The Understanding Severe Thunderstorms and Alberta Boundary Layers Experiment (UNSTABLE): overview and preliminary results

The Canadian Prairies are subjected to numerous severe thunderstorms with an average of 203 severe weather reports received by Environment Canada each summer. The Alberta foothills are a favoured region for thunderstorm development with more thunderstorm days occurring there than anywhere else within the Prairie Provinces. Most storms developing in the foothills move eastward to affect the Edmonton – Calgary corridor. This is one of the most densely populated and fastest growing regions in Canada. Alberta has proven to be particularly susceptible to costly thunderstorm events; Public Safety and Emergency Preparedness Canada estimate that since 1981 more than 40 lives and $2.5B have been lost due to severe thunderstorms. Its close proximity to the Rocky Mountains, poor understanding of boundary-layer processes and a scarcity of observations make forecasting in this region particularly challenging.

Various severe weather studies have stressed the importance of mesoscale convergence boundaries and boundary-layer water vapour in thunderstorm development. However, these boundaries and associated small-scale processes cannot be adequately resolved using existing synoptic-scale surface and upper-air observation networks on the Prairies. Previous thunderstorm research in Alberta has focused largely on storm structure and larger-scale upper-air processes. Conceptual models for severe storm outbreaks in Alberta are nearly 20 years old and do not focus explicitly on mesoscale boundaries that are known to be important for convective initiation (CI). Recent studies investigating the dryline in Alberta suggest that the genesis and evolution of this boundary may differ somewhat in comparison to conceptual models for the U.S. Plains. Gradients in latent and sensible heat fluxes arising from contrasting areas of soil moisture or land use have the potential to influence the development of mesoscale boundaries and CI in this region.

Environment Canada researchers and scientists from academia are conducting a field experiment over the Alberta foothills to investigate boundary-layer processes associated with CI and severe storms. The Understanding Severe Thunderstorms and Alberta Boundary Layers Experiment (UNSTABLE) focuses on three main areas related to improved forecasts of CI; boundary layer water vapour and convergence boundaries, land surface processes (i.e., sensible and latent heat fluxes) and associated circulations, and application of high-resolution numerical models as a forecast tool. The overall goals of UNSTABLE are to better understand the processes leading to the development of severe thunderstorms, refine conceptual models related to CI, and assess the utility of mesoscale numerical models to resolve physical processes over the Alberta foothills.

UNSTABLE consists of two field campaigns and associated modeling studies. During the summer of 2008, a pilot UNSTABLE field study was undertaken that included a two-week intensive observation period. Measurements from a special network of surface (fixed and mobile), upper-air, and airborne instruments, in combination with existing observing networks, were used to characterize processes associated with CI in the Alberta foothills during the peak summer severe weather season. Results from the pilot will be used to refine measurement and other strategies for a full-scale UNSTABLE experiment in 2011.

UNSTABLE will result in a dataset for future study with spatial and temporal resolution never before obtained in this region, or in other severe storm studies in Canada. Particular efforts will be made to transfer UNSTABLE results to forecast operations through traditional (e.g., journal articles and presentations at conferences and other fora) and experimental means such as Environment Canada's Research Support Desk initiative that allows research scientists to work directly in forecast operations with operational staff.