Abstract
Abstract. Extensive regions of marine boundary layer cloud impact the radiative balance through their significant shortwave albedo while having little impact on outgoing longwave radiation. Despite this importance, these cloud systems remain poorly represented in large-scale models due to difficulty in representing the processes that drive their life cycle and coverage. In particular, the mesoscale organization and cellular structure of marine boundary clouds have important implications for the subsequent cloud feedbacks. In this study, we use long-term (2013–2018) observations from the Atmospheric Radiation Measurement (ARM) Facility's Eastern North Atlantic (ENA) site on Graciosa Island, Azores, Portugal, to identify cloud cases with open- or closed-cellular organization. More than 500 h of each organization type are identified. The ARM observations are combined with reanalysis and satellite products to quantify the cloud, precipitation, aerosol, thermodynamic, and large-scale synoptic characteristics associated with these cloud types. Our analysis shows that both cloud organization populations occur during similar sea surface temperature conditions, but the open-cell cases are distinguished by stronger cold-air advection and large-scale subsidence compared to the closed-cell cases, consistent with their formation during cold-air outbreaks. We also find that the open-cell cases were associated with deeper boundary layers, stronger low-level winds, and higher rain rates compared to their closed-cell counterparts. Finally, raindrops with diameters larger than 1 mm were routinely recorded at the surface during both populations, with a higher number of large drops during the open-cellular cases. The similarities and differences noted herein provide important insights into the environmental and cloud characteristics during varying marine boundary layer cloud mesoscale organization and will be useful for the evaluation of model simulations for ENA marine clouds.
Highlights
It is well established that a small increase in the global coverage of marine boundary layer (MBL) stratocumulus clouds could offset warming associated with a doubling of CO2 (Hartmann and Short, 1980; Randall et al, 1984; Slingo, 1990)
Bony and Dufresne (2005) have shown that the simulation and response to the changing climate of MBL stratocumulus clouds represents the main source of uncertainty in cloud feedbacks simulated by Earth system models used for predicting the future climate
The eastern North Atlantic (ENA) broadly falls under the subsiding branch of the Hadley circulation, forcing a boundary layer inversion over a warmer sea surface that leads to shallow convection, it routinely experiences cold-air outbreaks (McCoy et al, 2017; Lamraoui et al, 2019; Ghate et al, 2020)
Summary
It is well established that a small increase in the global coverage of marine boundary layer (MBL) stratocumulus clouds could offset warming associated with a doubling of CO2 (Hartmann and Short, 1980; Randall et al, 1984; Slingo, 1990). The eastern North Atlantic (ENA) broadly falls under the subsiding branch of the Hadley circulation, forcing a boundary layer inversion over a warmer sea surface that leads to shallow convection, it routinely experiences cold-air outbreaks (McCoy et al, 2017; Lamraoui et al, 2019; Ghate et al, 2020) These cold-air outbreaks contain opencellular marine stratocumulus with the transition from the closed- to open-cellular cloud organization happening farther north.
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