The Forced Secondary Circulation of the Mei-yu Front

Abstract

Using National Centers for Environmental Prediction reanalysis data for the period 28 June to 12 July during 2001 to 2013, the secondary circulation (SC) associated with the mei-yu front was quantitatively diagnosed by numerically solving a primitive version of the Sawyer-Eliassen equation. Results demonstrate that a direct SC exists near the mei-yu front zone during mid-summer and the synoptic-scale geostrophic deformations are the main factors determining SC structures. About 94% of the sinking strength and 61% of the ascending strength in the SC are induced by the geostrophic deformations. Other terms, such as diabatic heating, ageostrophic dynamical forcing, and frictional forcing, mainly influence the fine flow pattern of the SC. The forced SC produces a frontogenesis area tilting to the north with altitude. Further diagnosis clarifies the positive feedback involving the geostrophic shear forcing and vorticity frontogenesis in the upper-level mei-yu front zone. Furthermore, statistical results indicate that all 34 deep convection cases that occurred in the warm region of the meiyu front over the period 2004–2013 experienced high-level frontogenesis associated with along-jet cold advection. The cyclonic shear forcing “moved” the monsoon SC’s subsidence branch to the warm side of the mei-yu front and caused the subsidence branch to extend downwards to the lower troposphere, conducive to the initiation of deep convection in the warm region of the mei-yu front.

摘 要

根据准地转理论, 地转风的水平汇流和切变、 非绝热加热、 边界层摩擦辐合、 次网格湍流混合会在高空锋的冷区和暖区分别强迫出上升和下沉运动, 构成垂直环流圈, 称为次级环流. 在一定的条件下, 次级环流可以促进锋生, 并提供天气尺度抬升条件触发对流与降水. 对于次级环流的研究多集中于天气和次天气尺度锋区附近. 梅雨锋作为次季节尺度准静止季风锋, 是否会通过热动力过程强迫出次级环流? 如果有次级环流出现, 它们又会对梅雨锋产生怎样的影响, 这是本文研究的动机. 我们应用 2001–2013 年的逐 6 小时再分析数据, 采用基于热成风平衡假设的原始形式的 Sawyer–Eliassen 方程定量诊断发现, 在梅雨锋区北侧存在一个平均态的热力直接次级环流. 该次级环流的上升支和下沉支主要是由高空大尺度地转变形场强迫生成的, 而潜热加热、 非地转过程、 摩擦作用只是次要强迫因子. 进一步的诊断分析揭示, 高空地转切变强迫次级环流加强并向暖区低空伸展, 暖区内的垂直运动梯度增强促进锋生, 而锋生过程又加强了高空地转切变, 从而形成了正反馈过程, 次级环流在其中起到了媒介作用. 统计分析的结果显示, 在 2004–2013 年发生在梅雨锋暖区的深对流事件均与这种正反馈的锋生过程有关, 说明次级环流的产生是促进梅雨锋降水的重要热动力过程. 同时, 研究发现与次级环流相关联的锋生区随高度相北倾斜, 这也为梅雨区大气中的对称不稳定的发生提供了适宜条件.