Synoptic flow patterns and large-scale characteristics of flash flood-producing rainstorms over northeast Bangladesh

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ORIGINAL PAPER

Synoptic flow patterns and large‑scale characteristics of flash flood‑producing rainstorms over northeast Bangladesh Mohan K. Das1 · A. K. M. Saiful Islam1   · Samarendra Karmakar2 · Md. Jamal Uddin Khan1 · Khaled Mohammed1 · G. M. Tarekul Islam1 · Sujit Kumar Bala1 · Thomas M. Hopson3 Received: 8 July 2018 / Accepted: 11 November 2019 © Springer-Verlag GmbH Austria, part of Springer Nature 2019

Abstract Flash flood and related hazards occurred over the Haor (wetland) areas of northeast Bangladesh during 17–18 April 2010. Prediction of this sudden incident is challenging when it happened on the high terrain of Meghalaya Plateau and adjoining Bangladesh. Flash flood event occurred when convective cells assembled into a mesoscale convective system (MCS) over the steep edge of the Plateau. The MCS obtained its extreme point after getting moisture support from the southerly flow of the Bay of Bengal (BoB). This study investigated the synoptic flow patterns and large-scale characteristics of the flash flood-producing storm and its associated tropospheric conditions in northeast Bangladesh using the Weather Research and Forecasting (WRF) model. The model used a 3-nested domain with the horizontal resolution of 27 km, 9 km, and 3 km, respectively. The study revealed that the model underestimated the strength of the flash flood in general in respect of rainfall. The 48-h simulated rainfall was about 152 mm for outer domain-1, about 195 mm for inner domain-2 and about 209 mm for the innermost domain-3 whereas actual rainfall was 223 mm as recorded by Bangladesh Meteorological Department (BMD). The southerly wind was strong at 950 hPa and the westerly wind prevailed at 500 hPa level. The model simulated results show that cloud water mixing ratio was 1.8 mg m−3 and extended vertically up to 17 km. Ice water mixing ratio was 200 mg m−3 and found in between 12 and 20 km, indicating the formation of ice in the upper troposphere. The maximum values of x, y, and z-wind components over Cherrapunji were − 11 ms−1, − 21 ms−1 and − 2.8 ms−1, respectively which indicated the strengthening of the convective system to produce flash flood.

1 Introduction

Responsible Editor: M. Telisman Prtenjak. Electronic supplementary material  The online version of this article (https​://doi.org/10.1007/s0070​3-019-00709​-1) contains supplementary material, which is available to authorized users.

The wetland areas of the northeastern parts of Bangladesh, locally known as “Haor”, experiences frequent flash floods during the pre-monsoon season. These flash floods occur due to the heavy rainfall associated with severe thunderstorms/Mesoscale Convective Systems (MCSs) within a

* A. K. M. Saiful Islam [email protected]; [email protected]

Sujit Kumar Bala [email protected]

Mohan K. Das [email protected]

Thomas M. Hopson [email protected]

Samarendra Karmakar [email protected]

1



Md. Jamal Uddin Khan [email protected]

Institute of Water and Flood Management (IWFM), Bangladesh University of Enginee