On the relationship between the snowflake type aloft and the surface precipitation types at temperatures near 0 °C

Abstract

Winter precipitation types can have major consequences on power outages, road conditions and air transportation. The type of precipitation reaching the surface depends strongly on the vertical temperature of the atmosphere, which is often composed of a warm layer aloft and a refreezing layer below it. A small variation of the vertical structure can lead to a change in the type of precipitation near the surface. It has been shown in previous studies that the type of precipitation depends also on the precipitation rate, which is directly linked to the particle size distribution and that a difference as low as 0.5°C in the vertical temperature profile could change the type of precipitation near the surface. Given the importance of better understanding the formation of winter precipitation type, the goal of this study is to assess the impact of the snowflake habit aloft on the type of precipitation reaching the surface when the vertical temperature is near 0°C. To address this, a one dimensional cloud model coupled with a bulk microphysics scheme was used. Four snowflake types (dendrite, bullet, column and graupel) have been added to the scheme. The production of precipitation at the surface from these types of snow has been compared to available observations. The results showed that the thickness of the snow-rain transition is four times deeper when columns and graupel only fall through the atmosphere compared to dendrites. Furthermore, a temperature of the melting layer that is three (four) times warmer is required to completely melt columns and graupel (dendrites). Finally, the formation of freezing rain is associated with the presence of lower density snowflakes (dendrites) aloft compared to the production of ice pellets (columns). Overall, this study demonstrated that the type of snowflakes has an impact on the type of precipitation reaching the surface when the temperature is near 0°C.