Measuring Ice Nucleating Particles from Space: Do they Influence Southern Ocean Mixed-phase Clouds?
Abstract
Mixed-phase clouds (MPCs) contain both liquid cloud droplets and ice crystals and control the radiative energy budget. Liquid clouds are optically thicker than ice clouds with a similar water content. Thus, the radiative properties of MPCs are strongly influenced by their thermodynamic phase composition. The cloud phase feedback in extratropical MPCs is associated with the large values for the equilibrium climate sensitivity in a large subset of models participating in the 6th Coupled Model Intercomparison Project. Despite their importance, MPCs are one of the most poorly understood cloud regimes.
In situ MPCs form when primary ice nucleation occurs at the top of supercooled liquid clouds. If the cloud top temperature (CTT) is above the homogeneous freezing temperature, ice nucleating particles (INPs) are required for ice to form. However, the temperature at which INPs nucleate ice depends on their physicochemical properties. The importance of INPs for forming ice in MPCs is undisputed. However, direct measurements of INP concentrations are restricted to single observations via field campaigns. Larger scale approaches, so far, have focused on the supercooled liquid fraction (SLF) to assess the influence of aerosol particles on the ice formation in MPCs. However, the SLF is prone to the influence of dynamics, which makes it hard to disentangle the effect of INPs. Here we present a new statistical approach combining CTTs for pure liquid and liquid-top MPCs observed from spaceborne remote sensing. In particular, we use data from CloudSat's radar and CALIPSO's lidar to distinguish between pure liquid and liquid-top MPCs. CTTs from reanalysis data are then examined on a profile-by-profile basis and resampled to a 1°x1° resolution. The frequency of different CTTs for the liquid only and liquid-top MPCs is used to estimate the temperature at which INPs are activated for each grid box. The large prevalence of clouds in the mixed-phase temperature region (0 to -38°C) makes the Southern Ocean a perfect region to study the influence of ice nucleation on MPC properties on a large scale. The proposed statistical approach is used to evaluate the importance of INPs in specific regions of the Southern Ocean and in different seasons. Thus, this study provides a new framework for assessing the influence of INPs on the formation of MPCs.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMA033.0004C
- Keywords:
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- 3310 Clouds and cloud feedbacks;
- ATMOSPHERIC PROCESSES;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 3349 Polar meteorology;
- ATMOSPHERIC PROCESSES;
- 3359 Radiative processes;
- ATMOSPHERIC PROCESSES