Spatial Variability and Composition of Ice Nucleating Particles over the Southern Ocean

Abstract

Supercooled liquid clouds are ubiquitous over the Southern Ocean (SO), even down to temperatures of -25 °C, and comprise a large fraction of the marine boundary layer clouds. Many Earth System Models and reanalysis products overestimate the occurrence of ice and have insufficient liquid cloud cover in the region, while recent simulations have found that the microphysical representation of ice nucleation and growth has a large impact on these properties. However, measurements of SO ice nucleating particles (INPs) to validate simulated ice nucleation are sparse, and many previous observations were limited to fairly warm temperatures (-15 or -20 °C). Observations of INPs are presented here from two simultaneous field campaigns in the SO during January-March 2018: the Clouds, Aerosols, Precipitation Radiation and atmospherIc Composition Over the southeRN ocean II (CAPRICORN-2) study on the CSIRO R/V Investigator, and the Southern Ocean Cloud Radiation Aerosol Transport Experimental Study (SOCRATES) on the NSF/NCAR G-V aircraft. The SOCRATES campaign is noteworthy for collecting the first in-situ observations in and above cloud in the SO. Measurements of INPs active in the immersion freezing mode were made during both projects in real time with Colorado State University (CSU) Continuous Flow Diffusion Chambers (CFDCs) at temperatures below -25 ℃, and via offline analyses of aerosol filter and seawater samples using the CSU Ice Spectrometers from -10 to -30 ℃. INP concentrations were at the lower bound of those from other ocean regions, and much lower than historical measurements in the SO collected prior to the early 1970s. Chemical treatments performed on the filter suspensions were used to infer the fraction of biological, organic, and mineral INPs, which varies with latitude and height, and indicate a variety of sources, including local marine aerosol and dust. Data from G-V overflights of the R/V Investigator were used to investigate the vertical structure of INPs in this region. Electron microscopy analyses of INPs collected from the CFDCs, along with back trajectories and aerosol measurements, provide additional information on INP composition and possible sources.