The Effect of Multi-Model Averaging of Climate Model Outputs on the Seasonality of Rainfall Over the Columbia River Basin

The rainfall seasonality index is the measure of precipitation distribution throughout the seasonal cycle. The aim of this study is to compare the effect of different multi-model averaging methods on the rainfall seasonality index at each 1/16 latitude-longitude cells covering the Columbia River Basin. In accordance with the same, ten different climate model outputs are selected from 45 available climate models from CMIP5 dataset. The reanalysis precipitation data is used to estimate the errors in rainfall seasonality for the climate model outputs. The inverse variance method and statistical multi criteria analysis (SMCA) method were used to estimate the weights for each climate model output. The precipitation amounts from the climate model outputs were then averaged using these model weights. The rainfall seasonality index was estimated from: (1) observed reanalysis data; (2) averaged precipitation amount from ten combinations of CMIP5 outputs for the current climate (1979–2005) using inverse variance method; (3) averaged precipitation amount from the ten combinations of CMIP5 outputs for the current climate using SMCA. The results showed the no/little differences in rainfall seasonality index for each climate model averaging. Moreover, the multi-modelling of climate models resulted in relative improvements in the performance of the rainfall seasonality over the Columbia River Basin as compared to individual models. The estimated model weights for the current climate can be useful to combine the model outputs for the future climate. Conclusion Two Multi-model Averaging Methods: Inverse Var and Weights Findings Acknowledgment  SI of observed dataset in CRB indicates a seasonal pattern in South-east and south-west region with rather seasonal with short drier season in extreme east part of the basin.  Rainfall is equally spread throughout the with a definitive wetter season in northern parts (rocky mountain chain) with central part depicting the pattern closer to eastern parts of the basin.  As per Multi-model climate series with both the techniques, the SI index is spatially different compared to observed.  Western region of the basin is preserving the behavior of seasonality of rainfall as represented by observations.  Eastern and Northern part reveals SI les than equal to 0.2 which represents that the rainfall is spread equally throughout the year.  Central region of CRB represents equal spread but with definite wetter season.  There is particularly no visible difference between the seasonality accessed from the two different multi-modelling methods.  GCM downscaled using statistical methods seem not been able to replicate the observations on finer resolution.  Though the Multimodal time series failed to replicate the observations but this is relative huge improvement when compared to individual model results. Rainfall Seasonality Index (SIi), estimated by Eq (4), shows the variability of precipitation through the year (Walsh and Lawler, 1981). SIi = 1 Ri Xin − Ri 12 n=12