、温、湿梯度以及辐射和土壤热通量资料在过去的几十年间被广泛地用于计算陆-气感热、潜热通量。然而,不同的计算方法之间存在一些差异,随意的选用可能会造成计算结果存在较大的误差。文中利用绿洲系统能量与水分循环过程观测试验的2005年6月甘肃金塔绿洲中部观测资料,在分析了观测期间近地层微气象特征的基础上,对鲍恩比法、空气动力学法(包含两种不同计算方案)和变分法计算的绿洲农田下垫面的湍流通量进行了比较,并通过敏感性实验和理论分析探讨了不同方法间计算结果的差异和误差的来源。结果表明:不同方法计算的湍流通量特征存在明显差异。鲍恩比法从理论上满足能量平衡关系,但在昼夜交替和夜晚时出现了计算不稳定现象,因此在这些时段不适用。变分法避免了鲍恩比法计算夜间湍流通量的不稳定性,而且二者计算的白天湍流通量有较好的相关性;同时变分法由于加入了辐射和土壤热通量信息,明显改善了空气动力学法白天的能量平衡状况;变分法计算得到的潜热通量对辐射和微气象条件有较为合理的响应;对数据误差的敏感性试验表明,鲍恩比法和空气动力学法应用于绿洲下垫面上对观测误差较为敏感,而变分法相对比较稳定。进一步的分析表明,鲍恩比法在-2<β<0的范围内由于方法本身局限而适用性较差;而空气动力学方法计算结果的差异和误差来源于所应用的相似性函数。因而与鲍恩比法和空气动力学法计算的湍流通量值相比较,变分法的计算结果更加合理和稳定,在绿洲下垫面有较好的适用性。该文研究方法和结果可为这些方法的使用提供参考。

The near-surface gradients of wind speed,temperature and specific humidity combined with the radiative and soil heat fluxes have been widely applied in estimating the surface-air turbulent exchange over decades.However,different methods may produce significant differences and errors in their results.Based on the analyses of the characteristics of micrometeorology in the surface layer,comparisons between the turbulent fluxes estimated by the Bowen ratio method（BRM）,the two aerodynamic methods（AMs） and the variational method（VM） are presented and analyzed.A sensitive study and theoretical analyses are conducted to reveal the sources of the differences between the results from different methods and their errors.The measurements from＂The Oasis System Energy and Water Cycle Field Experiment＂were taken over a farmland underlying surface in the Jinta oasis during a period from June 7 to June 17,2005.The results indicate that,（1） fluxes estimated by the BRM were strongly instable in the nighttime,thus the BRM is not applicable for estimating fluxes over oasis surface at night;（2） in comparison with the BRM,the VM could well restrain the instability for nighttime data,and show a good correlation with those from the BRM for daytime results;at the same time,the VM is found to be more accurate than the aerodynamic methods （AMs） in terms of satisfying the surface energy balance in the daytime,because of its complement of radiative and soil heat fluxes data;（3） time series of latent heat flux estimated by the VM responded more reasonably to the changes of radiation and micrometeorological conditions compared with those by the AMs;（4） the sensitivity tests show that the BRM and the AMs are sensitive to data errors,while the VM is much more stable;（5） further study indicates that the errors of the BRM could be amplified fast for -2〈β〈0,when the Bowen ratio is in the vicinity of -1 and that the accuracies of the AMs depend largely on their similarity functions used.In brief,the VM is found to be more rational and stable compared with the other methods. This study provides useful information for better applications of these methods.