Articles | Volume 16, issue 18
Research article
29 Sep 2016
Research article |  | 29 Sep 2016

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

Chung-Chieh Wang, Bing-Kui Chiou, George Tai-Jen Chen, Hung-Chi Kuo, and Ching-Hwang Liu

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.

Short summary
In this study, the back-building process of a quasistationary convective line with extreme rainfall is investigated using a cloud model. At the initiation stage of new cells, thermodynamic processes of near-surface latent heating coupled with adiabatic cooling above along the convergence line, rather than dynamic pressure perturbations, are found to be important. The stronger uplift and cooling aloft provided by old cells made their upstream areas more favorable for new cell development.
Final-revised paper