Turbulence profiles and cloud-surface coupling in Arctic stratiform clouds

Description

Stratiform clouds play a key role in Arctic climate through their high frequency of occurrence, persistence, variability of phase composition, and impacts on radiation and hydrology. Many of the key mechanisms that characterize these clouds are poorly understood due to complexities associated with a three-phase water system that is sensitive to aerosol properties and intricately tied to the atmospheric structure. Observations suggest that these clouds are at times thermodynamically coupled to the surface while at other times they are decoupled by an intervening stable layer. In this study we examine the coupling state of mixed-phase stratiform clouds during the fall and spring transition seasons at Barrow, Alaska.

Characteristic signatures of the coupling state can be derived from ground-based remote sensors such as Doppler cloud radar. For example, radar-derived turbulent dissipation rates that are quasi-constant in height from the cloud layer down to the surface suggest that both the cloud and surface contribute to the turbulence affecting the cloud layer. On the other hand, a decrease in dissipation rates below the cloud suggest that the cloud itself is primarily responsible for generating its own turbulence without much influence from the surface. Retrievals of the dissipation rate are first evaluated using independent measures from aircraft and tethered-balloon systems, demonstrating that the radar estimates are able to reasonably represent the dissipation rates and their vertical distribution. We then explore the impact that the coupling state has on a number of cloud properties such as depth, height, microphysical properties, and temporal variability. For example, it is generally found that surface-coupled clouds tend to have larger liquid water paths and stronger vertical motions than those that are decoupled. Overall, the cloud-surface coupling state can have significant impacts on the cloud structure and lifetime as it controls the balance of heat, moisture, and aerosols to the cloud from below and above.