An Observational and Modeling Study of Mesoscale Air Masses with High Theta-E
An Observational and Modeling Study of Mesoscale Air Masses with High Theta-E
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DOI：
10.1175/mwr-d-17-0389.1
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发表时间：
2017-07
影响因子：
3.2
通讯作者：
Wolfgang Hanft;A. Houston
中科院分区：
文献类型：
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作者：
Wolfgang Hanft;A. Houston
通常，气团边界的冷面由于其相关的负浮力对垂直运动是稳定的。然而，在某些条件下，边界冷侧的空气可以经历一个转变，其中它假设等效的位温和基于表面的对流有效势能高于边界暖侧的气团。由此产生的气团在这里称为具有高theta-e （MAHTE）的中尺度气团。结果来自一项观测和中尺度模拟研究，旨在研究MAHTE的特征和负责MAHTE形成和演化的过程。观测分析的重点是2016年6月20日在堪萨斯州西北部使用Mesonet和Tracker联合收集的MAHTE近地表观测数据。最高的等效位势温度比在暖区观测到的温度高15-20 K，位于边界冷侧2-5 km处。该案例还使用WRF-ARW进行了建模，以检查仅通过观察无法推断的MAHTE形成过程。模型分析表明，等效位温跨界的垂向微分平流对模拟的MAHTE地层具有重要意义。具体来说，暖区较深的垂直混合/平流减少了水分（等效势温），而边界冷侧的垂直运动/混合受到抑制，从而使等效势温在很大程度上得到了完全由日照驱动的增加。模型分析还表明，表面水分通量在模拟的MAHTE地层中并不重要。
Typically, the cool side of an airmass boundary is stable to vertical motions due to its associated negative buoyancy. However, under certain conditions, the air on the cool side of the boundary can undergo a transition wherein it assumes an equivalent potential temperature and surface-based convective available potential energy that are higher than those of the air mass on the warm side of the boundary. The resultant air mass is herein referred to as a mesoscale air mass with high theta-e (MAHTE). Results are presented from an observational and mesoscale modeling study designed to examine MAHTE characteristics and the processes responsible for MAHTE formation and evolution. Observational analysis focuses on near-surface observations of an MAHTE in northwestern Kansas on 20 June 2016 collected with a Combined Mesonet and Tracker. The highest equivalent potential temperature is found to be 15–20 K higher than what was observed in the warm sector and located 2–5 km on the cool side of the boundary. This case was also modeled using WRF-ARW to examine the processes involved in MAHTE formation that could not be inferred through observations alone. Model analysis indicates that differential vertical advection of equivalent potential temperature across the boundary is important for simulated MAHTE formation. Specifically, deeper vertical mixing/advection in the warm sector reduces moisture (equivalent potential temperature), while vertical motion/mixing is suppressed on the cool side of the boundary, thereby allowing largely unmitigated insolation-driven increases in equivalent potential temperature. Model analysis also suggests that surface moisture fluxes were unimportant in simulated MAHTE formation.