Characteristics of Ice and Mixed-Phase Clouds Based on in-Situ Observations over the Southern Ocean and Comparisons with the NCAR Community Atmosphere Model

The microphysical properties of ice and mixed-phase clouds have large spatial and temporal variabilities, which result in major challenges for the simulations/parameterizations of these types of clouds in the General Circulation Models (GCMs). Representations of ice and mixed-phase clouds in the GCMs are often simplified when parameterizing the cloud properties on the model grid scales (10s to 100s km). However, lack of in-situ observations over the Southern Ocean limited the evaluations of the representations of ice and mixed-phase clouds in climate model simulations in this region.

In this work, we will use the in-situ, airborne observations from the NSF O₂/N₂ Ratio and CO₂ Airborne Southern Ocean (ORCAS) Study, which was carried out by the NSF/NCAR Gulfstream-V research aircraft during January 15 – February 29, 2016. The 1-Hz, in-situ observations on the ~250 m horizontal resolution will be analyzed at both clear-sky and in-cloud conditions from ORCAS, which provides a unique dataset for understanding cloud formation over the Southern Ocean. Relative humidity distributions inside and outside of ice and mixed-phase clouds will be compared between the in-situ observations and the simulations of the NSF Community Atmosphere Model version 5 (CAM5) at three horizontal resolutions (i.e., 0.23°×0.31°, 0.47°×0.63°, and 0.9°×1.25°).

Previously, Diao et al. (2014) used the HIPPO Global campaign (2009-2011) to compare the frequency distributions of clear-sky ice supersaturation (i.e., RHi > 100%) between the Northern and Southern Hemispheres, yet no significant differences in the ISS frequency distributions were found. Here we will use the ORCAS dataset to further examine ice supersaturation - the prerequisite conditions for ice crystal formation - in relation to various microphysical properties of the coexisting ice particles and liquid droplets. We will examine the potential influences of aerosols on the transition from ice supersaturation to ice crystals by using various chemical tracers (i.e., O₃ and CO) and total aerosol number concentrations (e.g., aerosols > 0.1 μm and > 0.5 μm). In addition, the CAM5 model sensitivity to the horizontal grid spacings will be evaluated for their representations of ice and mixed-phase clouds. Furthermore, seasonal and latitudinal variabilities in the frequency distributions of ice supersaturation (i.e., relative humidity with respect to ice (RHi) > 100%) in and out of ice and mixed-phase clouds will be examined to address the potential impact of the spatial and temporal sampling biases in in-situ observations. These observational analyses will help to evaluate the coexistence of ice supersaturation with ice crystals and liquid droplets, the allowance of ice supersaturation at clear-sky and in-cloud conditions, and the representations of RHi spatial variabilities in CAM5 simulations.