Changing Cold Regions Network

Winter precipitation types. The impact of crystal type aloft on the type of precipitation reaching the surface was analyzed. To address this issue, many types of snow crystal have been included into a sophisticated microphysics scheme used to study the formation of winter precipitation produced at temperatures near 0°C. The production of precipitation at the surface from these types of snow has been compared to available observations. The thickness of the snow-rain transition was found to be four times deeper when columns and graupel only fall through the atmosphere compared to dendrites. Freezing rain at the surface is associated with the presence of fluffy snow aloft (ex: dendrites) whereas ice pellets are produced from the melting of graupel. Using this scheme it was shown that dense particles such as graupel would have led to solid precipitation at the surface during the Alberta Flooding events. This had an impact on the severity of the storm. Overall, this study demonstrated that the type of snowflakes impacts the type of precipitation reaching the surface when the temperature is near 0°C. 

Precipitation events in the Kananaksis Valley. During the March-April 2015, precipitation events in the Kananaskis Valley were documented. They were associated with both westerly (downslope) and easterly flow fields (upslope) aloft. It is generally considered that only easterly conditions are significant but 11 of the 17 events were associated with westerly flow. Even though the surface temperature was near 0°C throughout the study period, westerly flow events were generally associated with warmer and drier environmental conditions than easterly flow ones. This is consistent with general descent in westerly flows and ascent in easterly ones and it led to a higher occurrence of rain (35%) at the surface in westerly flows. Many types of precipitation were observed. Rain, snow and mixed phase precipitation all occurred at the surface and often within the same event. Solid precipitation particles, which 60% were rimed, reached the surface at temperatures up to 9°C and relative humidity < 40% as previously reported by Harder and Pomeroy (2013). This is critical because within the typically dry lower atmosphere, it was suggested that the more dense particles would be less likely to sublimate aloft and this contributed to the higher fraction of such particles at the surface as well as higher amounts of precipitation. To assess how these precipitation events will change in warmer and moister conditions, the WRF 4 km simulations, both historical and pseudo-global warming (PGW), produced by CCRN have been used. The WRF historical simulations reproduced most of the events documented in March-April 2015 (70%). In warmer and moister conditions, more precipitation is produced (26%) but the type of events did not changed. In some instances, the type of precipitation changed from snow to rain (3 events) but the amount of graupel remained constant. 

Future changes in freezing rain. To assess future changes in the occurrence of precipitation associated with temperatures near 0°C, in particular freezing rain, we used the fifth generation of the Canadian Regional Climate (CRCM5) model with a 0.44° grid mesh. We focused on the Canadian Prairies, which is the region of interest in the recent study on freezing rain by Kotchubadja et al. (2017). CRCM5 was first driven from ERA-Interim for the 1981-2000 period and by the GCM-MPI for the 1981-2000 and 2081-2100 periods using the scenario RCP 8.5. Freezing rain was diagnosed using Bourgouin (2000), which is used operationally at ECCC. The preliminary results indicate that CRCM5 reproduced fairly well the mean annual number of hours of freezing rain. It also suggested an increase of freezing rain occurrence in the Northwest Territories in the future (30 h/year) with changes south of the domain. This is due to higher occurrence of near 0°C temperatures to the north during the year and with a seasonal shift to the south.