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Variability of Rainfall Over the Coastal Area in Bangladesh by Using Climate Model

SHAHANA ISLAM
Department of Geography and Environment, Jagannath University, Dhaka-1100, Bangladesh

MD. MONIRUZZAMAN*
Department of Geography and Environment, Jagannath University, Dhaka-1100, Bangladesh

M. A. MANNAN
Bangladesh Meteorological Department, Agargaon, Dhaka- 1207, Bangladesh

Abstract/ Executive Summary:

The study attempt to understand the variability of rainfall by looking into the previous and future climate of the coastal area in Bangladesh from 1850 to 2100 by using the climate model (CMCC-CM- the Centro Euro-Mediterraneo Sui Cambiamenti Climatici Climate Model) of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC). Rainfall data were collected from CMCC-CM by R programming for two GHGs emission scenarios (RCP 4.5 and RCP 8.5) referred to as ‘Representative Concentration Pathways (RCPs)’. The analysis has been conducted based on four seasons and an annual basis by plotting model data in MS Excel and R programming. The model shows that the average annual rainfall will increase from 1055.6 mm (during 1850-1900) to 1368.1mm (during 2051-2100) for RCP 4.5 while it will reach 1569.7mm (during 2050-2100) for RCP 8.5. Rainfall is also increasing for all seasons except winter. In winter season, the average rainfall will increase from 35.37mm (during 1850-1900) to 41.75mm (during 2051-2100) for RCP 4.5, where it will decrease from 35.37mm (during 1850-1900) to 22.55mm (during 2051-2100) for RCP 8.5 in the study area. The increasing and decreasing trend are more in high GHGs emission scenarios than in the medium, which will be alarming. Accordingly, this projection will be helpful to understand the adverse impacts of climatic elements and take short and long-term planning of decision-makers in that area.

Keywords: Climate model, Rainfall variability, RCP, Coastal area.

Location: The coastal area in Bangladesh

Start Date: 00-00-1850

End Date: 00-00-2100

Program Area: Comparative study

Commodity: Modeling, Rainfall

Objectives:

The climate change studies of climatic variables using different models or tests to estimate future projections and uncertainties over the coastal regions of Bangladesh are urgently necessary. So, the study attempts to understand rainfall variability by looking into the previous and future climate from 1850 to 2100 using the climate model (CMCC-CM- The Centro EuroMediterraneo Sui Cambiamenti Climatic (Climate Model) of the coastal areas in Bangladesh. Model data were cracked and collected by R programming. A basic statistical method was used to understand the variability of rainfall by annual and seasonal analysis over the coastal area in Bangladesh.

Methodology:

Study Area: According to PDO-ICZMP (Project Development Office-Integrated Coastal Zone Management Project), 2003, the coastal areas of Bangladesh are classified by 19 coastal districts (147 Upazilas), where a total of 48 Upazilas in 12 districts are exposed to the sea and or lower estuaries, are defined as the exposed coast, and the remaining 99 Upazilas of the coastal districts are termed interior coast (Uddin and Kaudstaal 2003; Parvin et al. 2010). This 711 km long coastline consists of a vast network of river systems draining the vast flow of the Ganges-Brahmaputra-Meghna River system.

Data Source and Analysis: Rainfall data of the coastal area of Bangladesh were collected from a general circulation model (CMCC-CM) of the 5th Phase Coupled Model Intercomparison Project (CMIP5) within IPCC during the period of 1850 to 2100 where historical data have been divided into 1850 to 2005 and future data is divided into 2006 to 2100 within four different emission scenarios. CMIP5 has many different models, and every model has different surface variables data. Again, a study of Climatic Change (2011) has shown that Representative Concentration Pathways (RCPs), a set of four new emission scenarios (RCP 2.6, RCP 4.5, RCP 6.0 and RCP 8.5) developed as an appropriate approach for investigations of future climatic conditions (Van Vuuren et al. 2011). As southern coastal Bangladesh is a small region, we have been trying to use RCP 4.5 (medium stabilization scenarios of greenhouse gas emission) and RCP 8.5 (very high baseline emission scenarios) based model for looking into the previous climate and for future climate. According to literature, data availability and data resolutions, the selected model (CMCC-CM) of CMIP5 was used.

In this regard, NetCDF format data of selected variables were downloaded from the KNMI (Koninklijk Nederlands Meteorologisch Instituut) website and then cracked the NC file by using R programming. As these data are divided by grid points into the whole world, so picked the data of selected variables according to the appropriate 19 grid points (latitude-longitude) of coastal areas of Bangladesh for 1850 to 2100. Basic statistical method was used in this study and the analysis has been conducted based on four specific seasons: winter (December-February), pre-monsoon (March-May), monsoon (June-September) and post-monsoon (October-November); which was followed by different researchers and on an annual basis for CMCC-CM model for both RCPs (4.5 and 8.5) by plotting data in MS Excel and R programming. The figures are represented by graphs, charts, and WinSurfer, and GIS software was utilized in this research to make a map.

Source of Information: J. Asiat. Soc. Bangladesh, Sci. 47(2): 149-160, December 2021

Article Link (Online Journal): DOI: https://doi.org/10.3329/jasbs.v47i2.57278

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Bangladesh is considered as one of the most vulnerable countries to the impacts of climate changes. Still the coastal areas are exposed more to climate stresses and stay at a vulnerability front line. Every year Bangladesh faces an average of 1.8 tropical storms. However, the casualties in terms of the number of death of people are conspicuously lower than previous experience. As one of the main elements of climate change is rainfall, these estimations possibly help understand the future climatic conditions and make shortand long-term planning in that area. CMCC-CM model showed an increasing trend of rainfall for both emission scenarios (RCP 4.5 and RCP 8.5) during the baseline period (1850-2100). The annual rainfall has been increased from 1055.62 mm to 1368.10 mm from 1850 to 2100 for RCP 4.5, while it has been increased from 1055.62 mm to 1569.71 mm for RCP 8.5. In the summer season, rainfall has increased from 69.47 mm to 94.95 mm for RCP 4.5, while it has increased to 80.67 mm for RCP 8.5. Similarly, in the monsoon season, the average rainfall has increased from 779.51mm to 1008.92 mm for RCP 4.5, while it has increased to 1251.03 mm for RCP 8.5. But in the winter season, the scenario is different. The average rainfall has increased from 35.37 mm to 41.75 for RCP 4.5, where it has decreased from 35.37 mm to 22.55 mm for RCP 8.5 from 1850 to 2100. It has also been seen that July and February have been recorded as the highest and lowest rainfall month for the monsoon and winter seasons respectively.

From this finding, considering new CMCC-CM (CMIP5) projections will be more helpful for decision-makers as they have a comparatively better representation of the Earth’s physical processes. Generally, it shows an increasing trend of rainfall over coastal areas, which indicates more natural calamities in the future. Where the increase is more in high GHGs emission scenarios than the medium. But the rainfall also shows a decreasing trend for the only winter season in the high emission scenario (RCP 8.5), which indicates the barrier of crop productions and reducing groundwater level in the dry season. Consequently, if the GHGs emission can be minimized, the rise of temperature and disparity of rainfall could be controlled but if don't control, the agrarian economy of the coastal area will be more vulnerable.

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