On the Long Lasting “C‐Type” Structures in the Sodium Lidargram: The Lifetime of Kelvin‐Helmholtz Billows in the Mesosphere and Lower Thermosphere Region
On the Long Lasting “C‐Type” Structures in the Sodium Lidargram: The Lifetime of Kelvin‐Helmholtz Billows in the Mesosphere and Lower Thermosphere Region
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DOI：
10.1029/2019ja026630
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发表时间：
2019-04
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通讯作者：
S. Mondal;S. Sarkhel;Jay Agarwal;D. Chakrabarty;R. Sekar;T. Yuan;X. Cai;A. Liu;S. Nozawa;N. Saito;T. Kawahara;M. Mlynczak;J. Russell
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作者：
S. Mondal;S. Sarkhel;Jay Agarwal;D. Chakrabarty;R. Sekar;T. Yuan;X. Cai;A. Liu;S. Nozawa;N. Saito;T. Kawahara;M. Mlynczak;J. Russell
为了了解钠（Na）激光雷达图中持久的“C型”结构的特征，分析了来自不同观察位置的6个病例。从低纬度、中纬度和高纬度站点收集的Na激光雷达图显示了C型结构的长寿命，这被认为是中间层和低热层（MLT）区域的开尔文-亥姆霍兹（KH）波的表现。为了探索持久C型结构的特征，利用TIMED卫星上的Na激光雷达仪器和SABER仪器收集的温度廓线，推导了MLT区域Brunt-Väisälä频率平方的高度廓线。在C-型结构区域，所有六种情况下都是正的，这表明这些区域是对流稳定的。同时进行的风测量使我们能够计算三种情况下的理查森数和雷诺数，这表明出现C型结构的区域是动力稳定和非湍流的。本文提出了一个假设，即低温可以使普朗特数的大小增加，对流稳定大气区可以使雷诺数的大小减小。因此，先前产生的KH巨浪的残余物在几乎“冻结”的条件下可以通过这个有利的区域被背景风平流到不同的位置，在那里它们可以持续很长一段时间而没有太大的变形。这些在MLT区域的长期KH波最终将在Na激光图中显示长期的C型结构。
In order to understand the characteristics of long‐lasting “C‐type” structure in the Sodium (Na) lidargram, six cases from different observational locations have been analyzed. The Na lidargram, collected from low‐, middle‐, and high‐latitude sites, show long lifetime of the C‐type structures which is believed to be the manifestation of Kelvin‐Helmholtz (KH) billows in the Mesosphere and Lower Thermosphere (MLT) region. In order to explore the characteristics of the long‐lasting C‐type structures, the altitude profile of square of Brunt‐Väisälä frequency in the MLT region has been derived using the temperature profile collected from the Na lidar instruments and the SABER instrument onboard TIMED satellite. It is found to be positive in the C‐type structure region for all the six cases which indicates that the regions are convectively stable. Simultaneous wind measurements, which allowed us to calculate the Richardson numbers and Reynolds numbers for three cases, suggest that the regions where the C‐type structure appeared were dynamically stable and nonturbulent. This paper brings out a hypothesis wherein the low temperature can increase the magnitude of the Prandtl number and convectively stable atmospheric region can cause the magnitude of Reynolds number to decrease. As a consequence, the remnant of previously generated KH billows in nearly “frozen‐in” condition can be advected through this conducive region to a different location by the background wind where they can sustain for a long time without much deformation. These long‐lived KH billows in the MLT region will eventually manifest the long‐lasting C‐type structures in the Na lidargram.