Observations of atmospheric density currents using a tethered balloon‐borne turbulence probe system
Observations of atmospheric density currents using a tethered balloon‐borne turbulence probe system
复制标题
使用系留气球湍流探测器系统观测大气密度流
DOI：
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发表时间：
2000
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通讯作者：
Alapworth
中科院分区：
文献类型：
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作者：
Alapworth
一个系留风筝气球与多个仪器探针夹在系留电缆被用来调查大气密度-电流通过三个地点在不同的距离从海岸的前锋。该系统允许在锋面通过时同时对风、温度和湿度三个分量进行高达十个级别的高频测量。这使得能够构建电流及其循环的二维可视化。结果，通过直接观测揭示了各种各样的大气密度流的结构。描述了内环流和不同环境对海流及其运动的影响，以及湍流统计。其中一股洋流可能与沿海的扰动有关，在一个案例中，两股相反的海风似乎在内陆相撞。两个电流穿透表面反转从表面解离。其中一个是顺风飞行，并形成了一个强大的近地表射流。在两种情况下，发现相关的波和孔结构在水流上方的层中行进。在一种情况下，发现环境空气侵入头部区域并在内部对流，而在另一种情况下，空气从头部区域挤出到环境流中。测量阵列能够评估动态方程的不同项，并评估其相对贡献。热力学和动力学变量之间存在显著的相关性。对白天密度流内部通量平衡的研究表明，内部流在维持密度流的帽状逆温中起着重要作用。在界面区域观察到涡流，其中Richardson数显着大于0.25，证实了一些建模研究。在大多数情况下，前缘斜率与理论值雅阁，但在一个顺风情况下，斜率明显小于预测值。内陆洋流被发现比沿海洋流更深，这可能解释了当天晚些时候观察到的速度增加。水汽锋的结构一般不如热锋明显。
A tethered kite balloon with multiple instrumented probes clamped to the tethering cable is used to investigate atmospheric density‐current fronts passing through three sites at varying distances from the coast. The system allows high‐frequency measurements of the three components of wind, temperature and humidity to be made simultaneously at up to ten levels as the front passes through. This enables a two‐dimensional visualization of the current and its circulation to be constructed. As a result the structure of some of the wide variety of atmospheric density currents is revealed by direct observation. The internal circulations and the effects of varying environments on the currents and their movement are described, together with turbulence statistics. One current was associated with what may have been a coastal trapped disturbance, and in one case two opposing sea breezes appear to have collided inland. Two currents penetrating into surface inversions were dissociated from the surface. One of these was travelling downwind and formed a strong near‐surface jet. Associated wave and bore structures were found to be travelling in layers above the currents in two cases. In one case environmental air was found to intrude into the head region and convect internally, while, in another, air from the head region extruded into the environmental flow. The measurement array enabled the different terms of the dynamic equations to be evaluated and their relative contribution assessed. A remarkable correlation is found between thermal and dynamic variables. A study of the internal flux balance of daytime density currents shows that the internal flow plays a significant part in maintaining the capping inversion of the current. Eddies were observed at interfacial regions in which Richardson numbers were significantly greater than 0.25, confirming some modelling studies. Frontal slopes in most cases are in accord with theoretical values but in one downwind case the slope is considerably less than predicted. The inland currents are found to be deeper than coastal currents which may explain the increase in speed observed later in the day. Water vapour frontal structures are generally less marked than thermal fronts.