Processes Associated with Boundary Layer Cloud Ice Phase Precipitation in the High Southern Latitudes

Abstract

A high bias in absorbed solar radiation at the surface of the Southern Oceans (SO) due to low-biased cloud cover is at the root of many challenges in simulating the response of the Southern middle and high latitudes to climate change (Trenberth and Fasullo, 2010; Hwang and Frierson, 2012; Armour et al, 2016). It is the extensive shallow convective and extended stratiform cloudiness in the marine boundary layer (MBL) that give the Southern Oceans south of -60 degrees one of the highest cloud cover fractions of any comparable region on Earth (Mace et al., 2009; Mace and Zhang, 2014). These extensive shallow cloud systems are typically found in the cold air portions migratory middle latitude cyclones. In the high southern latitudes, the pristine aerosol environment results in super-cooled water persisting for a much longer time and to lower temperatures than in the northern hemisphere, which has important implications on ice- and liquid-phase precipitation processes and microphysical and radiative properties (Kanitz et al., 2011). In particular, the phase partitioning of the extended supercooled clouds is both governed by the small scale precipitation processes but also ultimately linked to the Earth’s climate sensitivity (Tan et al, 2016).
Understanding the climate sensitivity has, at its foundation, an understanding of the processes that occur on the aerosol and cloud scale where models must be capable of simulating either statistically or explicitly what is observed in nature. DOE ARM has conducted two significant deployments to the Southern Ocean recently that addresses this issue. The Macquarie Island Cloud and Radiation Experiment (MICRE) fielded a suite of remote sensors on Macquarie Island and collected unprecedented data for more than a year and presently the Measurements of Aerosols, Radiation and Clouds over the Southern Ocean (MARCUS) is being conducted with the AMF2 deployed on the Aurora Australis that is transiting between Australia and the Antarctic multiple times per year over the next two years. We are seeking funding to help analyze that data to address the following science questions.

1. What are the conditions that govern phase partitioning in supercooled boundary layer clouds over the high latitude Southern Oceans?
2. To what extent are supercooled mixed phase clouds responsible for the high cloud fractions over the Southern Ocean and do these mixed phase clouds contribute to the model biases in absorbed shortwave radiation?
3. Can the cloud properties and phase partitioning over the Southern Ocean be linked to the underlying aerosol environment?
4. How do the aerosol properties at the surface vary spatially and seasonally over the high latitude Southern Ocean and what processes control these features?
5. How well defined are the atmospheric compositional boundaries between Southern Ocean marine, Antarctic free tropospheric, and Antarctic surface airmasses, and how does this influence the chemistry and physics of the Southern Ocean atmosphere?

We are building on a unique base of experience to address the science questions. Both the PI and Co-I have spent many months aboard research vessels in the Southern Oceans collecting data that are similar to the data collected during MICRE and MARCUS. We have developed innovative data reduction and retrieval algorithms uniquely tailored to such data that we will apply to the MICRE and MARCUS data sets.