Sensitivity of Mesoscale Circulations during a July 2009 Heat Wave at Vancouver, British Columbia, Using a High-Resolution Model Simulation and a Pseudo-Global Warming Approach

Heatwaves represent a major hazard for mid-latitude climates. One such event at Vancouver, British Columbia in July 2009 had profound public health impacts since the region does not normally experience such extreme temperatures. Heatwave events over the region are driven by synoptic and mesoscale interaction from features such as the West Coast Thermal Trough (WCTT), coastal southerlies, sea-breezes and mountain-valley circulations. To gain better insight into such processes, and to understand how they may change in the future climate – high resolution (4 km) data from the Weather and Research and Forecasting (WRF) model were utilized for a control (CTRL) simulation and a future projection using a pseudo-global warming (PGW) approach for the period of the heatwave. Initial and lateral boundary conditions for the WRF CTRL simulation were provided by ERA-interim reanalysis (for a base period of 1976-2005), whereas those for the WRF PGW simulation were derived by applying mean monthly climate perturbations from an ensemble of global climate models (for a future projection from 2071-2100 under a mid-range climate change scenario) to the ERA-interim dataset. Results suggest that model error and changes to u-component flow are sensitive to v-component wind intensity; and that local deflection of positive v-wind anomalies (coastal southerlies) causes local suppression of the sea-breeze – further enhancing temperatures. Further, modulation from the WCTT and coastal southerlies have a greater impact on sea-breeze propagation than thermal parameters such as sensible heat-flux during the heatwave simulation. At the same time, mountain-valley circulations are shown to be sensitive to Planetary Boundary Layer height during the simulation period.