青藏高原大气边界层湍流特征的地基雷达遥感研究

Characterization of the turbulence in the atmospheric boundary layer over Tibetan Plateau from observations of vertically pointing radar

中文关键词:  大气边界层高度,高原,频谱分析,湍涡特征

英文关键词:Atmospheric boundary layer height, Tibetan Plateau, Spectrum analysis, Turbulence characteristics

中文摘要:

      资料和方法：基于2014年7月和8月那曲测站C波段调频连续波雷达（FMCW）、L波段探空以及梯度塔数据，目的：讨论和分析了边界层内的湍流特征。来自FMCW雷达的边界层高度(Z-zi)和探空数据的边界层高度(sd-zi)分别由雷达反射率因子（Z）的垂直廓线和理查森数法获得。本研究中也使用了再分析数据集的边界层高度(ERA5-zi)。然后这三个边界层高度（zi）数据集相互比较。也分析了不同感热通量(H)和稳定度(z/L或zi/L)条件下近地层和混合层湍流统计特征以及湍流谱。主要结果与结论如下：Z-zi与sd-zi的相关性略高于与ERA5-zi的相关性，大部分样本的一致性较好，但个别点差异较大。那曲Z-zi的日变化较为明显，Z-zi中值的日较差约为0.7~0.8 km。在北京时(BT) 16:00左右Z-zi达到最大值，样本中值约为1.2 km。随着H的增加，近地层、和RWT都在逐渐增加，增加的速率基本为线性的，而和RWT的增加的速率在H超过一定阈值后逐渐减小。当-z/L < 0.3时，和RWT都随-z/L数值增大而迅速增大；当-z/L > 0.3时增速明显趋缓。总体上混合层的最大值出现在约0.25~0.3zi的高度上，平均值约1.2~1.3 m2/s2。强不稳定时混合层内略小于弱不稳定时的结果。在混合层内的平均值随H的增大而增大。/随z/zi增高先迅速增大后逐渐减小，/极大值出现在0.35zi附近。随着z/L的减小，近地层归一化垂直速度谱谱峰左移，峰值变大。强不稳定时湍涡尺度变大，湍涡能量增强。根据泰勒湍涡冻结假设，估算出垂直方向以及与温度有关的湍涡的空间尺度分别约20 m和75 m。混合层归一化后的湍流垂直速度谱在0.5zi处的湍涡尺度和强度达到最大，其最大的时间尺度和空间尺度分别约10~15分钟和2.5 km。

英文摘要:

      Based on the C-band frequency modulated continuous wave radar (FMCW), L-band radiosonde and gradient tower data at Naqu station in July and August 2014, the characteristics of the turbulence in boundary layer are discussed and analyzed. The boundary layer height from FMCW radar (Z-zi) and radiosonde (sd-zi) are derived from the vertical profile of radar reflectivity factor (Z) and Richardson number method, respectively. The boundary layer height from reanalysis data set (ERA5-zi) is also used in this study. Then these three boundary layer height (zi) data sets are compared with each other. The turbulent statistical characteristics and spectrum of turbulence in surface layer and mixing layer under different sensible heat flux (H) and stability (z/L or zi/L) condition are also analyzed. The main results and conclusions are as follows: The correlation between Z-zi and sd-zi is slightly higher than that with ERA5-zi, with good consistency in most samples, but significant differences in individual points. The daily variation of Z-zi at Naqu is relatively obvious, with a daily range of approximately 0.7~0.8 km for the median of Z-zi. At around 16:00 Beijing time (BT), Z-zi reached its maximum value, with a median value of approximately 1.2 km. As H increases, both 、 and RWT near surface gradually increase, with a linear increasing rate, but the increasing rate of and RWT gradually decrease when H exceeds a certain threshold. When -z/L < 0.3, both and RWT rapidly increase as the -z/L increase; When -z/L > 0.3, the increasing rate significantly slows down. Overall, the maximum value of in the mixed layer occurs at a height of approximately 0.25~0.3zi, with an average value of approximately 1.2~1.3 m2/s2. The result of in the mixed layer under strong instability condition is slightly smaller than that under weak instability condition. The average value of in the mixed layer increases with the increasing of H. As z/zi increases, / first rapidly increases and then gradually decreases, with the maximum value appearing around 0.35zi. As z/L decreases, the peak of the normalized vertical velocity spectrum in the near-surface layer shifts to the left and becomes larger. When the instability is strong, the scale of the turbulent vortex becomes larger and the energy of the turbulent vortex is enhanced. Based on the Taylor turbulent vortex freezing hypothesis, the spatial scales of vertical and temperature dependent turbulent vortex are estimated to be approximately 20 m and 75 m, respectively. The normalized turbulent vertical velocity spectrum in the mixed layer reaches its maximum scale and intensity at 0.5zi, with the maximum time scale and spatial scale of about 10-15 minutes and 2.5 km, respectively.