基于地基C波段垂直雷达的青藏高原大气边界层湍流特征研究

Turbulence characteristics in the convective boundary layer over the Qingzang plateau observed by a ground-based C-band vertical detection radar

基于2014年7—8月那曲地区不同来源的边界层高度资料(包括C波段调频连续波垂直探测雷达(BLH_(CR))、L波段探空(BLH_(SD))、梯度塔观测及欧洲中心再分析数据集(BLH_(ERA5))),分析了边界层高度(BLH)、不同感热通量(SH)和稳定度(z/L或BLH_(CR)/L)条件下近地层和混合层湍流统计特征及湍流谱。主要结果如下:(1)BLH_(CR)、BLH_(SD)与BLH_(ERA5)互相之间有很好的相关,均方根偏差约为0.6 km。那曲地区BLH_(CR)的日变化较为明显,日较差中值为0.7—0.8 km。在北京时16时BLH_(CR)达最大,样本中值约为1.2 km。(2)随着感热通量增大,近地层垂直速度标准差(σ_(w))、温度标准差(σT)、温度与垂直速度相关系数(R_(wT))逐渐增大,σT增大速率呈线性,σ_(w)和R_(wT)的增大速率在感热通量超过一定阈值后逐渐减小。当-z/L<0.3时,σT和R_(wT)随-z/L增大迅速增大;-z/L≥0.3时增速明显趋缓。(3)混合层内湍流垂直速度方差(σ_(w)^(2))最大值出现在(0.25—0.3)×BLH_(CR)高度,平均值为1.2—1.3 m^(2)/s^(2)。强不稳定时混合层内σ_(w)^(2)略小于弱不稳定;σ_(w)^(2)平均值随感热通量增大而增大。归一化湍流垂直速度方差(σ_(w)^(2)/w_*2)随z/BLH_(CR)增大先迅速增大后逐渐减小,极大值出现在0.35×BLH_(CR)附近。(4)随着不稳定层结的增强,近地层和混合层的归一化垂直速度谱均谱峰左移,谱峰对应的归一化功率谱密度变大。强不稳定时湍流尺度变大、能量增强。

Based on observations of C-band frequency modulated continuous wave radar(BLH_(CR)),L-band radiosonde data(BLH_(SD)),gradient tower data and reanalysis data set(BLH_(ERA5))at Naqu station in July and August 2014,the boundary layer height(BLH),turbulent statistical characteristics and turbulent spectrum in surface layer and mixing layer under different sensible heat fluxes(SH)and stability(z/L or BLH_(CR)/L)conditions are analyzed.The main results are as follows:(1)There are significant correlation among BLH_(CR),BLH_(SD)and BLH_(ERA5),and the root mean square deviations among them are about 0.6 km.The BLH_(CR)have significant daytime variation at Naqu,and the daily range of the median of BLH_(CR)is approximately 0.7—0.8 km.At around 16:00 BT,BLH_(CR)σ_(w),σT RwTreaches its maximum value with a median value of approximately 1.2 km.(2)All of and in surface layer gradually increaseσTσ_(w)RwTwith increased SH.The generally increases linearly,and the increase rates of and gradually decrease when SH exceeds aσT RwTcertain threshold.When-z/L<0.3,both and rapidly increase with the increase of-z/L;when-z/L≥0.3,the increase rateσ2wsignificantly decline.(3)The maximum value of in mixing layer occurs at a height of approximately(0.25—0.3)×BLH_(CR),with anσ2waverage value of approximately 1.2—1.3 m^(2)/s^(2).The in mixing layer under strongly unstable stratification is slightly less than thatσ2wunder weakly unstable stratification;the average value of in mixing layer increases with the increasing of SH.With the increase ofσ2w w2∗z/BLH_(CR),/presents a first rapid increase and then gradually decreases,with the maximum value occurring at around 0.35×BLH_(CR).(4)With the increase of unstable stratification,the peak frequency of the normalized vertical velocity spectrum both in surface layer and mixing layer moves towards low frequency and its corresponding value(normalized power spectral density)increase.Both the scale of the turbulent vortex and turbulent intensity increase for strongly unstable stratification.