Classification of the Below-Cloud Mixing State Over the Southern Ocean
Using In-Situ and Remotely-Sensed Measurements
Abstract
We demonstrate that the relationship between the abundance of
particulate surface area observed at sea-level and measurements of
backscattered light by a ceilometer can be used to classify the mixing
state of the atmospheric layer beneath the lowest observed cloud, where
the relationship is defined by the Spearman Rank correlation. The
accuracy of this correlation-based method was compared to two methods of
detecting boundary layer decoupling based on radiosonde measurements. An
optimized version of the new methodology correctly determined the mixing
state of the below-cloud layer for 76 ± 4% of the radiosondes available
for comparison. Further, it was more accurate than an alternative
ground-based metric used to determine the below-cloud mixing state. For
the majority of the time series in which the correlation analysis could
be applied, the below-cloud boundary layer was well-mixed (54%), or
else fog was present (27%), which indicated that aerosol particles
observed at sea-level often have a direct pathway into low-cloud (81%).
In the remaining analysis period, the near-surface atmospheric layer was
stable and the atmospheric layer near the ocean surface was decoupled
from the overlying cloud (19%). Forecasts from the Antarctic Mesoscale
Prediction System also support our findings, showing that conditions
that mix aerosol particles from the ocean surface to the lowest observed
cloud occur 84% of the time over the open Southern Ocean. As a result,
aerosol particles measured near sea-level are often tightly coupled to
low-cloud formation over the Southern Ocean, highlighting the utility of
shipborne aerosol observations in the region.