Characterizing Pineapple Express storms in the Lower Mainland of British Columbia, Canada

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Authors: Spry, CM; Kohfeld, KE; Allen, DM; Dunkley, D; Lertzman, K
Year: 2014
Journal: Can. Water Resour. J. 39: 302-323   Article Link (DOI)
Title: Characterizing Pineapple Express storms in the Lower Mainland of British Columbia, Canada
Abstract: Pineapple Express (PE) storms are subtropical winter storm systems that are known to trigger precipitation-induced natural hazards. However, they have not previously been well characterized in the Lower Mainland of British Columbia. This study uses an established chronology of PE storms to identify and compare PE and non-PE storms, using (1) meteorological variables recorded at meteorological stations from the Vancouver International Airport and Capilano watershed, (2) streamflow variables from a hydrometric station in Capilano watershed and (3) precipitation samples collected for oxygen isotope analysis from 27 recent storms identified as PE or non-PE storms using satellite imagery. Historical trends in PE storms were then estimated using records of PE storm magnitude (as recorded at Vancouver International Airport) and frequency (as determined from the subset of PE storms that made landfall between 47.5 and 52.5 degrees N) for the period of 1948 to 2011. Results from a Mann-Whitney U-test suggest that median precipitation values of PE storms are significantly higher than those of non-PE storms at the Capilano watershed. Comparisons of the 75th, 80th, 85th, 90th and 95th quantiles suggest that both precipitation and streamflow rates at stations in the Capilano watershed were significantly higher for PE storms than for non-PE storms. Precipitation samples from three PE storms identified with satellite imagery exhibited substantially less-depleted oxygen isotope ratios compared with non-PE storms, consistent with a subtropical water vapour source inferred from satellite images. In effect, from 1948-2007, Vancouver experienced 0-7 days/year that are classified as PE storm days, and these storm days contributed an average of 11 and 17% to annual precipitation and stream discharge totals, respectively. Annual PE storm maxima (i.e. total precipitation for the largest PE storm day in a given year) show significant correlations with both storm frequency and annual average temperature, suggesting that the largest PE storms tend to occur in warmer years with increased storm activity. The study also suggests an inverse relationship between the percent contribution of PE storms to annual precipitation and the Madden-Julian Oscillation (MJO) index. These results suggest that water years (1 October-30 September) with substantial PE precipitation are correlated with the negative phase of the MJO when convection is enhanced in the western Pacific Ocean.
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