Abstract
Atmospheric marine particle concentrations impact cloud properties, which strongly impact the amount of solar radiation reflected back into space or absorbed by the ocean surface. While satellites can provide a snapshot of current conditions at the overpass time, models are necessary to simulate temporal variations in both particle and cloud properties. However, poor model accuracy limits the reliability with which these tools can be used to predict future climate. Here, we leverage the comprehensive ocean ecosystem and atmospheric aerosol-cloud data set obtained during the third deployment of the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES3). Airborne and ship-based measurements were collected in and around a cold-air outbreak during a three-day intensive operations period from September 17–19, 2017. Cold-air outbreaks are of keen interest for model validation because they are challenging to accurately simulate, which is due, in part, to the numerous feedbacks and sub-grid scale processes that influence aerosol and cloud evolution. The NAAMES observations are particularly valuable because the flight plans were tailored to lie along Lagrangian trajectories, making it possible to spatiotemporally connect upwind and downwind measurements with the state-of-the-art FLEXible PARTicle (FLEXPART) Lagrangian particle dispersion model and then calculate a rate of change in particle properties. Initial aerosol conditions spanning an east-west, closed-cell cloudy to clear air transition region of the cold-air outbreak indicate similar particle concentrations and properties. However, despite the similarities in the aerosol fields, the cloud properties downwind of each region evolved quite differently. One trajectory carried particles through a cold-air outbreak, resulting in a decrease in accumulation mode particle concentration (−42 %) and cloud droplet concentrations, while the other remained outside of the cold-air outbreak and experienced an increase in accumulation mode particle concentrations (+62 %). The variable meteorological conditions between these two adjacent trajectories result from differences in the local sea surface temperature altering stability of the marine atmospheric boundary layer because of the location of the Labrador Current. Further comparisons of historical satellite observations indicate that the observed pattern occurs annually in the region, making it an ideal location for future airborne Lagrangian studies tracking the evolution of aerosols and clouds over time under cold air outbreak conditions.
Highlights
Understanding marine aerosol, meteorological processes, and their impact on cloud properties is crucial for calculating the global radiative balance
Large-eddy simulations of cloud condensation nuclei (CCN)-poor stratocumulus clouds have shown that sedimentation of large droplets near cloud-top can reduce the amount of cloud-top evaporation and overall drying of the decoupled cloud layer, prolonging the life of the cloud (Ackerman et al, 2004; Bretherton et al, 2007; Hill et al, 2009)
Saliba et al (2020) indicates that there are almost always some continental and pollution particles in the North Atlantic marine boundary layer, accounting for >10% of the total organic and sulfate mass under what is otherwise identified as clean conditions
Summary
Understanding marine aerosol, meteorological processes, and their impact on cloud properties is crucial for calculating the global radiative balance. Cloud-top entrainment of free tropospheric air warms and dries the decoupled cloudy layer as the below-cloud surface-coupled layer continues to moisten from the latent heating, thereby strengthening the inversion between the two layers (Albrecht et al, 1995; Bates et al, 1998a; Bretherton and Wyant, 1997; Zhou et al, 2015) The progression of these two processes decreases the altitude of the lifting condensation level of rising air and increases the altitude of the lifting condensation level of sinking 95 air, respectively, causing the stratocumulus cloud base to rise until it has completely evaporated, while forming convectively driven cumulus clouds with lower cloud bases. Despite precipitation acting as a major sink for marine particle concentrations, the removal of aerosol can result in optimal conditions for new particle formation and high concentrations of small Aitken mode particles (Clarke, 1993; Clarke et al, 1999; Raes et al, 1997; Russell et al, 1998; Sanchez et al, 2018; Seinfeld and Pandis, 2006; Thornton et al, 1997; Yue and Deepak, 1982)
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