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

Abstract

Shortwave radiation biases over the Southern Ocean (SO) stem largely from a poor under- standing of low clouds in the cold sectors of extratropical cyclones, where rapid transitions between low cloud mesoscale morphologies are frequently observed. Stratus dominates the poleward regime of the cyclones, and it transitions into closed mesoscale cellular convection (MCC) downstream and then to open MCC behind the cold front. In the warm sector, clustered and suppressed cumulus are more abundant than in other parts of the cyclone domain. Aiming to improve the understanding of processes controlling morphologies over the SO, cyclone-centric composites are constructed to study how low cloud mesoscale mor- phologies relate to cloud controlling factors using principal component analysis. The first two principal components are strongly related to cyclone intensity and sea surface tem- perature respectively. Daily-average insolation at the top of the atmosphere is used as an additional independent predictor. Over the SO, insolation is an important factor control- ling low clouds, and likely controls seasonal variability through weakening cloud top cooling. Closed and open MCC are negatively correlated with insolation, while disorganized MCC and clustered cumulus are positively correlated with high insolation. This seasonality of morphologies is broadly consistent with summertime deepening of the boundary layer seen in other datasets. Stratus, on the other hand, displays a weaker seasonal cycle. Closed MCC, open MCC, and clustered cumulus tend to form in stronger cyclones, while stratus and suppressed cumulus tend to form in weaker cyclones. The two stratiform morphologies (closed MCC and stratus) are more abundant in cyclones over a colder sea surface, while the two cumuliform morphologies (clustered cumulus, suppressed cumulus) are more frequently found in cyclones over a warmer ocean. 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.