Abstract
Marine atmospheric boundary layer clouds cover vast areas of the Southern Ocean (SO), where they are commonly organized into mesoscale cellular convection (MCC). Using three years of Himawari-8 geostationary satellite observations, open and closed MCC structures are identified using a hybrid convolutional neural network. The results of the climatology show that open MCC clouds are roughly uniformly distributed over the SO storm track across mid-latitudes, while closed MCC clouds are most predominant in the southeast Indian Ocean with a second maximum along the storm track. The ocean polar front, derived from ECMWF-ERA5 sea surface temperature gradients, is found to be aligned with the southern boundaries for both MCC types. Along the storm track, both closed and open MCCs are commonly located in post-frontal, cold air masses. The hourly classification of closed MCC reveals a pronounced daily cycle, with a peak occurring late night/early morning. Seasonally, the diurnal cycle of closed MCC is most intense during the summer months (DJF). Conversely, almost no diurnal cycle is evident for open MCC.
Highlights
Marine atmospheric boundary layer (MABL) clouds play a primary role in defining the regional radiation budget over the Southern Ocean (SO) (Haynes et al, 2011) as they cover vast areas of the ocean surface (Trenberth and Fasullo, 2010) and 15 exert strong shortwave and longwave radiative effects (Hartmann and Short, 1980)
High-frequency geostationary satellite observations over the Southern Ocean (SO) are used to explore how marine atmospheric 330 boundary layer (MABL) clouds are organized in mesoscale cellular convection (MCC) morphologies
We first focus on developing a convolution neural network (CNN) model to identify and classify open and closed MCC clouds based on three years of Himawari-8 satellite data from 2016 to 2018, and to study their relationship to synoptic systems over the SO
Summary
Marine atmospheric boundary layer (MABL) clouds play a primary role in defining the regional radiation budget over the Southern Ocean (SO) (Haynes et al, 2011) as they cover vast areas of the ocean surface (Trenberth and Fasullo, 2010) and 15 exert strong shortwave and longwave radiative effects (Hartmann and Short, 1980). Despite the importance of MABL clouds, general circulation models (GCMs) and reanalysis products struggle to correctly simulate their complex microphysics and dynamics over the SO (Bodas-Salcedo et al, 2016; Kay et al, 2016) These biases commonly lead to the underestimation of shortwave radiation, in part because models produce less supercooled liquid water and lower cloud amount than observed, in the cold sector of extra-tropical cyclones and in marine cold air outbreaks (Bodas-Salcedo et al, 2012, 2016; 20 Field et al, 2014; Naud et al, 2014; Williams et al, 2013). MCC morphology types are phenological classifications, and an indication of underlying physical processes (Wang and Feingold, 2009b; Wood and Hartmann, 2006; Wood, 2012) These physical processes modulate fundamental features such as the overall cloud fraction and albedo, as well as microphysical properties such as precipitation rate, cloud droplet number concentrations and effective radius, affecting the radiation balance and precipitation efficiency of these clouds (Wood and Hartmann, 2006; Wood et al, 2011). Overnight the moisture fluxes from the surface will help rebuild the cloud deck
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