A numerical study of the June 2013 flood-producing extreme rainstorm over southern Alberta

A devastating flood-producing rainstorm occurred over southern Alberta of Canada during June 19-22 2013. The long-lived heavy rainfall from this event was a result of complex interplays between topographic, synoptic and convective processes, which rendered the accurate simulation of this event a challenging task. The Weather Research and Forecasting (WRF) Model was used to simulate this event. The results were evaluated by using CaPA (Canadian Precipitation Analysis) data and both the timing and location of the model precipitation features closely fit the observations, indicating that the WRF model is capable of reproducing this type of severe event. Sensitivity tests were carried out with reduced-mountain height to examine the effects of topographic lifting in producing the precipitation. In addition, Q-vector diagnostics were carried out to evaluate the relative contribution of orographic lifting, mid-troposphere cyclonic vorticity advections, lower tropospheric frontogenesis and latent heat release in producing the updrafts.

Regional water budget assessments and HYSPLIT back trajectory analysis using WRF simulation output suggests that the moisture for the precipitation was mainly from the large scale moisture convergence by advection across significant part of the continent, especially the recycling of the antecedent soil moisture through evapotranspiration over the Canadian Prairies and US Great Plains. Part of the moisture can be further traced back to the northeastern Pacific but direct uptake from the Gulf of Mexico was not a significant source of moisture for this event.