Abstract
Abstract. A key challenge for numerical weather prediction models is representing boundary layer clouds in cold air outbreaks (CAOs). One important aspect is the evolution of microphysical properties as stratocumulus transitions to open cellular convection. Abel et al. (2017) have shown, for the first time from in situ field observations, that the break-up in CAOs over the eastern Atlantic may be controlled by the development of precipitation in the cloud system while the boundary layer becomes decoupled. This paper describes that case and examines in situ measurements from three more CAOs. Flights were conducted using the UK Facility for Airborne Atmospheric Measurements (FAAM) British Aerospace-146 (BAe-146) aircraft in the North Atlantic region around the UK, making detailed microphysical measurements in the stratiform boundary layer. As the cloudy boundary layer evolves prior to break-up, increasing liquid water paths (LWPs) and drop sizes and the formation of liquid precipitation are observed. Small numbers of ice particles, typically a few per litre, are also observed. Eventually LWPs reduce significantly due to loss of water from the stratocumulus cloud (SC) layer. In three of the cases, aerosols are removed from the boundary layer across the transition. This process appears to be similar to those observed in warm clouds and pockets of open cells (POCs) in the subtropics. After break-up, deeper convective clouds form with bases warm enough for secondary ice production (SIP), leading to rapid glaciation. It is concluded that the precipitation is strongly associated with the break-up, with both weakening of the capping inversion and boundary layer decoupling also observed.
Highlights
Cold air outbreaks (CAOs) frequently impact the North Atlantic region during winter months
While this paper focuses on synoptic-scale weather events, the problem of reproducing mid-latitude stratocumulus cloud (SC) is an issue in climate modelling, in relation to the Southern Hemisphere (SH) radiation bias, a result of Southern Ocean (SO) clouds not reflecting enough short-wave radiation
This paper presents in situ observations during four CAOs of clouds deep within areas of SCs, and in the transition regions that are associated with the eventual evolution to open cellular convection
Summary
Cold air outbreaks (CAOs) frequently impact the North Atlantic region during winter months. Connolly et al (2013) used a large eddy simulation (LES) to examine the impact of gravity waves on the marine boundary layer (MBL), finding that the waves do cause additional drizzle formation, the entrainment of warm dry air into the cloud can cause a change from a closed cell structure to a patchy cloud regime. These include (i) the dynamical structure of the boundary layer; (ii) surface fluxes of heat and moisture; (iii) entrainment of dry air at the cloud top; (iv) the availability of CCN and ice nucleating particles (INPs); (v) the microphysical structure, which, in part is controlled by (iv); (vi) the development of both liquid and frozen precipitation particles; (vii) the possible influence of gravity waves on (iii) and (vi) Adequate description of these in models is key to accurately simulating cloud cover during these events and predicting the impact of the severe weather associated with them. The main focus is on the microphysics of these cloud layers and the aerosol properties in the boundary layer
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