Predicting the Frequency of Low Cloud Mesoscale Morphologies in Southern Ocean Extratropical Cyclones Using Cloud Controlling Factors

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Abstract Shortwave radiation biases over the Southern Ocean (SO) stem largely from a poor understanding of low clouds in the cold sectors of extratropical cyclones, where rapid transitions between low cloud mesoscale morphologies are frequent. Stratus dominates the poleward regime of the cyclones. It transitions into closed mesoscale cellular convection (MCC) downstream and then to open MCC in the cold sector of cyclones. Clustered and suppressed cumulus are often found in the warm sector. Principal component (PC) analysis is applied to a set of cloud controlling factors to characterize properties of the entire extratropical cyclone that are critical to low cloud mesoscale morphologies. The first two PCs are strongly related to cyclone intensity and sea surface temperature averaged over the cyclone domain, respectively. Daily average insolation at the top of the atmosphere, which has large seasonal and latitudinal variability over the SO, is used as an additional independent predictor. Closed and open MCC are negatively correlated with insolation, while disorganized MCC and clustered cumulus are positively correlated with insolation. In stronger cyclones, closed MCC, open MCC, and clustered cumulus tend to be more frequent, whereas stratus and suppressed cumulus tend to be less common. In cyclones over a colder sea surface, closed MCC and stratus are more abundant, and clustered cumulus and suppressed cumulus are less abundant. These results deepen the current understanding of low cloud processes and provide insights of transitions between morphologies, and thus changes in cloud radiative effects, over the SO in a changing climate.

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