Characterization of the Dryline in Alberta: Observations from UNSTABLE 2008

One of the most active thunderstorm initiation (TI) regions in Canada is the foothills of the Rocky Mountains in Alberta. Storms developing there often impact the Edmonton-Calgary corridor which is one of the most densely populated and fastest growing regions in Canada. In Alberta, 113 severe weather reports are received each summer (10-yr average) and since 1987 the economic cost of severe thunderstorm events in the province totals over $2.4 billion (adjusted to 2010 dollars). While forecasters recognize the importance of this region for TI and have a general understanding of the meteorology involved, some smaller-scale details of the relevant physical processes are not yet fully understood.

In the summer of 2008, Environment and Climate Change Canada (formerly Environment Canada) led a field campaign to investigate TI in the Alberta Foothills. The Understanding Severe Thunderstorms and Alberta Boundary Layers Experiment (UNSTABLE) 2008 was conducted from June to August 2008 with an intensive observation period (IOP) from 9 to 23 July. During the IOP, a network of fixed and mobile observation platforms were used to sample surface and upper-air conditions associated with the potential for TI. One specific goal of UNSTABLE 2008 was to better understand the dryline in Alberta and develop a more complete conceptual model of the dryline circulation and boundary with respect to the role it may play in TI. The UNSTABLE campaign offered the first opportunity to obtain both surface and upper-air observations at, and in close proximity to, drylines which until then had been inferred using limited surface observations, remote sensing, and NWP data.

From 9 days with observed drylines, results from various in-situ observation platforms are summarized and used to characterize the evolution of the dryline and near-dryline environment. As existing radar coverage and surface observations were limited over the project domain, targeted surface observations were needed to define dryline position. These data, along with mobile surface observations allowed sampling of contrasting dry and moist air, dryline passage, and cross-dryline structure. Sounding and aircraft data provided additional observations of the boundary layer on either side of the dryline.

The above data have refined our conceptual model of the dryline in Alberta. While based on a small sample size, the results provide forecasters with a more complete understanding of Alberta dryline structure and evolution. Analysis maps illustrate the linear extent and spatiotemporal variability of dryline events while surface observations and mobile transects indicate the true strength of thermodynamic gradients and “narrowness” of the boundary at the surface. Upper-air soundings show the vertical structure of the dryline circulation and highlight changes in stability across the dryline. The results illustrate details of dryline structure that cannot be resolved using operational networks and equip forecasters with knowledge that can be applied in the context of forecasting TI and severe weather.