4 th International Conference on Water & Flood Management (ICWFM-2013) Mohammad Alfi Hasan 1 *, A. K. M. Saiful Islam 2 and Bhaski Bhaskaran 3

Transcription

1 PREDICTING FUTURE PRECIPITATION AND TEMPERATURE OVER BANGLADESH USING HIGH RESOLUTION REGIONAL SCENARIOS GENERATED BY MULTI-MEMBER ENSEMBLE CLIMATE SIMULATIONS Mohammad Alfi Hasan 1 *, A. K. M. Saiful Islam 2 and Bhaski Bhaskaran 3 1 Institute of Water and Flood Management, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh, mdalfihasan19@gmail.com 2 Institute of Water and Flood Management, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh, akmsaifulislam@iwfm.buet.ac.bd 3 Met Office, Hadley Center, United Kingdom, b.bhaskaran@metoffice.gov.uk ABSTRACT In order to assess the implications of climate change, a study is conducted with a high resolution regional climate model (RCM), PRECIS (Providing Regional Climates for Impact Studies) over whole Indian subcontinent. The PRECIS simulations corresponding to the IPCC- SRES A1B emission scenario have been conducted with boundary data from all the 17 members of Perturbed Physics Ensemble of the Hadley Centre Coupled Model (HadCM3) for the Quantifying Uncertainty in Model Predictions (QUMP) project. All the runs are carried out at a 25km horizontal resolution for a continuous period The analysis of future rainfall and temperature are examined over three time slices, viz. short (2020s, i.e ), medium (2050s, i.e ) and long (2080s, i.e ) with baseline period ( ). All changes of future climate have been examined both annually and seasonally. During the monsoon season of all three future time slices, mean climate of the country will be wetter than present. Temperature will rise up to 6 C at the end of the century whilst winter will get warm faster than summer. Summer will have maximum 2.7 C increase during the 2050 s and 4.0 C during the 2080 s whilst winter temperature will change 4.2 C during the 2050 s and 6.0 C degree during the 2080 s. However, change of minimum temperature is more than the change of maximum temperature which results more number of hot nights rather than hot days at end of the century. In the 2020 s, during summer season there will be less chance of decrease of rainfall (about - 6.1%) than increase of rainfall (max about 49.0%). Majority of ensemble simulations suggest that at least ~15% increase of monsoon season rainfall expected at end of century which will intensify the possibility of the occurrence of floods in this region. Keywords: Bangladesh, climate change, ensemble, PRECIS, seasonal climate 1. INTRODUCTION Climate change is now a major environmental and developmental issue. Bangladesh is a densely populated country and is prone to a multitude of climate-related impacts such as floods, droughts, tropical cyclones and storm surges. Bangladesh is likely to be one of the most vulnerable countries of the world affected by climate change (Ali, 1999). Vulnerability of Climate change is also emphasized in recent forth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2007). With prevailing change of climate and increasing population Bangladesh is getting more vulnerable to food security, water management and disaster management. Future change of rainfall will eventually affect the water-stress problems in the small country like Bangladesh as 575

2 lots of challenges already associated with water supply situation (Rajib et al., 2008). Extreme weather events such as floods, cyclones, severe drought, and sea level rise are already intensified in the South Asia due to impact of temperature rise (Haq et al, 1998, Karim et al, 1998). In this respect, it is obvious that, long term planning is not possible without a clear idea of climate change of future. Quantifying future changes is essential to determine possible impact of climate change. Climate models are the main tools available for developing projection of future climate (Houghton et al 1995, 2001). There are a number of mathematical models of global circulation that indicate expectations of future climate change scenarios. Regional Climate model (RCM) is a dynamic downscaling tool for climate features in case of obtaining detailed information in particular region (Jones et al. 2004). Different studies already made using different regional climate models (RCM) over Bangladesh. PRECIS developed by Met office, UK, is one of regional climate model that is useful to predict future regional climate. Different countries like India, China, New Zealand, United Kingdom etc have used PRECIS model to quantify future possible change at regional scale made some analysis to project future climate change (Rajendran et al., 2008; Fowler and Kilsby, 2003; Drost et al., 2007). Different studies have already made over the country have been made using PRECIS climate model (Murshed et al., 2011; Islam et al., 2008; Rajib et al. 2011; Islam and Hasan., 2012). But none the studies used climate projection less then 50km resolution which is essential to acquire climate information from District administrative level of Bangladesh. In this context, Bangladesh University of Engineering and Technology and Met Office, UK has conducted a collaborative research to generate high resolution (25 km) climate change information using PRECIS model. A total of the 17 ensemble members of A1B SRES Scenarios form IPCC report have been simulated over Bangladesh from 1971 to This paper examines the future projections of rainfall and temperature in Bangladesh derived from regional climate model, PRECIS using the 17 QUMP ensembles lateral boundary condition data. 2. MODEL AND METHODOLOGY 2.1. Model Description The PRECIS is a hydrostatic, primitive equation grid point model containing 19 levels described by a hybrid vertical coordinate. PRECIS current version has ability to downscale up to 25 km resolution from GCM boundary data and can produce outputs for more than 150 parameters. One of the best tools of regional dynamic downscaling tools, PRECIS is made available for use by scientists of developing countries involved in vulnerability and adaptation studies. Basic aspects explicitly handled by the model are briefly outlined in (Noguer et al. 2002). A total of the 17 member perturbed physics ensemble (PPE) produced using Hadley Centre Coupled Model (HadCM3) under the Quantifying Uncertainty in Model Predictions (QUMP) project of Hadley Centre Met Office, UK, have been used as lateral boundary conditions (LBCs) for the 138year simulations of the PRECIS over Bangladesh. Availability of continuous data of 138 year provides an opportunity to assess the impact of climate change over Bangladesh Methodology PRECIS was run at 25 25km horizontal grid resolution in a large domain covers E and 0-41 N using lateral boundary data from GCM model of Hadley Centre, Met Office, UK. Model domain is shown in Figure 1 which covers the entire Indian sub-continent. Previously simulation has been conducted at the Institute of Water and Flood Management (IWFM) for a smaller domain with 50km horizontal resolution over Bangladesh using A1B scenario from special report on 576

3 emissions scenarios (SRES) of the Intergovernmental Panel on Climate Change (IPCC). To produce high-resolution fine scale information, a a domain of resolution 25km is used under a collaborative research project between Institute of Water and Flood Management, BUET and Met Office, UK. Seventeen ensemble experiments using lateral boundary conditions (LBCs) of the 138year ( ) is conducted at Met office using regional climate model, PRECIS. These simulations are based on the 17-member perturbed physics ensemble (PPE) produced by HadCM3 under the QUMP project of Hadley Canter Met office, UK. Figure 1: Domain used for the 17 perturbed QUMP ensemble simulations by PRECIS Model In this study, high resolution regional climate information generated using PRECIS with LBC s from QUMP simulations, has been examined in details to evaluate the model skills in representing the regional climatology over Bangladesh. Seasonal changes of rainfall and temperature have been observed using all the 17 ensemble simulations to capture all the possible changes of future. All the analysis have been conducted for the three future time periods of 2020 s (2011 to 2040), 2050 s (2041 to 2070) and 2080 s (2071 to 2100) to quantify near, medium and long term changes. Finally, quartile analysis has been conducted to represents probabilities of changes of future climate. 3. RESULTS 3.1. Rainfall Rainfall is a crucial natural process of climatic system. But rainfall process consists of a number of micro-scale processes like cloud formation or condensation that are not captured very well neither in GCM nor RCM. There are still large uncertainty lies to predict rainfall for future years. However, with all the 17 ensembles simulation it might be possible to capture the best probability of future climate. Table 1 gives the change of rainfall in percentage during all the four seasons. Reduction of rainfall is more prominent in the winter seasons than other seasons. Winter can be drier with reduction of rainfall from -1.4% during the 2020 s.. However, rainfall will increase during winter season at the end of the century. But this effect is not significant due to very little amount of rainfall at winter season in Bangladesh (Ahasan et al., 2010).. Mean ensembles shows a increase of summer rainfall of 12.3%, 21.4% and 26.4% for time slice of 2020 s, 2050 s and 2080 s which will increase the possibility of violent norwester summer thunder storm along with heavy rainfall. Mean ensembles suggest that monsoon will have increase of rainfall 21.8% at end of century which can increase chances of monsoon flooding. During the post monsoon, 577

4 rainfall will increase slightly about 7.5% at the end of the century. Range of rainfall change during the post monsoon lies between to 20.0%, to 30.6% and to 24.3% at 2020 s, 2050 s and 2080 s respectively. Table 1: Percentage change of mean rainfall for the future time periods of 2020 s (2011 to 2040), 2050 s (2041 to 2070) and 2080 s (2071 to 2100) with respect to baseline (1971 t0 200) during all the four seasons of Bangladesh Summer Monsoon Post-monsoon Winter 20s 50s 80s 20s 50s 80s 20s 50s 80s 20s 50s 80s Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Mean Max Min Figure 2 shows Box-whisker plot of future possibility of change of rainfall during the rainy, summer and monsoon season when majority of the rainfall occurred in Bangladesh. Variability of rainfall during the summer season is much higher than monsoon season. Majority (75%) of ensemble simulations suggested that rainfall will be continuously increased during the summer season for all the three future time-slices. In the near future (2020 s), around 85% of ensemble members suggested that summer rainfall will be increased and 50% suggested that it will be within 5% to 20% range. Again during the 2050 s, 50% ensemble members suggested that rainfall will be increased ranging from 18% to 38%. The probability of tropical storms during the summer season will be increased which is crucial for countries economy. In the near and medium future, a large variability in the change of rainfall is found which will be gradually reduced at the end of the century. At the 2020 s during the monsoon season the variability of rainfall is ranges from 5 to 38%. At 2050 s, 75% of ensemble simulations suggest that rainfall will be increased to 1 to 25%. The 2nd Quartile plot shows that rainfall will rise from 15% to 30% at the end of the century. Interestingly, the mean value of all the ensemble simulations is close to the median value, which represents a mean state of climate change for these three future time periods. Hence, it can also be said that mean state of change of rainfall during the summer and monsoon seasons over Bangladesh is well represented by high resolution climate model ensembles simulations. 578

5 Figure 2: box-whisker plot of change of rainfall during the summer (left) and monsoon (right) from all the 17 qump ensembles simulations for the 3 future time slice representing 2020s (2011 to 2040), 2050s (2041 to 2070) and 2080s(2071 to 2100) over Bangladesh. White dot represent mean value of the 17 ensemble simulations, black box represents second and gray box represents third quartile of all the 17 ensemble simulations 3.2. Temperature Future temperature rise will have profound impact on sea-level rise and cyclonic storm surge and consequently effects livelihoods, ecosystems and food security of the nation. A total of 17 QUMP ensemble simulations try to quantify the best possible results of future rise of temperature. Changes of temperature in the 2020s, 2050 s and 2080 s with respect to base line are shown in Table 2. Increase of temperature during winter season will be more prominent than other seasons. Mean increase of temperature during winter season will be increased about 4.7ºC though it will be increased up to 6ºC at the end of century. In near future (2020s), minimum increase of mean temperature will be 1.1ºC which will cause yield reduction of clod loving crops. Monsoon season will experience less amount of increase of temperature. The mean temperature of ensemble simulations will increase about 1.1ºC, 2.4ºC, 3.7ºC for the 2020 s, 2050 s and 2080 s, respectively. 579

6 Table 2: Projected future change of mean temperature for the three future time periods of 2020 s (2011 to 2040), 2050 s (2041 to 2070) and 2080 s (2071 to 2100) with respect to baseline (1971 t0 200) for all the four seasons over Bangladesh Winter Summer Monsoon Post-monsoon 20s 50s 80s 20s 50s 80s 20s 50s 80s 20s 50s 80s Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Mean Max Min Figure 3: Box-whisker plot of change of mean temperature during the winter, summer, monsoon, post-monsoon seasons of Bangladesh (from left to right respectively) for all the 17 ensemble simulations for the 3 future time slices representing 2020s (2011 to 2040), 2050s (2041 to 2070) and 2080s (2071 to 2100). White dot represents mean, black box represents second and gray box represents third quartile of the 17 ensemble simulations 580

7 Future changes of mean temperature are shown in box whisker plots in the Figure 3. Variability of changes of temperature during summer and monsoon seasons is less than other two seasons by showing more confident projections of future. Temperature will be increased ranging from 0.9ºC to 1.9ºC during the 2020 s. In 2020 s, during summer season, temperature will be increased ranging from 0.9ºC to 1.2ºC. However, during the post-monsoon and winter seasons temperature will be increased ranging from 1.5ºC to 1.8ºC. In mid 2050 s, during the summer season, temperature will be increased ranging from 2.2ºC to 2.6ºC. During the winter and post-monsoon, variability of temperature will greatly vary at the end of the century. 4. CONCLUSIONS The expected future changes of the characteristics of rainfall and temperate over Bangladesh under IPCC SRES-A1B scenario based on the 17 QUMP simulations has been assessed using PRECIS is presented on the study. A number of conclusions can be made from this study which are listed below: From all ensemble simulations the mean value of temperature and precipitation can be represented as mean state of future change of climate as their median value is closer to mean values for all the seasons of Bangladesh. However, according to IPCC and QUMP project, any of these simulations has equal probability to future. Bangladesh will experience a significant rise in mean temperature during winter and postmonsoon season about C during the mid 2050 s and about 4.5 ºC at the end of the century. It has been found that temperature will increase more during the dry season than wet season which will increase warm nights than hot days. Temperature will increase at least 2.5 ºC during all the seasons at the end of century. During the summer rainfall will increase by about 15% during the 2020 sthough some of the ensemble member s shows slightly decreasing tendency. Majority of the ensemble simulations suggest that monsoon rainfall will be increased ranges from 15% to 30% at the end of the century. Seventeen simulations of QUMP project are indicative of the expected range of climate change over Bangladesh as QUMP project aimed to capture all possible uncertainties of future. However, there are some limitations to capture mesoscale climatology at regional scale from Global Circulation Model (GCM). In future, finer resolution GCM can be used to produce more accurate climate projection. This will allow us to quantify the uncertainties better and also gain more confidence in the projected future climate over Bangladesh. ACKNOWLEDGEMENTS The PRECIS simulations made at Met Office, UK were facilitated under a collaborative research project entitled High-resolution Regional Climate Change Information for Bangladesh to inform Impacts Assessments, Vulnerability indicators and Adaptation Policies of BUET with Hadley Centre, Met office, UK funded by DFID. REFERENCES Ali A. (1999). Climate change impacts and adaptation assessment in Bangladesh. Climate Research, Vol. 12,

8 Ahsan, M.N., Chowdhary, Md.A.M., and Quadir, D.A. (2010). Variability and Trends of Summer monsoon Rainfall over Bangladesh Journal of Hydrology and Meteorology, Vol. 7, No.1, Drost, F., Renwick, J., Bhaskaran, B., Oliver, H., and McGregor, J. (2007). Simulation of New Zealand s climate using a high-resolution nested regional climate model. International Journal of Climatology, Vol.27, Fowler, J.H. and Kilsby, C.G. (2003). Implication of seasonal and annual extreme rainfall. Geophysical Research Letters, Vol.30, No.13, 53(1-4). Haq. S., Karim. Z., Asaduzzaman. and M., Mahtab. F. (1998). Vulnerability and Adaptation to Climate Change for Bangladesh. Kluwer Academic Publishers, Dordrecht, 135 Houghton, JT., Meira Filho, LG.,Callander BA.,Harris, N., Kattenberg, A., Maskell, K. (1995) Climate Change 1995, Intergovernmental Panel on Climate Change (IPCC), Cambridge University Press: Cambridge. IPCC, (2007). Climate Change The Physical Science Basis Contribution of Working Group I to the Fourth Assessment Report of the IPCC, Cambridge University Press, 996 Islam, M.N., Rafiuddin, M., Ahmed A.U., and Kolli, R.K. (2008). Calibration of PRECIS in employing future scenarios in Bangladesh, International Journal of Climatology, RMetS, Vol. 28, Islam, A.K.M., and Hasan, M.A. (2012). Climate Induced Changes of Precipitation Extremes over Bangladesh Proceeding of 3 rd International Conference on Environmental Aspect of Bangladesh. Japan, ID 110, Murshed, SB., Islam, A.K.M., and Khan, M. (2011). Impact of Climate Change of Rainfall Intensity in Bangladesh 3 rd International Conferences on Water and Flood Management. Dhaka, Bangladesh. Jones, R.G., Noguer, M., Hassell, D.C., Hudson, D., Wilson, S.S., Jekins, G.J., and Mitchell, J.F.B., (2004). Generating High Resolution Climate Change Scenarios Using PRECIS. Met Office Hadley Center, Exeter, UK. Jones, R.G., Noguer, M., and Murphy. JM., (1995). Simulation of climate change over Europe using a nested regional climate model. Part I: assessment of control climate, including sensitivity to location of lateral boundaries, Quarterly Journal of Royal Meteorological Society, Vol. 121, Karim Z, Hussain Sk G, Ahmed A U. (1998). Climate change vulnerability of crop agriculture. In Vulnerability and Adaptation to Climate Change for Bangladesh, Kluwer Academic Publishers, Dordrecht, Kumar, K.K., Patwaedhan, S.K., Kulkarni, A., Kamala, K., Koteswara, K.R. and Jones, R. (2011). Simulated projections for summer monsoon climate over India by a high-resolution regional climate model (PRECIS), Current Science, Vol. 101, No. 3, Noguer, M., Jones, R., Hessel, D., Hudson, D., Wilson, S., Jenkins, J. and Mitchell, J., (2002). Workbook on generating high-resolution climate change scenarios using PRECIS., Hadley Centre for Climate Prediction and Research, Met Office, Bracknell, UK, 43 Rahman, M., Islam, N., Ahmed, A., and Georgi, F. (2012). Rainfall and temperature scenarios for Bangladesh for the middle of the 21 st century using RegCM., Journal of Earth System Science, 121, No.2, Rajib, M.A., Rahman, M.M., Islam, A.K.M, Edward, A. McBean (2011). Analyzing the Future monthly Precipitaion Pattern in Bangladesh from Mulit-Model Projections using both GCM and RCM., Proceeding of the World Environment and Water resources Congress, ASCE, 2011,