An evaluation of kilometer-scale ICON simulations of mixed-phase
stratocumuli over the Southern Ocean during CAPRICORN
Abstract
This study investigates the representation of stratocumulus (Sc) clouds,
cloud variability, and precipitation statistics over the Southern Ocean
(SO) to understand the dominant ice processes within the Icosahedral
Nonhydrostatic (ICON) model at the kilometer scale using real case
simulations. The simulations are evaluated using the shipborne
observations as open-cell stratocumuli were continuously observed during
two days (26th -27th of March 2016), south of Tasmania. The radar
retrievals are used to effectively analyze the forward-
simulated radar signatures from Passive and Active Microwave TRAnsfer
(PAMTRA). We contrast cloud-precipitation statistics, and microphysical
process rates between simulations performed with one-moment (1M) and
two-moment (2M) microphysics schemes. We further analyze their
sensitivity to primary and secondary ice-phase processes
(Hallett–Mossop and collisional breakup). Both processes have
previously been shown to improve the ice properties of simulated shallow
mixed-phase clouds over the SO in other models. We find that only
simulations with continuous formation, growth, and subsequent melting of
graupel, and the effective riming of in-cloud rain by graupel, capture
the observed cloud-precipitation vertical structure. In particular, the
2M microphysics scheme requires additional tuning for graupel processes
in SO stratocumuli. Lowering the assumed graupel density and terminal
velocity, in combination with secondary ice processes, enhances graupel
formation in 2M microphysics ICON simulations. Overall, all simulations
capture the observed intermittency of precipitation irrespective of the
microphysics scheme used, and most of them sparsely distribute intense
precipitation (>1mm h-1 ) events. Furthermore, the
simulated clouds are too reflective as they are optically thick and/or
have high cloud cover.