Understanding Southern Ocean Cloud Controlling Factors on Daily Time Scales and Their Connections to Midlatitude Weather Systems

Clouds and their associated radiative effects are a large source of uncertainty in global climate models. Understanding of these processes is thus important to constraining model equilibrium climate sensitivity and improving future climate projections. One region with particularly large model biases in shortwave cloud radiative effects (CRE) is the Southern Ocean. Previous research has shown that many dynamical “cloud controlling factors” influence shortwave CRE on monthly time scales, and that two important cloud controlling factors over the Southern Ocean are mid-tropospheric vertical velocity and estimated inversion strength (EIS). Model errors may thus arise from biases in representing cloud controlling factors (atmospheric dynamics), representing how clouds respond to those cloud controlling factors (cloud parametrizations), or some combination thereof.
This study extends previous work by examining cloud controlling factors over the Southern Ocean on daily time scales in both observations and global climate models. This allows the cloud controlling factors to be examined in the context of transient weather systems. Composites of EIS and mid-tropospheric vertical velocity are constructed around extratropical cyclones and anticyclones to examine how the different dynamical cloud controlling factors influence shortwave CRE around midlatitude weather systems and to assess how models compare to observations. On average, models tend to produce a realistic cyclone and anticyclone, when compared to observations, in terms of the dynamical cloud controlling factors. The difference between observations and models instead lies in how the models’ shortwave CRE responds to the dynamics. In particular, the models’ shortwave CRE are too sensitive to perturbations in mid-tropospheric vertical velocity and, thus, they tend to produce clouds that are too bright in the cold frontal region of the cyclone and too dim in the center of the anticyclone.