Airflows and turbulent flux measurements in mountainous terrain: Part 2: Mesoscale effects
Airflows and turbulent flux measurements in mountainous terrain: Part 2: Mesoscale effects
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山区气流和湍流通量测量：第 2 部分：中尺度效应
DOI：
10.1016/j.agrformet.2004.04.007
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发表时间：
2004
影响因子：
6.2
通讯作者：
R. Monson
中科院分区：
文献类型：
--
作者：
A. Turnipseed;D. E. Anderson;S. Burns;P. Blanken;R. Monson
尼沃特岭Ameriflux站点位于落基山脉内，使其受到山区地形常见气流的影响。在这项研究中，我们研究了这些中尺度特征对当地湍流通量测量的影响，最值得注意的是，形成山谷/山区流动和山区背风侧波。山谷/山区的流动造成局部非平稳的风流造成的背风侧收敛区（LCZ）的通道，其中上坡和下坡流满足附近的测量塔。在2001年6月至8月期间，可能的背风面收敛被标记为所有半小时白天通量测量期间的26%。然而，在这些期间没有明显的通量损失。在一些相对稳定的夏季夜晚，湍流（用σw表示）和标量波动（例如温度和CO2）表现出的周期性似乎与低频重力波的通过一致（τ <20 min）。在垂直速度和标量谱中都观察到了0.0008Hz（20分钟）的谱峰，并表明低频波占总标量协方差的25-50%。在某些强西风时期（主要是在冬季），观测到大型山地重力波形成。通常情况下，通量塔位于下坡“射击流”的区域内，这会产生高湍流，但根据有效的湍流统计数据和几乎完全的能量预算闭合，对局部通量测量没有不利影响。定期，我们发现证据的再循环，转子风的风数据的时间序列，从Ameriflux塔网站和第二个气象网站位于8公里上坡和西方的同时。在2000-2001年冬季的4个月里，我们只检查了14%的半小时时间段，表明可能存在转子风。平均而言，与没有转子的情况相比，在转子发生期间，能量预算关闭减少了20%。从这项研究的结果表明，在这个网站上的本地半小时通量测量的中尺度风流型相对罕见的，但显着的影响的潜力存在。这些事件的发生可以通过检查正常湍流统计参数来检测。
The location of the Niwot Ridge Ameriflux site within the rocky mountains subjects it to airflows which are common in mountainous terrain. In this study, we examine the effects of some of these mesoscale features on local turbulent flux measurements; most notably, the formation of valley/mountain flows and mountain lee-side waves. The valley/mountain flows created local non-stationarities in the wind flow caused by the passage of a lee-side convergence zone (LCZ) in which upslope and downslope flows met in the vicinity of the measurement tower. During June–August, 2001, possible lee-side convergences were flagged for ∼26% of all half-hour daytime flux measurement periods. However, there was no apparent loss of flux during these periods. On some relatively stable, summer nights, turbulence (designated via σw), and scalar fluctuations (temperature and CO2, for example) exhibited periodicities that appeared congruent with passage of low frequency gravity waves (τ ∼ 20min). Spectral peaks at 0.0008Hz (20min) in both vertical velocity and scalar spectra were observed and indicated that 25–50% of the total scalar covariances were accounted for by the low frequency waves. During some periods of strong westerly winds (predominantly in winter), large mountain gravity waves were observed to form. Typically, the flux tower resided within a region of downslope “shooting flow”, which created high turbulence, but had no detrimental effect on local flux measurements based on valid turbulence statistics and nearly complete energy budget closure. Periodically, we found evidence for re-circulating, rotor winds in the simultaneous time series of wind data from the Ameriflux tower site and a second meteorological site situated 8km upslope and to the West. Only 14% of the half-hour time periods that we examined for a 4 month period in the winter of 2000–2001 indicated the possible existence of rotor winds. On average, energy budget closure was ∼20% less during periods with rotor occurrence compared to those without. Results from this study demonstrate that the potential exists for relatively rare, yet significant influences of mesoscale wind flow patterns on the local half-hour flux measurements at this site. Occurrence of these events could be detected through examination of normal turbulence statistical parameters.