Wintertime In-situ Cloud Microphysical Properties in the Mixed-phase Temperature Regime over the Mid-latitude Southern Ocean

The Southern Ocean (SO) is a critical component of the Earth’s climate system. Yet both climate models and reanalysis products continue to suffer from large biases in the radiation budget over the SO, which have been attributed to errors in the representation of cloud, aerosol, precipitation and their interactions over this region.

A poor physical understanding of these processes is due, to a significant degree, to sparse field observations in this remote region. This challenge is exacerbated in winter months when the roaring winds and extreme waves of the storm track prevent a direct access to the harsh environment. Satellite products, too, suffer from greater uncertainties in winter. In particular, retrieved cloud properties based on visible channels of passive remote sensors are sensitive to solar zenith angle, with the largest uncertainties during winter months when zenith angles are too high. Therefore, in-situ measurements are of important value for evaluating satellite products and constrain the retrieval assumptions.

In this study, unique in-situ observations made by 20 flights in the vicinity of Tasmania over the SO during three Austral winter seasons (June-October, 2013-2015) are analyzed to investigate the microphysical properties of the mid-latitude SO clouds. Instruments aboard the aircraft include a Droplet Measurement Technologies Cloud Aerosol and Precipitation Spectrometer (CAPS) which incorporates a hot-wire liquid water sensor, a single particle light scattering Cloud and Aerosol Spectrometer (CAS), and a Cloud Imaging Probe (CIP). Bulk cloud water content was measured by a Science Engineering Associates WCM-2000 Multi-Element Water Content System and thermodynamic variables were measured by a Meteolabor TP-3S sensor.

In this analysis, we primarily focus on the mixed-phase temperature regime between 0 and −15°C, in which liquid droplets and ice crystals coexist. When only pristine / baseline condition is considered, 61% of the 1-Hz cloud samples fall in this temperature regime. Using the ice-water fraction computed from the WCM-2000 measurements, we classify the cloud samples into three broad categories: liquid, mixed-phase, and heavily glaciated clouds. It is found that, within the temperature range considered, these three categories of clouds were observed 37%, 60%, and 3% of the time, respectively, with mixed-phase clouds being most prevalent.

To quantitatively investigate the microphysical properties of the mixed-phase clouds, particle number concentration, size distribution, and asphericity measured / computed from the CAPS data are analyzed. Evidence of secondary ice production (e.g. fresh ice particles such as needles and columns) was commonly observed in the Hallett–Mossop temperature zone (−3 to −8°C), with high number concentrations recorded up to 54 L⁻¹ . These mixed-phase clouds were also commonly observed to be precipitating. Using aerosol particle size measurements and number concentrations measured by the CAS in clear-air passes below and above the sampled clouds, ice nucleating particle (INP) number concentrations are estimated with a recognized ice nuclei parametrization scheme. It is found that the observed ice number concentrations are typically a few orders of magnitude higher than the estimated INP concentrations. The observed high ice number concentrations are largely consistent with the theoretical values when ice crystals are produced via a splinter production. Our analysis suggests that, similar to other regions of the world, secondary ice processes (likely the Hallett–Mossop ice multiplication mechanism) may be playing a key role in ice (and precipitation) production in shallow convective clouds during winter, where the in-situ observations were made.

This study offers complementary insights to the preliminary findings from the recent “Southern Ocean Cloud Radiation Aerosol Transport Experimental Study” (SOCRATES) field campaign, where supercooled liquid water (SLW) was found to be prevalent over the high-latitude SO during the summer months (Jan - Feb) of 2018. It has been commonly hypothesized in the literature that the poor representation of SLW clouds contributes substantially to the simulated cloud and radiative biases in climate models over the SO. Yet an open question remains as to under what conditions and to what extend SLW becomes predominant. Our study suggests that a physical understanding of the production, evolution and transformation of SLW is critical to support the ongoing effort towards a better representation of clouds, aerosol, precipitation and their interactions over this climatically important region.