The life cycle of a pocket of open cells and its response to biomass burning aerosol

Wednesday, December 12, 2018 - 08:00
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Pockets of open cells (POCs) are regions of broken, precipitating stratocumulus clouds embedded into an overcast stratocumulus cloud deck with little or no drizzle. Owing to their low cloud fraction, POCs have a reduced cloud shortwave radiative effect compared to the surrounding stratocumulus cloud. POCs can persist for several days, but respond to meteorological and microphysical (aerosol) controls. This work simulates, for the first time, the complete life cycle of a POC. The POC was observed in the South-East Atlantic during the Cloud Aerosol Radiation Interactions and Forcing field campaign (CLARIFY). Lagrangian cloud-system resolving simulations which track the POC are evaluated with geostationary and sun-synchronous satellite observations and in-situ data. The role and significance of a biomass burning aerosol layer for the POC, sampled during the CLARIFY campaign, are investigated. The in-situ observations show that biomass burning aerosol layer was located close to (above) the inversion in the late stage of the POC life cycle, but with little evidence of significant entrainment of biomass burning aerosol into the boundary layer. At earlier stages of the POC life cycle, the boundary layer was shallower, creating a gap between the inversion and the biomass burning aerosol layer, rendering entrainment of aerosol from the free troposphere into the boundary layer unlikely. Simulations with a reduced vertical separation between the biomass burning aerosol layer and the inversion over the course of the POC life cycle show suppression of precipitation and a transition from the open cell state into a high cloud fraction cloud state with a high cloud shortwave radiative effect in the POC.

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