Aerosol in Pockets of Open Cells
Low lying marine stratiform clouds exhibit an important net cooling effect
on the global climate system. Thus understanding climate change depends in part
on our insight how these clouds from, are maintained,
and dissipate. Research over the last two decades suggests that aerosols
modulate these processes.
From an aerosol perspective,
this work elucidates peculiar “holes” (a.k.a. Pockets of Open
Cells, POCs) embedded in an otherwise stratiform cloud sheet encountered during the second
research flight of the second Dynamics and Chemistry of Marine Stratocumulus
experiment (DYCOMS-II). Utilizing in situ and satellite data we demonstrate
that the cores of these pockets are associated with a boundary layer consisting
of a below-cloud layer, a thin cloud layer with variable cloud-top heights, and
a gap zone between the cloud-top and the top of the marine boundary layer. This
structure is shown to be associated with extremely low accumulation mode
aerosol (ACM) concentrations (1-30 cm-3). Also associated with POCs were Aitken mode particles
whose mode diameter was approximately 0.025 mm. These are thought due to a particle
nucleation event which occurred several hours prior to the research flight and
the in situ observations suggest that the event occurred within the remote
marine boundary layer. The Aitken mode particles were
not effective cloud droplet nuclei and seem to be derived from an oceanic,
non-hygroscopic precursor.
Our results suggest that ACM
concentrations, cloud morphology, and new particle nucleation are tightly
interwoven. If this is true, then small changes in the physicochemistry
of marine aerosol, e.g. due to increased wind speeds or anthropogenic
emissions, may affect the planetary albedo more
strongly than previously thought.