Hazardous Freezing Rain and Wet Snow in the Past and Future Climate of Manitoba, Canada, Using a Pseudo-Global Warming Approach

Freezing precipitation events are a major hazard across much of Canada, usually occurring in the form of freezing rain and wet snow and causing significant damage to transportation networks, infrastructure, and vegetation. Under future warming climatic conditions, the spatio-temporal characteristics of this precipitation may change but there is great uncertainty. The focus of this study is characterizing damaging freezing precipitation events within the province of Manitoba and examining their future occurrence within a warmer climate.

To examine these issues, 8 events were identified that impacted Manitoba. Three of these events had freezing rain only, 3 had wet snow only, while the final 2 had a combination of both. These were characterized using several Environment and Climate Change Canada (ECCC) datasets, and data from the Japanese 55-year Reanalysis (JRA-55). As well, two high resolution 4 km Weather Research and Forecasting (WRF) simulations performed by the National Center for Atmospheric Research (NCAR) were utilized, spanning a period of October 2000 to September 2013 (Liu et al., 2016). These were a retrospective control (CTRL) and a pseudo-global warming (PGW) simulation covering CONUS and Canada up to approximately 57°N. The CTRL simulation was forced with initial and boundary conditions from the European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA) Interim data and the PGW simulation duplicated the CTRL simulation but modified it with the monthly ensemble mean thermodynamic and dynamic change from a 19 member CMIP5 ensemble at the end of the 21st century, using the high emissions RCP 8.5 scenario.

Large scale and local factors were associated with these events. Most (7 of 8) showed consistent large scale forcing: a midlatitude cyclone with 500 hPa trough and jet exit enhancing lift, low surface pressure centre nearby, and an atmospheric river. Local factors, such as the elevated terrain of Riding Mountain National Park (approximately 450 m above the surrounding area), influenced 3 or 4 of the events. The terrain acted as a focal point for the hazardous precipitation by altering low level temperature and/or wind fields. This terrain is also somewhat co-located with areas of severe ice loading, as calculated by the Canadian Standards Association.

In the PGW simulations, these events changed significantly. In the 3 freezing rain events, 2 (1) increased (decreased) in extent, precipitation accumulation, and duration. In the 3 wet snow events, all decreased in extent, duration, and accumulation. In the combination events, 1 event increased in extent, duration, and accumulation, while the other decreased in extent and duration, and had decreased or the same accumulation. In addition, there were 3 instances in December and January in which there was no wet snow in the actual events (CTRL), but there was with pseudo-global warming (PGW).