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. A key challenge for numerical weather predictionmodels is representing boundary layer clouds in cold air outbreaks (CAOs).One important aspect is the evolution of microphysical properties asstratocumulus transitions to open cellular convection. Abel et al. (2017)have shown, for the first time from in situ field observations, that thebreak-up in CAOs over the eastern Atlantic may be controlled by thedevelopment of precipitation in the cloud system while the boundary layerbecomes decoupled. This paper describes that case and examines in situmeasurements from three more CAOs. Flights were conducted using the UKFacility for Airborne Atmospheric Measurements (FAAM) British Aerospace-146(BAe-146) aircraft in the North Atlantic region around the UK, makingdetailed microphysical measurements in the stratiform boundary layer. As thecloudy boundary layer evolves prior to break-up, increasing liquid waterpaths (LWPs) and drop sizes and the formation of liquid precipitation areobserved. Small numbers of ice particles, typically a few per litre, are alsoobserved. Eventually LWPs reduce significantly due to loss of water from thestratocumulus cloud (SC) layer. In three of the cases, aerosols are removedfrom the boundary layer across the transition. This process appears to besimilar to those observed in warm clouds and pockets of open cells (POCs) inthe subtropics. After break-up, deeper convective clouds form with bases warmenough for secondary ice production (SIP), leading to rapid glaciation. It isconcluded that the precipitation is strongly associated with the break-up,with both weakening of the capping inversion and boundary layer decouplingalso observed.

atmospheric chemistry and physics

In situ measurements of cloud microphysical and aerosol properties during the break-up of stratocumulus cloud layers in cold air outbreaks over the North Atlantic