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不同下垫面大孔径闪烁仪观测数据处理与分析
Results from Measurements of Large Aperture Scintillometer over Different Surfaces
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摘要: 大孔径闪烁仪是近年兴起的测量大尺度（500 m~10 km）地表通量的仪器。北京师范大学等单位分别于2002年、2004年在北京昌平小汤山开展了大孔径闪烁仪短期观测实验, 2006年6月又在北京密云建立了长期观测站。利用这些数据, 对大孔径闪烁仪观测数据进行处理与分析, 结果表明：闪烁仪光径高度和风速是影响观测显热通量的关键因子。当地表粗糙元的高度变化相对于光径高度不可忽略时, 零平面位移需要精确确定。波文比在湿润地表需要准确确定, 而气温、气压和动力学粗糙度则为不敏感因子。计算中所需的大气稳定度可用理查孙数判断, 也可借助日出日落时间或净辐射观测值确定。稳定条件下的普适函数目前无统一表达式, 可采用仪器说明书推荐的函数。通过几个站点闪烁仪观测显热通量与涡动相关仪测量值的比较表明：大孔径闪烁仪在均匀和非均匀地表都能得到合理的显热通量观测值。Abstract: For surface flux measurements, large aperture scintillometer (LAS) has become more and more popular in recent years. Compared with traditional observation techniques, it can measure surface fluxes on a larger scale (500 m-10 km). At present, LAS observation is not common in China. Beijing Normal University, associated with other institutes, has carried out several short term measurements of LAS at Xiao tangshan (Beijing, 2002, 2004). A long-term LAS site has also been constructed at Miyun (Beijing) in June, 2006. Sensible heat flux (H) calculation with the LAS data of above measurements shows that the beam height of LAS and wind speed are sensitive factors for sensible heat flux measurement (HLAs), zeroplane displacement height is crucial unless the beam height is much lager than it, and Bowen ratio needs to be determined accurately over wet surface, while air temperature, air pressure and aerodynamic roughness length are not sensitive for HLAs. There are two key points of HLAS calculation under stable condition. First, there is much less agreement on the form which universal stability function fT should take. In this study, the fT function proposed by Andreas (as in the LAS manual) is used. Further more, since the iteration process of HLAS can not be convergence when low wind speed and very stable conditions appearing at night, ψm≥-5 is specified. Second, the scintillometer is unable to determine the sign of the heat flux. Richardson Number Ri can be used to determine atmospheric stability and fix the sign of HLAS. Besides, the sunrise-sunset time and net radiation could be used for this purpose if there is no wind and air temperature profiles. Therefore, a calculation scheme of 24-hour sensible heat flux observed by LAS is obtained after settling the above two key points. According to the observations mentioned above, LAS can measure surface fluxes both over homogeneous and heterogeneous surfaces. The daily and monthly variation of HLAs is analyzed. And the observation differences between eddy covariance system and LAS, need to be studied further combined with footprint model and energy unbalance of eddy covariance system observation.
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图 1 利用HEC(HLAS1)、梯度理查孙数Ri(HLAS2)、日出日落时间 (HLAS3) 和净辐射 (HLAS4) 确定HLAS符号得到的HLAS与HEC以及净辐射Rn的比较
Fig. 1 Daily variation of HLAS, HEC and Rn with the sign of HLAS determined by the measured HEC(HLAS1), Richardson number (HLAS2), sunrise-sunset time (HLAS3) and net radiation (HLAS4)
图 2 均匀、非均匀地表上HLAS, HEC与净辐射Rn在白天的变化
Fig. 2 Daily variation of HLAS, HEC and Rn over homogeneous, heterogeneous surfaces
表 1 波文比不同取值条件下得到的显热通量HLAS与参考值的比较
Table 1 Statistics of HLAS flux estimated with different β against the reference value
表 2 风速u和空气温度T不同取值条件下得到的显热通量HLAS与参考值的比较
Table 2 Statistics of HLAS estimated under different conditions of u and T against the reference value
表 3 d, z0m和zLAS不同取值条件下得到的显热通量HLAS与参考值的比较
Table 3 Statistics of HLAS estimated under different conditions of d, z0m and zLAS against the reference value
表 4 稳定条件下不同普适函数与fT_Andreas函数得到的HLAS值之间的比较
Table 4 Statistics of HLAS estimated with different fT_functions against the one with fT_Andreas under stable condition
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