Thursday, 16 January 2025: 9:15 AM
224 (New Orleans Ernest N. Morial Convention Center)
Decades of observations of ice crystal number concentrations dwarfing number concentrations of ice nucleating particles reveal the ubiquity of secondary ice production in mixed-phase clouds. However, secondary ice production is rarely included in global atmospheric models, potentially leading to deficient ice crystals in mixed-phase clouds, and biases in cloud and radiative properties.
Here, we investigate the impact of ice crystal number concentration on the cloud fraction and radiative properties of mixed-phase Southern Ocean cumuli simulated by a global storm-resolving model with 2-4 km horizontal grid spacing, run with five different microphysics schemes, which all have different ice production processes. We evaluate cloud microphysics with aircraft observations from the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES), cloud fraction with the Himawari geostationary satellite, and cloud radiative effects with Clouds and the Earth's Radiant Energy System (CERES). Simulations with larger ice crystal number concentrations, which agree better with SOCRATES observations, have smaller mixed-phase cumulus cloud fractions that agree better with Himawari, and weaker shortwave cloud radiative effects that are less biased compared with CERES. This is because large ice crystal concentrations glaciate cumuli more significantly, reducing their horizontal extents and the amount of sunlight that they reflect away from the Southern Ocean.
Using a pair of simulations that are identical except that one includes secondary ice production and the other does not, we show that the inclusion of secondary ice production increases ice crystal number concentrations in cumulus clouds, decreases cumulus cloud fraction by 18%, and weakens shortwave cloud radiative effects over the Southern Ocean by 10 W m−2. We also demonstrate the key role that global storm-resolving models can play in detangling the effects of clouds on Earth's climate across scales, making it possible to trace the impact of changes in individual cumulus cloud anvils (10 km2) on the radiative budget of the massive Southern Ocean basin (107 km2).
Here, we investigate the impact of ice crystal number concentration on the cloud fraction and radiative properties of mixed-phase Southern Ocean cumuli simulated by a global storm-resolving model with 2-4 km horizontal grid spacing, run with five different microphysics schemes, which all have different ice production processes. We evaluate cloud microphysics with aircraft observations from the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES), cloud fraction with the Himawari geostationary satellite, and cloud radiative effects with Clouds and the Earth's Radiant Energy System (CERES). Simulations with larger ice crystal number concentrations, which agree better with SOCRATES observations, have smaller mixed-phase cumulus cloud fractions that agree better with Himawari, and weaker shortwave cloud radiative effects that are less biased compared with CERES. This is because large ice crystal concentrations glaciate cumuli more significantly, reducing their horizontal extents and the amount of sunlight that they reflect away from the Southern Ocean.
Using a pair of simulations that are identical except that one includes secondary ice production and the other does not, we show that the inclusion of secondary ice production increases ice crystal number concentrations in cumulus clouds, decreases cumulus cloud fraction by 18%, and weakens shortwave cloud radiative effects over the Southern Ocean by 10 W m−2. We also demonstrate the key role that global storm-resolving models can play in detangling the effects of clouds on Earth's climate across scales, making it possible to trace the impact of changes in individual cumulus cloud anvils (10 km2) on the radiative budget of the massive Southern Ocean basin (107 km2).
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