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Atmospheric river storms in western North America: attribution of the November 2021 BC event, impact-relevant diagnostic variables, and implications for communicating future projections

On 14th November 2021, a strong landfalling atmospheric river (AR) brought two days of intense rainfall to southwestern British Columbia. This led to landslides, extensive flooding, and loss of life and property. A rapid attribution study conducted after the event found 1) that AR storms with similar characteristics (i.e., with event total integrated vapour transport, TIVT, of the same magnitude and trajectory) are approximately one in 10 year events in the current climate of this region, 2) that such events have been made at least 60% more likely by the effects of human-induced climate change, and 3) that further warming will lead to increased frequency of such events in the future.

However, when the storm was characterized in terms of 2-day precipitation (RX2day) over the region, the event was found to have a rarity of 1 in 50 to 100 years in the current climate — substantially more rare than the AR itself. To help account for this discrepancy, characterization of the AR storm was expanded to include horizontal moisture convergence, column relative humidity, and the associated principal condensation rate. While the AR storm was strong but not unprecedented in terms of IVT and TIVT, its principal condensation rate, which is found to be highly correlated with extreme rainfall over the region, was substantially more rare, with a return period consistent with that found for RX2day. What does this result say about the usefulness of variables like IVT and TIVT that have traditionally been used to diagnose ARs and their potential impacts?

More broadly, are standard diagnostics and operational AR storm classification systems that rely on IVT useful for real-time warnings and communicating future projections of ARs with climate change? To answer, the correspondence between extreme precipitation and IVT, moisture convergence, and principal condensation rate is explored over western North America in historical reanalyses and a large initial condition ensemble of Environment and Climate Change Canada's CanRCM4 regional climate model. Initial results suggest that AR impacts are weakly associated with IVT and total IVT and that principal condensation rate is a useful supplement to better characterize extreme precipitation during AR storms.

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