Using Canadian GEM output for forecasts of convective initiation on the Canadian prairies: experimental techniques under development in the Hydrometeorology and Arctic Lab (HAL)

Upper-air climatology studies over the western Canadian prairies and the U.S. High Plains show that, during the summer months, the troposphere is conditionally unstable on most days. This highlights the importance of Atmospheric Boundary Layer (ABL) evolution for the initiation or inhibition of deep convection. Recent observational and modeling studies suggest that improved understanding of small-scale processes related to ABL water vapour, convergence, and lower tropospheric vertical wind shear is necessary for improved forecasts of convective initiation (CI). These processes are not readily resolved observationally by existing synoptic-scale networks.

Operational forecasters routinely utilize NWP output to complement a thorough analysis of observational data, calculate derived stability and other parameters, and fill in data-void areas. A variety of model-derived fields, parameters, and indices are available to characterize the evolution of the troposphere and environments conducive to the development of severe storms. Few of these focus on ABL processes, and fewer still on the problem of CI specifically. As part of Environment Canada's Research Support Desk (RSD) initiative, the Hydrometeorology and Arctic Lab (HAL) have designed and are evaluating a suite of experimental model fields in an effort to improve forecasts of CI. These fields are derived using full-resolution (58 eta levels, 15 km horizontal spacing) hourly output from the Canadian Meteorological Centre's Global Environmental Multi-scale (GEM) Regional model and focus on processes related to ABL water vapour, convergence, and lower-tropospheric wind shear. During the summer of 2007, the experimental (and other severe weather) fields were made available to Prairie and Arctic Storm Prediction Centre operations via an RSD web page and formed the basis for daily experimental forecasts and discussions on CI. Operational forecasters were invited to explore them on an experimental-only basis and provide feedback directly to the developers.

Given the sizeable area of the Canadian prairie domain, only partial radar coverage, and limitations of satellite imagery and manned observations, lightning data was selected to be used for field verification. Objective evaluation of the experimental fields is being conducted using cloud-to-ground lightning data from the Canadian Lightning Detection Network (CLDN) as a proxy for CI. First, each flash over the Canadian Prairies from the summer of 2007 was correlated with CI field values interpolated in both space and time. Second, an objective method of identifying the initial lightning strike for each storm was developed and applied to associate “CI flashes” with experimental field values. Results will be used to refine formulation of the fields and to explore applications to objective CI and lightning probability forecasts.