Role of entrainment in surface-atmosphere interactions over the
Role of entrainment in surface-atmosphere interactions over the
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DOI：
10.1029/97jd02236
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发表时间：
1997-12
期刊：
影响因子：
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通讯作者：
K. Davis;D. Lenschow;S. Oncley;C. Kiemle;G. Ehret;A. Giez
中科院分区：
文献类型：
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作者：
K. Davis;D. Lenschow;S. Oncley;C. Kiemle;G. Ehret;A. Giez
本文描述了1994年北方生态系统-大气研究（BOREAS）期间，美国国家大气研究中心（NCAR）的Electra研究飞机观测到的萨斯喀彻温省和马尼托巴省北方森林对流边界层（CBL）的演变过程。所有的观测都是在1530到2230 UT（当地太阳时间0930-1630）之间进行的。研究表明，CBL通量发散通常导致CBL在一天中干燥，在1000-1200 LST的早晨观测到最大的干燥（接近0.5 g kg - 1 h - 1），随着时间的推移，到下午（1500-1600 LST）几乎没有干燥（0-0.1 g kg - 1 h - 1）。最大增温（0.45 Kh−1）也出现在上午，到下午中期略有下降，约为0.4 Kh−1。CBL蒸汽压差（VPD）在一天的过程中增加。这种增加的很大一部分可以用垂直通量散度来解释，尽管水平平流似乎也很重要。我们认为边界层增长、垂直通量辐散和边界层云形成之间存在联系，云活动在正午达到峰值，以响应CBL的快速增长，然后在一天的晚些时候有所减少，以响应CBL变暖和增长减少。我们还看到VPD增加和气孔控制之间的反馈证据。我们使用来自Electra的涡流协方差通量测量值来计算虚拟温度夹带比Ar。计算的平均值为0.08±0.12，略低于通常假设的值0.2，也低于BOREAS的其他估计值。该值对CBL深度的测定非常敏感。我们发现Ar随穿越CBL顶部的平均风跃变的增加而增加。水汽的夹带通量与时间的关系最为密切（负相关）。水汽夹带量与地表通量之比为1.57±0.25。机载激光雷达对CBL顶部的观测显示，CBL顶部的“厚度”比简单理论预计的要小，但与过去的激光雷达观测结果一致。归一化厚度发现具有非常一致的值h¯/h -1/0.116±0.008，其中12例进行了检查。本文给出了一种计算CBL顶部可变性的新方法，并且我们表明，CBL深度的这种方差也随深度缩放，但这种归一化方差的值与过去文献中定义的“厚度”有很大不同。
We present a description of the evolution of the convective boundary layer (CBL) over the boreal forests of Saskatchewan and Manitoba, as observed by the National Center for Atmospheric Research (NCAR) Electra research aircraft during the 1994 Boreal Ecosystem-Atmosphere Study (BOREAS). All observations were made between 1530 and 2230 UT (0930–1630 local solar time (LST)). We show that the CBL flux divergence often led to drying of the CBL over the course of the day, with the greatest drying (approaching 0.5 g kg−1 h−1) observed in the morning, 1000–1200 LST, and decreasing over time to nearly no drying (0–0.1 g kg−1 h−1) by midafternoon (1500–1600 LST). The maximum warming (0.45 K h−1 ) also occurred in the morning and decreased slightly to about 0.4 Kh−1 by midafternoon. The CBL vapor pressure deficit (VPD) increased over the course of the day. A significant portion of this increase can be explained by the vertical flux divergence, though horizontal advection also appears to be important. We suggest a linkage among boundary layer growth, the vertical flux divergences, and boundary layer cloud formation, with cloud activity peaking at midday in response to rapid CBL growth, then decreasing somewhat later in the day in response to CBL warming and decreased growth. We also see evidence of feedback between increasing VPD and stomatal control. We use eddy-covariance flux measurements from the Electra to compute the virtual temperature entrainment ratio Ar. The computed mean value of 0.08±0.12 is somewhat lower than the commonly assumed value of 0.2, as well as with other estimates from BOREAS. This value is very sensitive to the determination of CBL depth. We find that Ar increases with an increasing jump in mean wind across the CBL top. The entrainment flux of water vapor is found to be most dependent on time of day (negative correlation). The ratio of entrainment to surface flux of water vapor is 1.57±0.25. Airborne lidar observations of the CBL top reveal a CBL top “thickness” that is smaller than would be expected from simple theory but consistent with past lidar observations. The normalized thickness is found to have a very consistent value h¯/h0-1/0.116±0.008, where 12 cases were examined. A new method of computing the variability of the CBL top is illustrated, and we show that this variance in the CBL depth also scales with the depth but that the value of this normalized variance differs substantially from the “thickness” defined in past literature.