Investigation of the Surface Coupling of Marine Clouds and its Interactions with Aerosols Over the Southern Ocean

Abstract

The persistent presence of marine low clouds (cloud tops below 3 km) depends on their coupling with the surface which supplies the energy, water, and cloud condensation nuclei (CCN) needed for their formation and development. In an ultra-clean environment like the Southern Ocean, clouds and precipitation are particularly susceptible to aerosol-cloud interactions through the modification of the cloud droplet number concentration (Nd). The cloud-sea surface coupling affects Nd in two ways. First, it provides the pathway through which surface-generated aerosols can reach cloud bases to serve as cloud condensation nuclei. Second, by modifying the moisture and energy budget of the marine boundary layer, the degree to which marine low clouds couple with the surface influences precipitation and cloud-top entrainment which controls Nd through scavenging and entrainment processes. A better understanding of this coupling and its impact on Nd and the ensuing effect on aerosol-cloud interactions may shed light on the cause of a widely-known problem of global climate models (GCMs): underestimation of cloud cover over the Southern Ocean, although there may be other contributing factors such as ice or mixed-phase clouds. Most, if not all, such models are incapable of treating coupling processes and their impacts on Nd which considerably affects aerosol-cloud interactions. By exploiting the measurements made during the Measurements of Aerosols, Radiation, and Clouds in the Southern Ocean (MARCUS) field campaign and large-eddy simulations (LES), we will address the following questions:

1. Under what large-scale conditions are marine low clouds coupled/decoupled with the surface?
2. How does the coupling regulate the transport of aerosols and their impacts on the Nd
3. What is the combined effect of coupling and the modified Nd on cloud cover?

The proposed work involves characterizing and quantifying both the thermodynamic and dynamical coupling of marine low clouds with the sea surface using Atmospheric Radiation Measurement (ARM) observations during MARCUS field campaign and an LES model. ARM data from multiple sensors will be used (e.g., Doppler cloud radar, ceilometer, and microwave radiometer) to characterize the marine low cloud-surface coupling by virtue of the vertical structure and integrated quantities of boundary-layer clouds, aerosols, as well as atmospheric profiles and surface meteorology. We will use a combination of case studies and statistics-based composite analyses to find any linkages between large-scale dynamics, marine low cloud-surface coupling, and cloud and boundary layer properties, the sequence of which reflects the chain of causality. An LES model with explicit aerosol physics, which includes the cycle of aerosols by being consumed as cloud condensation nuclei and be regenerated following cloud droplet evaporation, will be run to determine the sources and sinks of Nd and their dependence on the degree of marine low cloud-surface coupling. After sorting marine low clouds into (surface) coupled and non-coupled systems, we shall attempt to examine any systematic differences in both Nd and cloud occurrence between the two clusters under different meteorological conditions and further examine their respective roles, as well as causal relationships by means of LES modeling. This study shall help improve our understanding of the aerosol-cloud interactions by differentiating the dynamic role of the coupling and cloud physics denoted by Nd. It may thus bridge a broken linkage in the chain toward understanding mechanisms governing the persistence of marine low clouds over the Southern Ocean. Such an improved understanding will help solve the long-lasting problem of the cloud cover underestimation over the Southern Ocean by GCMs.
The proposed study is in direct response to the current call: Conduct studies using ARM funded data to improve understanding of cloud, aerosol, and/or surface-interaction processes in southern high latitudes.