Climate model downscaling of Vancouver Island precipitation using a synoptic typing approach

A statistical downscaling technique is employed to link atmospheric circulation produced by climate models at the large-scale to precipitation recorded at individual weather stations on Vancouver Island. Relationships between the different spatial scales are established with synoptic typing, coupled with non-homogeneous Markov models to simulate precipitation intensity and occurrence in historical and future periods. Types are generated through a clustering algorithm which processes daily precipitation observations recorded by Environment Canada weather stations spanning 1971 to 2000. Large-scale atmospheric circulation data is taken from an ensemble of climate model projections made under the IPCC AR4 SRES A2 scenario through the end of the 21st century. Atmospheric predictors used to influence the Markov model are derived from two versions of the data: Averages of model grid cells selected by correlation maps of circulation and precipitation data; a new approach involving Common Empirical Orthogonal Functions (EOFs) calculated from model output over the Northeast Pacific Ocean. Circulation-based predictors capture the role of sea level pressure (SLP), and winds in influencing coastal precipitation over Vancouver Island. The magnitude and spatial distribution of the projected differences are dependent on the predictors used. Projections for 2081 to 2100 made using common EOFs result in most stations reporting no statistically significant change compared to the baseline period (1971 to 2000) in both seasons. Projections using averaged grid cells find winter season (Nov-Feb) precipitation anomalies produce values that are modestly positive, with typical gains of 6.5% in average precipitation, typical increases of 7.5% rising up to 15% in extreme precipitation, and little spatial dependence. In contrast, average and extreme summer precipitation intensity (Jun-Sep) declines negligibly at most island weather stations with the exception of those on the southern and western sections, which experience reductions of up to 20% relative to the latter thirty years of the twentieth century. Precipitation occurrence decreases slightly in both seasons at all stations with declines in the total days with measurable precipitation ranging from 2% to 8% with reductions also seen in the length of extended periods of precipitation in both seasons.

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