Abstract
Marine stratocumuli are the most dominant cloud type by area coverage in the Southern Ocean (SO). They can be divided into different self-organized cellular morphological regimes known as open and closed mesoscale-cellular convec- tive (MCC) clouds. Open and closed cells are the two most frequent types of organizational regimes in the SO. Using the liDAR- raDAR (DARDAR) version 2 retrievals, we quantify 59 % of all MCC clouds in this region as mixed-phase clouds (MPCs) during a 4-year time period from 2007 to 2010. The net radiative effect of SO MCC clouds is governed by changes in cloud albedo. Both, cloud morphology and phase, have previously been shown to impact cloud albedo individually, but their interac- tions and their combined impact on cloud albedo remain unclear. Here, we investigate the relationships between cloud phase, organizational patterns, and their differences regarding their cloud radiative properties in the SO. The mixed-phase fraction, which is defined as the number of MPCs divided by the sum of MPC and supercooled liquid cloud (SLC) pixels, of all MCC clouds at a given cloud-top temperature (CTT) varies considerably between austral summer and winter. We further find that seasonal changes in cloud phase at a given CTT across all latitudes are largely independent of cloud morphology and are thus seemingly constrained by other external factors. Overall, our results show a stronger dependence of cloud phase on cloud-top height (CTH) than CTT for clouds below 2.5 km in altitude. Preconditioning through ice-phase processes in MPCs has been observed to accelerate individual closed to open cell transitions in extratropical stratocumuli. The hypothesis of preconditioning has been further substantiated in large-eddy simulations of open and closed MPCs. In this study, we do not find preconditioning to primarily impact climatological SO cloud mor- phology statistics. Meanwhile, in-cloud albedo analysis reveals stronger changes in open and closed cell albedo in SLCs than MPCs. In particular few optically thick (cloud optical thickness > 10) open cell stratocumuli are characterized as ice-free SLCs. Theses differences in in-cloud albedo are found to alter the cloud radiative effect in the SO by 12 W m−2 to 39 W m−2 depending on season and cloud phase.
Highlights
In the Southern Ocean (SO), marine stratiform low clouds cover between 40 % to 60 % of the ocean surface (Wood, 2015) and due to their high albedo, they play a key role in the radiative balance of the Earth (Randall et al, 1984; Ramanathan 25 et al, 1989; Hartmann et al, 1992; Chen et al, 2000)
According to our cloud phase classifications, most mixed-phase clouds (MPCs) are characterized by a Mix cloud layer with ice-phase precipitation below cloud base in the SO
Note that spaceborne studies can either be based on passive instruments which typically only cover the cloud top phase (Morrison et al, 2011) or include active instruments like lidar or radar (Hu et al, 2010; Huang et al, 2012; Ahn et al, 2018; Mace et al, 2021) which can penetrate into layers below cloud top
Summary
In the Southern Ocean (SO), marine stratiform low clouds cover between 40 % to 60 % of the ocean surface (Wood, 2015) and due to their high albedo, they play a key role in the radiative balance of the Earth (Randall et al, 1984; Ramanathan 25 et al, 1989; Hartmann et al, 1992; Chen et al, 2000). In austral winter, open MCC reach their highest occurrence frequency whereas closed MCC occur more often in summer. Due to their organizational differences, the cloud fraction of closed MCC clouds is on average about 30 % higher than for open MCC clouds (Wood and Hartmann, 2006) and closed MCC clouds reflect more incoming shortwave radiation. All three studies utilise field observations of particular situations together with numerical models allowing them to disentangle the potential impact of different processes in greater detail It remains to be seen whether preconditioning due to 60 ice-phase processes occurs often and widely enough to impact statistics of cloud morphology and cloud reflectivity.
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