Tracking ice nucleating particles to improve climate projections
Final Report Abstract
Clouds-aerosol interactions impact the Earth’s thermostat and precipitation. Clouds scatter solar radiation and absorb terrestrial radiation. The cloud thermodynamic phase — whether a cloud is composed of liquid or ice in the mixed-phase temperature regime (-35°C to 0°C) — affects the overall cloud radiative effect, as liquid droplets reflect more solar radiation than ice particles. In the mixed-phase temperature regime, if the climate warms and ice clouds are partially replaced by liquid clouds, then the cloud albedo increases as a result. This process is known as the negative cloud-phase feedback. More importantly, it has become increasingly clear that the spread of climate projections (+1.8 to +6.5 K) in the new generation of climate models depends strongly on the simulated cloud-phase feedback. Reducing the spread of climate projections has been associated with societal benefits estimated at over 10^13 USD. Aerosols are especially important for mixed-phase clouds, as certain aerosol particles can act like Ice Nucleating Particles (INPs) triggering droplet freezing, increasing the frequency of ice clouds, and decreasing the cloud cover and water content as ice particles grow at the expense of cloud droplets and then fall down, enhancing precipitation. However, aerosol-driven droplet freezing is poorly constrained in climate models, leading to high uncertainties in the frequency of ice clouds and, therefore, in climate projections. In mixed-phase clouds, different parameterizations for immersion freezing lead to a significant spread between models regarding the mean-state cloud ice frequency and the radiative effect of dust-INPs. Current freezing schemes extrapolate laboratory results to larger atmospheric scales often failing to replicate the spatiotemporal variability in cloud-phase. Thus, to advance our understanding of INPs and project climate change with higher accuracy, it is crucial to reconcile dust aerosol-driven droplet freezing — a microphysical process — with cloud-phase — a large-scale proxy for cloud glaciation. During the project, we showed for the first time that the ice-to-liquid cloud top frequency (ILF) can be attributed to dust-driven immersion droplet freezing between -15°C and -30°C in the Northern Hemisphere. By separating 35 years of space-borne cloud top observations according to cloud top temperature, we found that dust aerosol is strongly correlated with ILF between 30 and 90°N, both spatially and temporally. Furthermore, we find that the sensitivities of ILF to temperature and dust are in a ratio that agrees within a factor of 2 with laboratory measurements of droplet freezing. Thus, our results provide for the first time a standardized benchmark for assessing dust effects on ILF and for evaluating cloud glaciation in climate models.
Publications
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Important role of stratospheric injection height for the distribution and radiative forcing of smoke aerosol from the 2019–2020 Australian wildfires. Atmospheric Chemistry and Physics, 22(15), 9969-9985.
Heinold, Bernd; Baars, Holger; Barja, Boris; Christensen, Matthew; Kubin, Anne; Ohneiser, Kevin; Schepanski, Kerstin; Schutgens, Nick; Senf, Fabian; Schrödner, Roland; Villanueva, Diego & Tegen, Ina
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Mixed-phase regime cloud thinning could help restore sea ice. Environmental Research Letters, 17(11), 114057.
Villanueva, D.; Possner, A.; Neubauer, D.; Gasparini, B.; Lohmann, U. & Tesche, M.
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Does prognostic seeding along flight tracks produce the desired effects of cirrus cloud thinning?. Atmospheric Chemistry and Physics, 23(13), 7673-7698.
Tully, Colin; Neubauer, David; Villanueva, Diego & Lohmann, Ulrike
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Ice-nucleating particles in northern Greenland: annual cycles, biological contribution and parameterizations. Atmospheric Chemistry and Physics, 23(8), 4741-4761.
Sze, Kevin C. H.; Wex, Heike; Hartmann, Markus; Skov, Henrik; Massling, Andreas; Villanueva, Diego & Stratmann, Frank
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Retrieving global cloud top phase patterns: The reliability of spaceborne retrievals. Copernicus GmbH.
Villanueva, Diego
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The competing effect of aerosols on stratiform mixed-phase clouds. Copernicus GmbH.
Villanueva, Diego
