Numerical simulation of pockets of open cells formation in the South-east Pacific during VOCALS-Rex

Low stratocumulus clouds in the marine boundary layer play an important role for the Earth's radiative energy balance by influencing the effective planetary albedo. As such stratocumulus regions impose a negative shortwave cloud forcing for the global climate system.

Observations from satellites show a variety of cloud field morphologies in stratocumulus regions ranging from homogeneous and overcast stratus sheets to broken stratocumulus decks with either open or closed cellular appearance. Pockets of open cells embedded in otherwise unbroken stratocumulus decks (POCs) are also regularly observed cloud features. These different cloud morphologies modulate the overall albedo of stratocumulus regions on a daily to inter-seasonal timescale.

Yet, the interactions and feedbacks between the processes that trigger the formation of POCs at small-scales and the mesoscale circulations that control their life-cycle at larger scales are not very well understood. Especially unclear is the interplay between aerosols, clouds and precipitation that may be responsible for the observed microphysical and macrophysical properties of the POCs.

In this contribution we aim to simulate cases of POC formation observed in the South-east Pacific off the Chilean coast during the VOCALS-Rex field campaign. The numerical simulations are performed with the Consortium for Small-Scale Modeling's (COSMO) model which is a mesoscale non-hydrostatic limited-area model with a coupled double-moment aerosol-cloud-microphysics parameterization. The model is driven by meteorological initial and boundary conditions taken from the ECMWF analysis archive. A sequence of nested simulations with grid spacings ranging from 10km to 2km are performed to capture the formation of POCs and their organization within the mesoscale environment. The model simulations are evaluated and compared against satellite imagery and in-situ microphysical measurements from the NCAR/NSF C-130 research flights.