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Regional Climate Model for Rainfall Projection in Bangladesh

Md. Nazrul Islam*
SAARC Meteorological Research Centre, Agargaon, Dhaka-1207, Bangladesh

Abstract/ Executive Summary:

Rainfall is projected using regional climate model (RCM) called PRECIS (Providing REgional Climates for Impact Studies) for Bangladesh. The model control run (1961-1990) is completed in 50 km horizontal grid resolution and model generated rainfall is calibrated with ground-based rainfall measurement at 27 observational sites throughout Bangladesh. The model is again run for 2000-2020 to carry out validation and forecast of rainfall. It is found that PRECIS over-performed by about 4.47% during 2000-2006 in estimating rainfall over Bangladesh with R² is 0.81 and correlation coefficient is 0.90. This admirable performance of PRECIS encourages using it for projection of rainfall as well as surface water in the country. The rainfall projection in 2009 is found surplus 2.03% and 14.02% in monsoon (JJAS) and post-monsoon (ON) periods respectively. It is deficit 2.08% and 1.44% in pre-monsoon (MAM) and dry periods (DJF) respectively. Surface water over Bangladesh is calculated 40.68×10⁹ kg for baseline period. It varies from -0.98% (2013) to 5.53% (2018) during 2009 – 2020.

Keywords: Climate Change, Regional Climate Model, Forecast, Rainfall, Surface Water.

Location: All over Bangladesh

Start Date:

End Date:

Program Area: Socio-economic and Policy

Non-commodity: Rainfall

Objectives:

The aim of PRECIS is to allow developing countries, or groups of developing countries, to generate their own national scenarios of climate change for use in impacts studies. This will allow countries using PRECIS are in a better position to validate the model using their own observations. PRECIS is a hydrostatic, primitive equation grid point model containing 19 levels described by a hybrid vertical coordinate.

Methodology:

PRECIS is a hydrostatic, primitive equation grid point model containing 19 levels described by a hybrid vertical coordinate (Simmons and Burridge, 1981; Simon et al., 2004). In this study PRECIS runs in generating forecast rainfall scenarios using the special report on emissions scenarios (SRES) A2 of ECHAM4 LBC in a domain of 65–103°E and 6–35°N to cover entire SAARC region. The Bangladesh Meteorological Department (BMD) collected surface rainfall throughout the country has been used for the calibration of PRECIS generated rainfall. For the application of PRECIS to climate change impact studies in Bangladesh, it is important to find out the appropriate calibration method. Having this in mind, analyses have been performed on point-to-point basis (Islam et al., 2008). In this procedure, observed data at a particular site is considered as the representative of that location (Islam and Uyeda, 2006). Grid value of the model data is compared with the observed data representing that grid. If more than one observation sites exist within a grid, average value of all the observational sites is considered as representative value for that grid. Rain-gauge daily rainfall data collected by BMD are processed to obtain monthly, seasonal, annual, decadal and long-term values. The model data of rainfall is extracted at 27 observational sites of BMD and then are converted to monthly, seasonal, annual, decadal and long-term values. Surface water is calculated from rainfall using rain rate and area of the country.

Source of Information: 2nd International Conference on Water & Flood Management (ICWFM-2009), BUET, Dhaka, Bangladesh

Article Link (Online Journal):

Funding Source:

1. Government

Budget:

Total Budget (Tk.) :

Achievement/Findings:

A regional climate model named PRECIS is employed in generating rainfall scenarios for Bangladesh. PRECIS generated rainfall scenario is calibrated with ground-based observed rainfall during 1961-1990. The regression coefficients are utilized in validation of PRECIS generated rainfall during 2000-2006. Better performance of PRECIS through validation encourages employing it in rainfall forecasting for Bangladesh. In this work rainfall forecast for Bangladesh is performed experimentally for 2009. The monsoon rainfall is projected surplus 0.29 mm/day or 2.03% in 2009 and it will be surplus 0.44 mm/day or 14.02% in post-monsoon. It will deficit 0.11 mm/day (2.08%) and 0.01 mm/day (1.44%) in pre-monsoon and dry season respectively. Through the analysis of monsoon rainfall in Bangladesh this work discloses that PRECIS simulated rainfall is not directly useful in application purposes. However, after performing calibration, admirable result is obtained in estimating rainfall in Bangladesh with R² is 0.81 and correlation coefficient is 0.90. Forecasted surface water will vary from -0.98% (2013) to 5.53% (2018) in Bangladesh during 2000-2020.

Knowledge Management: Report/Proceedings