Deficiencies in marine subtropical low-cloud properties in general circulation models have been shown to affect radiative and dynamic interactions in the atmosphere, as well as the atmospheric and oceanic coupling. However, diagnosing the reasons for these deficiencies through comparison with data has been difficult because previous datasets have lacked the necessary scope in either time or in space to properly characterize variability on synoptic and larger scales. In this study, we remedy this by using global satellite-retrieved cloud properties.; Spectral analysis reveals that most of the time variability of cloud properties occurs on seasonal to annual time scales. The length of these dominant time scales suggests that the majority of the variability is influenced by the general circulation and its interaction with boundary layer turbulence, rather than a product of boundary layer turbulence alone.; Further analysis indicates that shifts in cloud-type in the subtropics occur for different reasons. In particular, the Northern Hemisphere (NH) subtropics undergo a change in dynamic regime during the local winter season. This change appears in the cloud fields as a shift from the more commonly seen lower-altitude, thicker optical thickness clouds to higher-altitude, thinner clouds. The latter cloud-type is associated with the lower sea level pressure, upward vertical velocity phase of the synoptic wave.; A single-column model (SCM) is used to investigate how the passage of a synoptic wave modulates both cloud properties and the turbulent processes of the boundary layer. We confirm some of the same relationships between clouds and meteorology found in our data analysis and from previous studies. We also observe that changes in the large-scale forcings alter the rate at which clouds modulate interactions between the atmosphere and the ocean, and the boundary layer and the upper troposphere as well as the nature of the feedbacks of the system. In particular, we found that the cloudy boundary layer is initially established by large-scale condensation in a statically stable layer. As the large-scale stability decreases (in this case, by large-scale advection), moist convection becomes a more dominant process, ventilating the lower layers of the atmosphere and increasing the relative humidity aloft. Although our SCM atmosphere begins to wander away from “reality”, it appears that the advective tendencies act to reestablish the vertical stability as the atmosphere returned to the higher sea level pressure anomaly phase of the wave.
展开▼
