Effects of secondary ice processes on a stratocumulus to cumulus transition during a cold-air outbreak

The representation of boundary layer clouds during marine Cold-Air Outbreaks (CAO) remains a great challenge for weather prediction models. Recent studies have shown that the representation of the transition from closed stratocumulus clouds to convective cumulus open cells largely depends on microphysical and precipitation processes, which secondary ice production (SIP) may strongly modulate. In this study we use the Weather Research and Forecasting model to investigate the impact of the most well-known SIP mechanisms (HallettMossop, mechanical break-up upon collisions between ice particles and drop-shattering) on a CAO case observed north of the United Kingdom in 2013. While Hallett-Mossop is the only SIP process extensively implemented in atmospheric models, our results indicate that the other two SIP mechanisms are also favored in the examined conditions. Activation of drop-shattering and especially collisional break-up can result in enhanced riming, ice depositional growth and/or ice aggregation. The first two processes quicken liquid depletion in the stratocumulus cloud, while along with aggregation, they enhance precipitation. The increased precipitation results in enhanced evaporation/sublimation in the sub-cloud layer, promoting boundary-layer decoupling, which further accelerates the onset of the stratocumulus break-up. However, the strong sensitivity to the expression of terminal velocity of the precipitating particles and the rimed fraction of cloud ice/snow suggests that the robust implementation of SIP to improve CAO predictions requires data from a large number of CAO events.