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Large Scale Processes for Understanding Extreme Rain Storm Over Rangamati on 12 June 2017-a Case Study

Samarendra Karmakar
Former Director
Bangladesh Meteorological Department, Dhaka, Bangladesh

Mohan Kumar Das
Institute of Water and Flood Management, BUET, Dhaka, Bangladesh

Haripada Sarker
Water and Flood Management, BUET, Dhaka, Bangladesh

Abstract/ Executive Summary:

Attempts have been made to study the large-scale surface and upper air synoptic processes associated with a monsoon depression during 11-12 June 2017. In this study, Grid Analysis and Display System (GrADS) software has been used to prepare the large-scale sea level pressure and upper flow patterns by analyzing the FNL re-analysis data. In this analysis, the FNL dataset is used to characterize the rainstorms, with key hydrometeorological variables describing the prior conditions of the very heavy rainfall event presented in the study. National Center for Environmental Prediction (NCEP) Final (FNL) analysis data of 1o by 1o grids for every 6 hours are used for large-scale synoptic analysis. The disastrous event was a strong monsoon depression in the early period of southwest monsoon 2017. Due to this depression, very heavy rainfall occurred in southeastern Bangladesh. Rangamati recorded 343 mm of rainfall in the 24 hours on 12 June 2017. Massive landslides occurred in three districts such as Rangamati, Bandarban and Chittagong. The analysis of surface and upper air synoptic conditions has revealed that a well-marked low was formed over the northwest Bay of Bengal within the low-pressure belt passing from Somalia coast extending through southern Pakistan, and India up to the east-central Bay of Bengal and adjoining Bangladesh. The wind speed is calculated from the pressure distribution and is found to be 24.23ms-1, which is at par with the observed one. The well-marked low was subsequently intensified into a depression and moved northeastwards over Bangladesh. Strong south south-westerly winds were associated in the eastern side of the depression, especially over the Chittagong Hill Tracts. The depression was found to extend up to 500 hPa level as seen from the distribution of geopotential and strong circulation around the centre. Winds were advecting from large continental and Ocean areas over South Asia. Strong winds and moisture influx, the strong narrow coma-like trough from a micro low at the surface to 500 hPa level as well as strong wind shear were responsible for the heavy rainfall, disastrous effects and massive landslides over Rangamati and adjoining areas.

Keywords: Large scale; Coma-like trough; Heavy rainfall; Geopotential; Anticyclone; Circulation.

Location: Rangamati, Bangladesh

Start Date: 00-00-2017

End Date:

Program Area: Risk Management in Agriculture

Commodity: Extreme climatic event, Rainfall, Storm

Objectives:

The present study has been undertaken to investigate the large-scale processes associated with very heavy rainfall over Rangamati that occurred on 12 June 2017. The study includes the synoptic and upper air conditions responsible for the occurrence of very heavy rainfall over Rangamati during the early phase of southwest monsoon in Bangladesh in 2017. Rangamati is located in the Chittagong Division. It is bordered by the Tripura state of India to the north, Bandarban District to the south, Mizoram State of India and the Chin State of Myanmar to the east, and Khagrachari and Chittagong Districts to the west. Rangamati is the only district in Bangladesh with international borders with two countries: India and Myanmar. The area of the district is 6116 km2 of which 1292 km² is riverine and 4825 km² is under forest vegetation.

Methodology:

2.1 Data used In this research, National Center for Environmental Prediction (NCEP) Final (FNL) analysis data of 1o by 1o grids for every 6 hours are used (NCEP FNL; hereafter, FNL) for large scale synoptic analysis. The FNL product is generated from the Global Data Assimilation System (GDAS). GDAS is a continuous collection of observational data through GTS and different sources. In the FNL products, analyses are presented from 1000 to 10 hPa. The common parameters are surface pressure, sea level pressure, geopotential height, temperature, sea surface temperature, soil values, ice cover, relative humidity, u- and v- winds, vertical motion, vorticity and ozone. In addition, three hourly synoptic observations and synoptic charts of the Bangladesh Meteorological Department (BMD) are analyzed in this study for understating the extreme rainstorm event over Rangamati and adjoining areas.

2.2 Methodology The large-scale sea level pressure and upper flow patterns have been prepared by analyzing the FNL re-analysis data with the help of the software GrADS. In this analysis, the FNL dataset is used to characterize the rainstorms, with key hydro-meteorological variables describing the prior conditions of the very heavy rainfall event presented in the study. The outputs have helped to investigate the synoptic and environmental characteristics of the depression which affected Rangamati severely causing landslides and damaging infrastructures along with the loss of agricultural crops and human lives. The distribution of sea level pressure, geopotential and wind flow at different constant pressure levels and different synoptic hours along with the synoptic charts obtained from BMD have been investigated during 11-12 June 2017.

Source of Information: Journal of Engineering Science 11(2), 2020, 87-97

Article Link (Online Journal): DOI: https://doi.org/10.3329/jes.v11i2.50900

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

On the basis of the present study, the following conclusions can be drawn: i. The disastrous event is a strong monsoon depression in the early period of southwest monsoon 2017, responsible for very heavy rainfall in southeastern Bangladesh with 343 mm rainfall at Rangamati in 24 hours on 12 June 2017 and massive landslides in three districts such as Rangamati, Bandarban and Chittagong. ii. The central pressure of the well-marked low in the Northwest BoB has a minimum pressure of about 995 hPa on 11 June 2017. There is a belt of the low-pressure system from Somalia coast extending through southern Pakistan, and India up to east-central BoB and adjoining Bangladesh. In the south and east (Eastern Asia) of the low-pressure belt, the sea level pressure ranges from 1004 hPa over the Arabian Sea and BoB to more than 1010 hPa over the South Indian Ocean. The range of 1010 to 1021 hPa pressure is found to exist in Tibetan Plateau and adjoining areas. The wind speed is calculated from the pressure distribution and is 24.23ms-1, which is at par with the observed one. iii. The existence of the low-pressure belt over Somalia-Pakistan-India-Bangladesh and North BoB is responsible to bring strong southwesterly winds over the Arabian and the BoB, intensifying the southwest monsoon over South Asia and converging the winds into the well-marked low as well as over the low-pressure belt. The well-marked low has intensified into a depression at 0000 UTC on 12 June 2017 and moved subsequently in a northeasterly direction over Bangladesh. iv. At 950 hPa, the depression centre has a low geopotential height of 380 m over Cumilla and adjoining area and a separate micro-cell of low geopotential height formed in the extended trough over Rangamati region and a separate micro-cell of low geopotential height formed in the extended trough over Rangamati region at 0600 UTC on 12 June 2017. There was intense circulation over eastern Cumilla and Rangamati with a stronger wind speed than that 10 m level. The winds are found to advect into the centre of the depression from large continental and ocean areas; the micro low becomes more prominent with a narrow trough extended to north Rangamati at 1200 UTC. v. On 12 June at 0000 UTC, strong winds of 20-30 ms-1 are found at 850 hPa level over the BoB, which has been responsible for strong moisture influx over the depression area, causing very heavy rainfall as well as disastrous landslides in the Chittagong Hill Tracts. There is a narrow band of maximum wind speed of 25-30 ms-1 at 850 hPa over the Rangamati region, creating a strong wind shear at the surface and damaging significantly the structural properties and natural resources there. vi. At 0600 UTC on 12 June 2017, there is a micro low of geopotential height of 1350 m in the shape of a comma at 850 hPa in the trough over Rangamati and adjoining area. This type of low is generally associated with disastrous weather events and it justifies the situation, which has occurred in the Rangamati region. vii. The depression is to extend vertically upwards up to 500 hPa at 0000 UTC on 11 June with a lower geopotential height of 5800-5850 m over the northwest Bay, having a trough towards the southwest covering the Arabian Sea. Relatively lower geopotential height was found to exist over India, the Arabian Sea and adjoining BoB. viii. The winds in the circulation over the BoB are weaker with 5 ms-1 at 500 hPa at 0000 UTC on 11 June but the winds are stronger (about 20 ms-1) and southwesterly in the southeastern coast of Bangladesh and southerly over the Chittagong Hill Tracts. The cross-equatorial flow was over the Arabian Sea is absent at 500 hPa but there was cross-equatorial flow over the BoB. Two strong anticyclones are found: one over the Arabian region with an extended ridge eastward up to Pakistan and the other over the Pacific Ocean east of the Philippines. A strong westerly trough is found at 500 hPa over Uttar Pradesh and south of Nepal; this westerly trough is responsible for the intensification of the depression. ix. A narrow strong trough of minimum geopotential height of 5780 m is extended from the eastern side of the depression towards the south over the Chittagong Hill Tracts with stronger southerly winds of 20-25 ms-1 over the eastern side of the depression. The extended narrow strong coma-shaped trough and the stronger southerly winds over the Chittagong Hill Tracts could be attributed to the ruinous damages in Rangamati and adjoining areas. x. A well-defined westerly trough is present at 200 hPa over northern India and an anticyclone over Myanmar and adjoining eastern Bangladesh, having southerly wind diverging towards northeast over Bangladesh. This southerly wind diverging towards the northeast over Bangladesh guided the depression as the steering wind move north-northeasterly direction.

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