LAS测算森林冠层上方温度结构参数的可行性

利用位于河南省济源市的华北低丘山地30年生栓皮栎-侧柏-刺槐人工混交林2010年4月至8月每月连续7d,LAS直接测算的森林冠层上方湍流结构参数,与经过湍流谱方法计算处理的三维超声风速/温度仪的观测数据比较,分析LAS测算低丘山地森林冠层温度湍流结构的可行性。结果表明:水平风速和温度湍流谱都有明显的惯性区出现(斜率-2/3);LAS直接测算的湍流温度结构参数与利用该惯性区的数据计算的结果具有较好的一致性,说明在起伏非均匀下垫面上,采用LAS观测湍流结构的变化情况具有较好的可行性。

The feasibility of using LAS measurements of the turbulence structure parameters of temperature above a forest canopy

The atmospheric boundary layer is the transport pathway of energy, momentum and material and directly influenced by the surface and forced on a timescale of about one hour or less. It is one of the major determining factors in material and energy exchange between forest ecosystems and internal boundary layers. The refractive index structure parameter (C_n²) is used to indicate the turbulence intensity. C_n² is determined by structure parameters of temperature (C_T²) and momentum (C_v²). The eddy-covariance (EC) method is considered as one of standard tools estimating surface fluxes because it can directly measure the energy transporting eddies and no similarity theory must be applied. Although the EC method is straightforward conceptually, there are many shortages and additional corrections that need to be applied. In the late 1970s, the assumption of using the scintillator to measure water and heat fluxes was first proposed. A scintillometer consists of a transmitter and a receiver. The receiver measures intensity fluctuations in the radiation emitted by the transmitter caused by refractive scattering of turbulent eddies in the scintillometer path. The measurement spatial scale of the scintillometer is lager than that of the EC method, making the mean time of fluxes less than one minute. In addition, the scintillometer is sensible to one dominant eddy size and interpolate the rest of the turbulent scales by using a theoretical form of the spectrum, rather than integrating over all measured eddy scales. Raw data was processed as follows: a) the de-spike criterion set to remove signals that were more than twice the standard deviation at half-hour time scale; b) the instantaneous flow was averaged to define a set of mean streamlines, which was set as the x coordinate line, then the fluctuation components of wind and temperature (u', v', w'and T') were calculated; c) spectrum of turbulences was computed by using Fast Fourier transform and Hanning filtering methods; d) the running mean was used to normalize spectrum density and the values of corresponding normalized frequency adopt the middle frequency of this waveband; e) the inertial region was defined inside which the slope of the spectrum curve was -2/3. Based on the turbulent data measured above a 30-year aged mixed plantation canopy in the hilly zone of the North China in the daytime (9:00-15:00) from April to August in 2010, C_T² was measured and calculated using the large aperture scintillometer(LAS) method and compared with the results obtained using three-dimensional sonic anemometers/thermometers. The objective was to investigate the feasibility of using LAS to measure the turbulence structure parameters of temperature above the forest canopy. The results showed that the inertial range could be observed in the spectra of temperature and horizontal component of high-frequency part of wind speed. It was consistent with the -2/3 power law. The C_T² calculated using the LAS method had a high aggrement with the results given by the turbulence spectral method, indicating that it was feasible to use the LAS method to measure turbulence fluxes over the heterogeneous surface. As the LAS measurement has the spatial scale from a few hundred meters up to several kilometers which match relatively well with the scale of satellite remote sensing pixel, it is widely used to validate the results obtianed by satellite remote sensing technology.