Evaluation And Projection Of Extreme Precipitation And Drought Using CMIP6 High ResMIP Simulations Over West Africa

Climate study over West Africa is a rigorous activity due to topography of the region.The climate models used has to be hi tech simulation and one of the determinant of the model performance is its resolution.This thesis evaluates eight models from High Resolution Multiple-model Intercomparison Project（High Res MIP）,a consortium of Couple Model Intercomparison Project Phase 6（CMIP6）.The aim is to investigate the performance of improved horizontal（higher）resolution on the corresponding low resolution simulations in reproducing extreme precipitation and drought over West Africa.Projection of extreme precipitation and drought were also performed after models evaluation.The methods employed in this study include calculating the arithmetic average of low and high resolution simulations,precipitation anomaly,Probability Density Function（PDF）,Taylor diagram,Pearson correlation coefficient（r）,standard deviation（STD）,Standardized Precipitation Evapotranspiration Index（SPEI）,Precipitation Concentration Index（PCI）,Mann Kendall trends analysis,Z-statistics.The study covers the historical period of 1950 – 2014 and 2025 –2049 for projection.Precipitation dataset from Global Precipitation Climatology Centre（GPCC）,Climate Research Unit（CRU）TSv4.03,and University Delaware（UDEL）v5.01 were used as reference with the purpose of accounting for uncertainty.The findings are listed below1.The observed precipitation annual peak over Guinea Coast,Sahel and entire West Africa is around 250,150,and 120 mm/m during September,August,and August,respectively.The High Res MIP models overestimates the precipitation onset over the Guinea Coast,but show a high skill over Sahel.The GCMs simulated early annual rainfall peak around May over Guinea coast.The high rainy area from observation is southeast and southwest,the pattern is adequately reproduced by the GCMs,although this is overestimated by some of the models during the monsoon period（June to September）.The pattern of observed extreme precipitation over southeast and southwest of the region seems to be adequately reproduced by most of the high and low resolution simulations,although with variations in the simulated amount.However,the Mann Kendall trends analysis for extreme precipitation shows the ability of high resolutions simulations in capturing the significant increasing trends over the two subregions.The observed highest 1-day precipitation seems to be overestimated by majority of models with higher resolution and underestimation by the simulation with low resolution.During June – September（JJAS）,CMIP6 simulations showed a high correlation coefficient,low Root Mean Square Error（RMSE）and ratio of standard deviation around 1 for mean climatology and spatial correlation coefficient relatively lower during extreme events.Meanwhile,the orography induced rainfall is adequately reproduce by high resolution models even when observation could not capture it.The improved horizontal resolution does not remove biases in the GCMs,therefore the bias correction employed improve the model performance.2.The projected precipitation annual cycle reveal precipitation increase from 130 to 150mm/m over Sahel while the peak month still remains at August.Over Guinea coast,the pattern of High Res MIP simulation is not consistent with the regular pattern of precipitation over the region（i.e.,the projected annual peak is May which appears to be early）.Over entire West Africa,precipitation annual peak also increase by 50 mm/m in the projected simulations with its peak around September.The seasonal precipitation change with reference to historical period shows positive change over Sahel by most of High Res MIP,although simulation with low resolution projected more positive change than high resolution simulations.The standardized precipitation anomaly reveal 2025– 2030 precipitation may be below mean value（i.e.,standardized precipitation anomaly is projected to be-2 during this period）while subsequently it turns to wet period.Precipitation higher than mean value is projected around 2030 – 2035 and reach its peak（i.e.,with projected standardized precipitation anomaly around +3）during 2035.The95 th percentile precipitation,highest one day precipitation,heavy and very heavy precipitation day projected positive change of precipitation over Sahel.CDD（CWD）increases（decreases）in the temporal evolution of extreme precipitation.3.The historical evaluation of drought shows Potential Evapotranspiration（PET）is at its lowest estimate over Guinea coast,Sahel,and entire West Africa during annual rainfall peak.The value is less than 20,less than 70,and 55 over Guinea coast,Sahel,and West Africa entirely.It reaches its highest value during the dry period and the corresponding value in those regions is greater than 20,greater than 80,and greater than 60 mm/m.All the models underestimate PET over Guinea coast and entire West Africa which is an indication of underestimation of maximum and minimum temperatures in estimating PET.The skill of high and low resolution simulations is low in reproducing precipitation pattern over Guinea coast,and that affected the CWA pattern which is not the case over Sahel.The study also reveal the Guinea coast is plagued with hydrological drought than the remaining two drought types considered.The climatological distribution of SPEI-12 and the Mann Kendall（MK）trends test indicate significant wet condition over most region except Nigeria.It is also reveal that moderate precipitation distribution dominates Guinea coast and strong irregular over the northern part of Sahel.ENSEMBLE-H outperform the GCMs especially over Sahel in simulating similar pattern with the reference data.The performance of the High Res MIP simulations in reproducing drought features quantitatively reveals PET and CWA produce a higher correlation coefficient with observation while the correlation between the simulation and observation in SPEI analysis is generally low.4.The drought projection shows no significant PET increase over Sahel and Guinea coast except MPI-H and MPI-L where there is slight increase in comparison with historical.The study shows there is projected increase of CWA over Sahel and entire West Africa indicating the increase in precipitation over the region might not allow dry condition to be sustain.Further projection analysis of SPEI distribution shows wet condition all over West Africa in meteorological and agricultural drought.But hydrological drought depicts dry condition over Guinea coast region.The projected trends reveal inter-model difference.But there is strong agreement of significant drought over Niger.The result from this thesis serves as baseline study of performance of CMIP6 High Res MIP models over West Africa.It shows no doubt that improvement in model horizontal resolution is crucial exercise.No single model whether low or high can be used to study all features and predicts climate over West Africa（i.e.,FGOAL-H and FGOAL-L performed better in annual cycle,MRI-H and MRI-L performed better in climatological mean distribution and extreme,ENSEMBEL-H and ENSEMBLE-L performed better quantitatively while EC-EARTH-H outperform in drought analysis）.It is recommended that each model institution should determine to improve the models performance,also improving the performance of the models does not limit to improving the horizontal resolution alone.A lot of biases is inherent in some of the simulations.