Effects of atmospheric conditions on the formation of winter precipitation types

Winter storms produce major problems for society and their precipitation is often the key factor responsible. The objective of this thesis is to better understand the formation of this precipitation. A multi-moment bulk microphysics scheme coupled with a cloud model has been developed to address this issue. It predicts the mass mixing ratio and total number concentration for many hydrometeor categories including rain, snow, freezing rain, wet snow, slush and ice pellets. Semi-melted particles have been incorporated into the bulk scheme since they are commonly formed at temperatures near 0°C and they influence the formation of other types of precipitation within the atmosphere and reaching the surface. Considering a vertical profile in the atmosphere, the precipitation type characteristics during the 1998 Ice Storm in the Montreal area have been compared with aircraft measurements. Many of the observed characteristics were reproduced by the model. Also, sensitivity tests on the precipitation types formed during the Ice Storm were performed. The results show that small variations (<0.5°C) in the temperature profiles as well as the precipitation rate can have major impacts on the types of precipitation formed at the surface in such a catastrophic event. Using a two-dimensional cloud model, the effects of the background wind on the precipitation type evolution within the atmosphere and at the surface have likewise been investigated. These results were compared with observations taken during a field project held at the Centre for Atmospheric Research Experiments (CARE) in the Toronto area during the winter 2006-2007. The results reproduced many of the precipitation types observed. The background wind field and snowfield aloft influence the type and the amount of precipitation reaching the surface. Overall, the environmental factors such as the temperature, the degree of saturation and the background wind critically affect the type of winter precipitation formed. This study demonstrates how sensitive the precipitation types formed across a winter storm precipitation transition region are to these environmental factors and it also shows the complexity of the prediction of winter precipitation types. These insights lead to strong requirements on prediction models but they also show the limitations of empirical techniques mainly using temperature profiles to diagnose the type of precipitation.