Stratocumulus cloud clearings: statistics from satellites, reanalysis models, and airborne measurements
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Published:2020-04-21
Issue:8
Volume:20
Page:4637-4665
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Dadashazar HosseinORCID, Crosbie Ewan, Majdi Mohammad S., Panahi Milad, Moghaddam Mohammad A., Behrangi Ali, Brunke Michael, Zeng XubinORCID, Jonsson Haflidi H., Sorooshian ArminORCID
Abstract
Abstract. This study provides a detailed characterization of stratocumulus clearings
off the US West Coast using remote sensing, reanalysis, and airborne in
situ data. Ten years (2009–2018) of Geostationary Operational Environmental
Satellite (GOES) imagery data are used to quantify the monthly frequency,
growth rate of total area (GRArea), and dimensional characteristics of
306 total clearings. While there is interannual variability, the summer
(winter) months experienced the most (least) clearing events, with the lowest
cloud fractions being in close proximity to coastal topographical features along the central
to northern coast of California, including especially just south of Cape
Mendocino and Cape Blanco. From 09:00 to 18:00 (PST), the median length,
width, and area of clearings increased from 680 to 1231, 193 to 443,
and ∼67 000 to ∼250 000 km2,
respectively. Machine learning was applied to identify the most influential
factors governing the GRArea of clearings between 09:00 and 12:00 PST, which
is the time frame of most rapid clearing expansion. The results from
gradient-boosted regression tree (GBRT) modeling revealed that air
temperature at 850 hPa (T850), specific humidity at 950 hPa
(q950), sea surface temperature (SST), and anomaly in mean sea level
pressure (MSLPanom) were probably most impactful in enhancing
GRArea using two scoring schemes. Clearings have distinguishing
features such as an enhanced Pacific high shifted more towards northern
California, offshore air that is warm and dry, stronger coastal surface
winds, enhanced lower-tropospheric static stability, and increased
subsidence. Although clearings are associated obviously with reduced cloud
fraction where they reside, the domain-averaged cloud albedo was actually
slightly higher on clearing days as compared to non-clearing days. To
validate speculated processes linking environmental parameters to clearing
growth rates based on satellite and reanalysis data, airborne data from
three case flights were examined. Measurements were compared on both sides
of the clear–cloudy border of clearings at multiple altitudes in the
boundary layer and free troposphere, with results helping to support links
suggested by this study's model simulations. More specifically, airborne
data revealed the influence of the coastal low-level jet and extensive
horizontal shear at cloud-relevant altitudes that promoted mixing between
clear and cloudy air. Vertical profile data provide support for warm and dry
air in the free troposphere, additionally promoting expansion of clearings.
Airborne data revealed greater evidence of sea salt in clouds on clearing
days, pointing to a possible role for, or simply the presence of, this
aerosol type in clearing areas coincident with stronger coastal winds.
Funder
Office of Naval Research National Aeronautics and Space Administration
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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