Wintertime In-situ Cloud Microphysical Properties of Mixed-phase Clouds
over the Southern Ocean
Abstract
In-situ observations made over twenty flights during three Austral
winters (Jun–Oct, 2013–15) were analyzed to characterize the cloud
microphysical properties and natural variability of mid-latitude shallow
convective clouds over the Southern Ocean (SO), with a focus on pristine
conditions and the mixed-phase temperature range (MPTR, 0 to -31ºC).
Liquid, mixed-phase, and ice cloud fractions were observed 39%, 44%,
and 17% of the time, respectively, under various meteorological
settings. Liquid phase clouds were typically characterized by low
droplet number concentrations and the common presence of drizzle.
Supercooled liquid water was prevalent in the MPTR, while freezing of
supercooled raindrops likely formed the primary ice nucleation mechanism
in these shallow clouds. Ice particles of various habits were present in
the mature/maturing convective cloud cells, suggesting the operation of
multiple particle growth regimes. Increased ice particle concentrations
(exceeding 100 L-1), well in excess of the expected
ice nuclei concentrations, were measured at temperature warmer than
approximately -12℃, signaling the operation of secondary ice production
mechanisms. However, these cloud segments were spatiotemporally
inhomogeneous, suggesting the chaotic and turbulent nature of the
secondary ice-forming processes. Accurately representing these processes
in global models, while necessary, is likely a challenge. Our analysis
also found marked inconsistencies between several satellite-based cloud
phase products that have underpinned recent developments of model
parameterization frameworks. Understanding and addressing these
inconsistencies are critical towards improving the representation of SO
clouds and their radiative properties in climate models.