A method to use RCM generated climate scenarios in application to water related issues in Bangladesh

Md. Nazrul Islam1* Nasreen Akter1, Ahsan Uddin Ahmed2 and Taiichi Hayashi3

1Department of Physics, Bangladesh University of Engineering & Technology, Dhaka, Bangladesh
2Centre for Global Change, Shaymoli, Dhaka, Bangladesh
3Disaster Prevention Research Center, Kyoto University, Japan

Abstract- Now-a-days regional climate models (RCMs) are considered as useful tools in generating future climate scenarios. The future scenarios are intended to be very useful for the planners at national level for the agriculture dependent and natural disaster affected country like Bangladesh. The question arises how to use the climate scenarios generated by various RCMs. It generally depends on their inherent uncertainties and reliability which should be within a considerable range. In this direction, the RCM outputs need to be calibrated with the observed data. In Bangladesh two RCMs, one is Providing Regional Climates for Impacts Studies (PRECIS), developed by Hadley Center, UK and the other is Regional Climate Model (RegCM), developed by ICTP, Italy, are running in the Department of Physics, Bangladesh University of Engineering & Technology. Model runs have been performed within the coverage of south Asian domain (65 - 117°E; 5 - 40°N). The NCEP lateral boundary conditions data are used to drive the model in different closure parameterizations. The PRECIS and RegCM outputs are calibrated with CRU (climate research unit) and surface observational data for the baseline period 1961-1990. Closure parameterizations are checked out to find out the best fit with reference to observed baseline rainfall and temperature data for this region. Based on the calibration, correction factors are obtained to prepare Look-up tables for climate parameters such as rainfall and temperature (both maximum and minimum). The correction factors are tabulated at 33 locations throughout Bangladesh with reference to the observed data. Using the proposed Look-up tables the future scenarios have been generated by RCMs and PRECIS. The scenarios can easily be utilized for the projection of rainfall and temperature for any future reference timeframe. There are slight differences in outputs of the two models, which are within about 5% with respect to observed data. A systematic cold bias is found for the simulation of temperature, while strong seasonal dependency is found for the simulation of rainfall. Model overestimated rainfall in pre-monsoon and underestimated it in monsoon period. The future scenarios generated by PRECIS and RegCM can be useful in planning purposes for a particular location with the help of proposed Look-up table. Details of calibration and selection of parameterization for both PRECIS and RegCM will be discussed during the seminar.

*Correspondence to: E-mail: mnislam@phy.buet.ac.bd        nazrul_buet@yahoo.com   Fax: +8802-8613046