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

Abstract

devastating flood-producing rainstorm occurred over southern Alberta, Canada from 19-22 June 2013. The long-lived heavy rainfall event was a result of complex interplays between topographic, synoptic and convective processes which rendered an accurate simulation of this event a challenging task. In this study the Weather Research and Forecasting (WRF) Model was used to simulate this event and was validated against several observation datasets. Both the timing and location of the model precipitation agree closely with the observations, indicating that the WRF model is capable of reproducing this type of severe event. Sensitivity tests with different microphysics schemes were conducted and evaluated using equitable threat and bias frequency scores. The double moment scheme (WDM6) generally performed better when compared with other schemes. The application of a conventional convective/stratiform separation algorithm shows that convective activity was dominant during the early stages then evolved into predominantly stratiform precipitation later in the event. The HYSPLIT back-trajectory analysis and regional water budget assessments using WRF simulation output suggest that the moisture for the precipitation was mainly from recycling antecedent soil moisture through evaporation and evapotranspiration over the Canadian Prairies and the US Great Plains. This analysis also shows that a small fraction of the moisture can be traced back to the northeastern Pacific; and direct uptake from the Gulf of Mexico was not a significant source in this event.