Potential vorticity analysis of quasi-biweekly rainfall events over the Yangtze Basin in summer 2014

Abstract

A series of heavy rainfall events occurred over the Yangtze River Valley (YRV) in summer 2014, which were modulated by the 10–20-day quasi-biweekly oscillation (QBWO). Thus, the strongest QBWO cycle for the period 10–24 July was used as a representative case to reveal the dynamical mechanism for the QBWO of the YRV rainfall from the potential vorticity (PV) perspective and based on MERRA-2 reanalysis data. The quasi-biweekly YRV rainfall was found to depend closely on the QBWO of the upper-tropospheric South Asian high (SAH), with the SAH configuration modified by the southward-intruding midlatitude high PV stream along with southwestward-advected high PV, altering the divergent condition over the YRV. Quantitative diagnoses for the anomalous vertical motion demonstrated that, in the wet phase of the QBWO cycle, the upper-tropospheric southward-intruding high PV stream acted as a positive PV advection, while negative PV advection was generated due to the lower-tropospheric southerlies, thereby forming a positive vertical gradient of horizontal PV advection to induce evident isentropic-displacement ascending motion. On the other hand, the southward-intruding high PV stream extended downward to the middle troposphere, causing the isentropic surfaces to become more sloping, thus producing a strong isentropic-gliding ascending component. Subsequently, the stronger diabatic heating-related ascending motion was induced to generate positive rainfall anomalies over the YRV. The opposite situation arose in the dry phase, with weak descending motion in magnitude.摘要2014年夏季长江流域(YRV)发生的多次阶段性强降水事件显著地受10–20天准双周振荡所调控. 代表性振荡过程与合成分析表明, 准双周湿位相主要取决于对流层高层南侵的高位势涡度(PV)与低纬度向西南平流的高PV导致南亚高压形态改变而产生的YRV高空辐散. 高空向南的正PV平流与低层南风输送的负PV平流在YRV上空形成正的PV平流垂直梯度, 激发出等熵面位移的上升运动分量; 高空的高PV同时向中低层伸展, 导致等熵面坡度增大从而增强气块沿等熵面上滑的上升运动分量, 由此产生更强的非绝热有关的上升运动分量. 干位相情况则相反.