Evidence for secondary ice production in Southern Ocean open cellular convection

Abstract

Ice particles present at temperatures warmer than −9 °C were encountered in unexpectedly high number concentrations (up to 54 L−1) by an instrumented aircraft over the Southern Ocean (SO), off the southwest coast of Tasmania, Australia, on 7 September 2013. The sampled clouds were precipitating, characterized by mixed‐phase, open‐cellular shallow convection. These clouds were present within a large‐scale environment characterized by cold air advection, in a pristine air mass for over 72 h. Using a Cloud and Aerosol Spectrometer, aerosol particles (diameters > 0.6 µm) size and number concentrations were measured and ice nucleating particle (INP) number concentrations were estimated with a recognized ice nuclei parametrization scheme. The estimated INP number concentrations were in the range of 10−5–10−1 L−1 at temperature above −9 °C, which is up to three orders of magnitude less than the ice number concentrations typically observed. The high ice number concentrations are largely consistent with the theoretical values when ice crystals are produced via a splinter production. The evidence suggests that secondary ice processes (likely the Hallett–Mossop mechanism) were playing a key role in generating the high ice number concentrations observed. Satellite observations from an A‐Train overpass in the neighbourhood during the flight period reveal a qualitatively consistent story, with patchy, mixed‐phase (but predominantly supercooled liquid water) clouds observed at cloud‐top temperatures around −6 °C. Using back trajectory calculations, these clouds are tracked over 23 and 46 h with A‐Train observations. The presence of these clouds is found to be common over the SO during this period of time. This suggests that the ice particles present in a relatively warm temperature range could potentially be commonplace, within the widespread (up to thousands of kilometres) shallow convective cloud fields over the SO. These clouds may have important implication for the energy budget and precipitation production over this climatically important region.

A Comparison of Cloud Microphysical Properties Derived From MODIS and CALIPSO With In Situ Measurements Over the Wintertime Southern Ocean

“…The System for Optical Array Probe Data Analysis (SODA) software developed at the National Center for Atmospheric Research was used to analyze the microphysical properties measured by the CIP. Further details of the CIP data processing are described in the supporting information of A17 and Huang et al ().…”

“…The frequent presence of ice in these clouds adds further uncertainty to the retrievals. Huang et al () examined one of the A17 flights in detail, exploring the potential for secondary ice production (specifically, the Hallett‐Mossop process) to account for the widespread presence of mixed‐phase clouds in open MCC.…”

Characteristics of the Marine Atmospheric Boundary Layer Over the Southern Ocean in Response to the Synoptic Forcing

“…Further, large uncertainties exist in the estimated amount of precipitation over the SO (Behrangi et al, ), which also has the potential to contribute to the regional biases. The ubiquitous boundary layer clouds over the SO (Huang, Siems, Manton, Hande, & Haynes, ; Mace et al, ; Muhlbauer et al, ) have been linked to both the incoming solar radiation bias (Bodas‐Salcedo et al, ) and, more recently, to frequent drizzle/light precipitation (Ahn et al, ; Huang et al, ). In order to better understand potential biases in the boundary layer clouds, it is necessary to understand if the boundary layer structure is properly represented in the widely used climate data sets.…”

Air‐Sea Heat and Momentum Fluxes in the Southern Ocean

“…However, high rain rates are rare and light precipitation (<1.5 mm/hr) dominates the SO as observed by Wang et al (). The region is also renowned for secondary ice production as observed in previous studies (Huang et al, ; Mace & Protat, ), which might produce much higher H r if hail were to reach the ocean. This aspect remains unaccounted for in the COARE 3.5 model. Stable stratification is not a common occurrence over the SO sector and was primarily observed over the cold eddy and in the warm sector of frontal passages.…”