Entrainment and Aerosol Effects on Marine Boundary-Layer Clouds: An Investigation Using ACE-ENA Data From HOLODEC, G1, Pico and ACTOS

Abstract

Marine boundary-layer clouds cover large regions of the globe, and are known to strongly influence radiative balances.  The microphysical properties and persistence of these clouds are tightly coupled with cloud-top entrainment and aerosol properties within both the boundary layer and the overlying free troposphere. This proposal addresses the microphysical response to entrainment and aerosol properties in marine stratocumulus clouds, using data from the recently-completed Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) project. Specifically, emphasis will be placed on extensive in situ measurements taken with the Atmospheric Radiation Measurement (ARM) G1 aircraft, including the Holographic Detector for Clouds (HOLODEC) instrument operated by the principal investigator’s research group. In addition, that data set will be augmented with additional airborne and above-boundary-layer, mountain-top measurements taken during the first phase of ACE-ENA: the airborne measurements were obtained using the helicopter-borne Airborne Cloud Turbulence Observation System (ACTOS) operated by the Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig, Germany, and the mountain-based measurements were obtained at the Pico Mountain Observatory by the research group of the co-investigator and scientists from TROPOS. The proposed study builds on the ARM-funded deployment of the HOLODEC instrument. It is aligned with Research Topic 2 “Warm Boundary-Layer Atmospheric Processes” and will contribute to the understanding of cloud microphysical processes observed at the ARM Eastern North Atlantic site, where long-lived stratocumulus clouds are extensive.

The proposed research objectives are aimed at addressing the following scientific questions, all of which are related to the multifaceted roles of entrainment in determining microphysical properties of stratocumulus clouds. They can be concisely summarized as follows:

• Recent research has shown that relative dispersion of the cloud droplet diameter tends to increase with decreasing cloud condensation nucleus (CCN) concentration. This manifestation of the dispersion aerosol indirect effect is relevant to cloud optical properties. To what extent is this dispersion effect related to stochastic condensation, and what role does entrainment play?
• How do boundary-layer versus free-tropospheric aerosol properties influence cloud microphysical properties? In particular, what is the role of secondary activation when free-tropospheric CCN concentrations are high? What are the physico-chemical properties of single free tropospheric particles? And are free-tropospheric aerosol an important source of CCN in the remote marine boundary layer?
• How does cloud-top entrainment influence cloud microphysical properties, through the competing limits of homogeneous and inhomogeneous mixing? Recent research suggests that these limits can be better quantified using a coordinate system based on the liquid water content and the phase relaxation time. We will explore the possibility of a transition from inhomogeneous to homogeneous mixing with increasing distance from cloud top.
• What is the role of cloud-top shear in generating non-buoyancy-driven turbulence, which then contributes to vertical cycling and development of super-adiabatic drops? Relevant to this question is quantifying the turbulent kinetic energy dissipation rate using G1 and ACTOS data; an effort will be made to compare measured dissipation rates to those retrieved from remote sensing measurements at the ARM Eastern North Atlantic field site.
• What shape are cloud droplet size distributions, and what is the influence of entrainment on single versus two-mode size distributions? What are the implications for droplets in the autoconversion range?