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Isentropic Analysis of Regional Cold Events over Northern China
Advances in Atmospheric Sciences ( IF 6.5 ) Pub Date : 2020-05-13 , DOI: 10.1007/s00376-020-9226-3 Qingyi Liu , Qian Liu , Guixing Chen
Advances in Atmospheric Sciences ( IF 6.5 ) Pub Date : 2020-05-13 , DOI: 10.1007/s00376-020-9226-3 Qingyi Liu , Qian Liu , Guixing Chen
From the perspective of cold air mass (CAM) analysis, we examine the characteristics and mechanisms of regional cold events (RCEs) over northwestern and northeastern China in the past 58 years (1958/59–2015/16). The RCEs in northwestern (northeastern) China are shown to have an average duration of 6.8 (4.7) days with a moderate (sharp) temperature drop. We quantitatively estimate the RCE-related CAM, for the first time, using an isentropic analysis method. Before an RCE in northwestern China, CAM is accumulated in western Siberia with convergent CAM flux under a blocking pattern in the Urals region. During RCE outbreak, CAM penetrates the valleys of the Tianshan-Altay Mountains to the Tarim Basin and Hexi Corridor. The CAM moves slowly because of the blocking pattern and orographic effect, which explains the relatively long duration of RCEs. Comparatively, during RCEs in northeastern China, the CAM depth anomaly originates more to the east and quickly passes the Mongolian Plateau guided by an eastward-moving trough. Diagnostic analyses further show that adiabatic processes play a crucial role in regulating the local change of CAM depth during the two kinds of RCEs. The advection term of adiabatic processes mainly increases the CAM depth during RCE outbreak, while the convergence term increases (reduces) CAM depth before (after) RCE outbreak. Both terms are relatively strong during RCEs in northeastern China, resulting in the rate of change in CAM depth being ∼50% larger than for those in northwestern China. Therefore, the variations of RCEs in duration and intensity can be well explained by the different evolution of CAM depth and flux. 本文从冷空气团质量分析角度, 研究了过去 58 年 (1958–2016年) 冬季中国西北和东北的区域低温事件的活动特征和演变机制. 结果表明: 西北 (东北) 地区的低温事件平均维持 6.8 (4.7) 天, 降温过程相对温和 (剧烈). 基于等熵分析方法, 首次定量估算了与两类区域低温事件有关的冷空气团厚度和流量. 在西北地区的低温事件开始之前, 阻塞高压出现在乌拉尔地区, 冷空气团在西伯利亚的西部地区出现厚度堆积和流量辐合. 冷空气团之后向南穿过天山与阿尔泰山之间的山谷, 进入河西走廊, 导致中国西北出现区域性低温. 受阻塞高压移动缓慢和地形阻挡效应的影响, 冷空气团的移动速度较慢, 导致低温事件在西北地区有着较长的持续时间. 与之相比, 与东北地区低温事件有关的冷空气团的堆积位置偏东, 之后经由蒙古高原快速南下. 在无明显地形阻挡和快速东移的高空槽引导下, 冷空气团的移动速度较快, 造成东北地区较为剧烈的降温. 进一步诊断分析表明, 动力过程主导两类低温事件中的冷空气团厚度的局地变化, 而热力过程的贡献较小. 其中, 水平平流项对低温事件爆发阶段的冷空气团厚度增加起主要贡献, 水平辐散项则在低温事件发生前 (后) 导致冷空气团厚度的增加 (减少). 这两项在东北低温事件过程中都较强, 导致冷空气团厚度在东北地区的局地变化速率比西北地区要快 50% 左右. 由此可见, 冷空气团的厚度及其通量演变在中国东北和西北地区呈现明显差异, 很好地解释了区域低温事件的持续时间和强度.
更新日期：2020-05-13
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