A numerical study of back-building process in a quasistationary rainband with extreme rainfall over northern Taiwan during 11–12 June 2012

Abstract

Abstract. During 11-12 June 2012, quasistationary linear mesoscale convective systems (MCSs) developed near northern Taiwan and produced extreme rainfall up to 510 mm and severe flooding in Taipei. In the midst of background forcing of low-level convergence, the back-building (BB) process in these MCSs contributed to the extreme rainfall and thus is investigated using a cloud-resolving model in the case study here. Specifically, as the cold pool mechanism is not responsible for the triggering of new BB cells in this subtropical event during the meiyu season, we seek answers to the question why the location about 15-30 km upstream from the old cell is still often more favorable for new cell initiation than other places in the MCS.With a horizontal grid size of 1.5 km, the linear MCS and the BB process in this case are successfully reproduced, and the latter is found to be influenced more by the thermodynamic and less by dynamic effects based on a detailed analysis of convective-scale pressure perturbations. During initiation in a background with convective instability and near-surface convergence, new cells are associated with positive (negative) buoyancy below (above) due to latent heating (adiabatic cooling), which represents a gradual destabilization. At the beginning, the new development is close to the old convection, which provides stronger warming below and additional cooling at mid-levels from evaporation of condensates in the downdraft at the rear flank, thus yielding a more rapid destabilization. This enhanced upward decrease in buoyancy at low levels eventually creates an upward perturbation pressure gradient force to drive further development along with the positive buoyancy itself. After the new cell has gained sufficient strength, the old cell's rear-flank downdraft also acts to separate the new cell to about 20 km upstream. Therefore, the advantages of the location in the BB process can be explained even without the lifting at the leading edge of the cold outflow.

Mechanisms for a Record‐Breaking Rainfall in the Coastal Metropolitan City of Guangzhou, China: Observation Analysis and Nested Very Large Eddy Simulation With the WRF Model

“…Schumacher and Johnson () examined the organization and environmental properties of extreme‐rain‐producing MCSs in the United States over a 3‐year period and showed that extreme local rainfall is often associated with back‐building/quasi‐stationary MCSs, which occur when new convective cells repeatedly form upstream of their predecessors and pass over a particular area. The continuous development of upstream convection requires triggering factors, for example, frontal lifting (Schumacher et al, ), outflow boundaries (Houston & Wilhelmson, ; Houze, ; Jeong et al, ; Schumacher, ; Schumacher & Peters, ; Wang et al, ), interaction between the low‐level jet and the midlevel circulation (mesoscale convective vortices; Schumacher, ; Schumacher & Johnson, , ), orographic lifting (Duffourg et al, ; Soderholm et al, ), and thermodynamic effects associated with latent heating/cooling (Wang et al, ). Schumacher and Johnson () also indicated that back‐building/quasi‐stationary MCSs are more dependent on mesoscale and storm‐scale processes, particularly lifting provided by cold outflows from previous convection, than on preexisting synoptic boundaries.…”

Budget Analyses of a Record‐Breaking Rainfall Event in the Coastal Metropolitan City of Guangzhou, China

“…Numerically, , and can be diagnosed by solving the three‐dimensional Poisson equations – with appropriate boundary conditions (Coffer & Parker, ; Wang et al, ). Then the nonlinear dynamic perturbation pressure can be obtained as the residual .…”

“…From (8), relatively low perturbation pressure is found on the downshear side of a localized updraft, and high perturbation pressure on the upshear side. (5)-(7) with appropriate boundary conditions (Coffer & Parker, 2015;Wang et al, 2016). Then the nonlinear dynamic perturbation pressure p ′ DNL can be obtained as the residual p ′ DNL ¼ p ′ D −p ′ DL .…”

Case Study of a Morning Convective Rainfall Event over Southwestern Taiwan in the Mei-Yu Season under Weak Synoptic Conditions

“…Known as the meiyu season, this unique weather and climate phenomenon typically lasts from midMay to midJune in Taiwan, when the front often extends from the vicinity of Japan through Taiwan and into southern China (Chang and Chen 1995;Chen 2004). Under its influence (e.g., Chen 1993), meso scale convective systems (MCSs) often form along or near the front and bring heavy rainfall to Taiwan (e.g., Kuo and Chen 1990;Chen 1992Chen , 2004Yeh and Chen 1998;Chen et al 2008), particularly when the prefrontal southwesterly monsoon flow strengthens and increases the moisture supply (e.g., Chen and Yu 1988;Chen and Li 1995;Chen et al 2005a;Jou et al 2011;Wang et al 2016).…”

“…6 of Ruppert et al 2013), heavy rainfall events can occur from organized MCSs (or even scattered but vigorous convection) that are often fed by strong, moisturerich prevailing southwesterly flow (e.g., Jou and Deng 1992;Chen et al 2005a;Davis and Lee 2012;Xu et al 2012;Tu et al 2014;Wang et al 2014a) and enhanced by topographic uplift (e.g., Johnson and Bresch 1991;Lin 1993;Ruppert et al 2013;Wang et al 2017). Besides the thermal effect that modulates the timing of rainfall (also Wang et al 2014b), the dynamic effect of terrain blocking of Taiwan may also affect the development of convection and shift rainfall locations (e.g., Li and Chen 1998;Wang et al 2005aWang et al , 2016. All the studies mentioned above, among others, have contributed toward a better understanding of the mechanisms of heavy rainfall in the meiyu season and how Taiwan's terrain affects its development and occurrence.…”