Fundamentals of winter precipitation: theory and measurements

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Near-freezing temperatures and a mixture of precipitation types such as snow, freezing rain and wet snow are often linked to cold season high-impact events. One example is the 1998 Ice Storm where up to 100 mm accumulated at the surface. The difficulty in both their simulation and their detection arises because the precipitation types reaching the surface depend on the atmospheric conditions and they can be formed of liquid, solid or a mixture. The long-term objective is to improve the fundamental representation of winter precipitation type formation into models, including their detailed characteristics, as well as their interactions with other particles and the environment. This research program will fund 11 highly qualified personals (HQPs). They will use a combination of numerical experiments using high-resolution atmospheric models, field measurements and theoretical approaches to address the following three key issues. First, the key formation processes will be studied theoretically and by using well-observed storms. In particular, the processes leading to the freezing of falling precipitation particles will be investigated to differentiate between ice pellets and freezing rain. Second, the detailed characteristics of precipitation particles in various regions of Canada will be assessed using information on the particles' sizes and fall speeds measured in the field by an optical detector of precipitation particles. Long-term datasets will be analysed to compare the characteristic of the particles in different climatic regions of Canada (ex: Alberta and southern Québec). This information will be used to determine the performance of automatic snow gauges. Third, the precipitation type characteristics in the context of climate change over southern Québec will be evaluated. Targeted high-resolution climate simulations, which include the formation of freezing rain, will be conducted. Threshold values of the precipitation particle characteristics such as their sizes and falls peed will be assessed and linked to the storms atmospheric conditions. Overall, the issue of winter precipitation is of major importance to Canada where winter storms are often experienced across the country. Precipitation is one of the most important factors as the specific type of precipitation and intensity can have catastrophic consequences on the environment and on infrastructures. Each weather event contributes to the regional climatology, hence, further investigation on winter storms and associated precipitation will be a key step to bridge the gap with climate studies and better anticipate their changes in the future. The outcome of this research program will serve to a broad community such as hydrology, policy makers for risk assessment, and the private sector, in particular electrical utilities, to develop mitigation plan.