Severe lake-effect snowstorms over the Great Lakes: A climatological, numerical and forecasting approach (Ontario)

Lake-effect snowstorms are an important source of severe weather over the Great Lakes region, which typically occur as the cold and dry arctic air flows over the warm lakes after the passage of a synoptic-scale low-pressure system. The intense transfer of heat and moisture can trigger atmospheric convection that is typically organized into long quasi-two-dimensional features known as cloud streets, roll clouds or lake-effect snowbands. This type of cloud also occurs over the high-latitude ocean during cold air breaks. In this study, through developing a climatology of lake-effect snowstorms over Southern Ontario for the period 1992-99, we identify the distinguishing characteristics of the synoptic-scale environment associated with intense lake-effect snowstorms. We also find that the track of a low-pressure system can have a significant impact on the development or lack thereof of lake-effect snowstorms over southern Ontario. A cloud-resolving atmosphere model is employed to simulate the development of cloud streets over the Labrador Sea. The simulations were initialized and validated using observations during a cold air outbreak over the Labrador Sea. The model was able to reproduce the observed downstream evolution of the roll clouds, which indicates that the model can successfully capture the secondary flow associated with the roll clouds that results in significant differences in the temperature, humidity and momentum fields between the updrafts and downdrafts. After that, numerical simulations are employed to investigate the influence of synoptic-scale atmospheric conditions and ice concentrations on roll cloud development. The results indicate that the development of lake-effect snowstorms is significantly modified by the upstream airflow conditions including stability, humidity, air-lake, temperature difference and wind speed etc. The results also show that the sea-ice zone has a significant impact on the atmospheric boundary layer development, which can be seen in both the evolution of the cloud field and the development of heat and moisture transfer patterns. Finally through simulating an observed lake-effect snowstorm event, we demonstrate that future forecasting of such high impact weather systems with a high-resolution cloud-resolving model.