Tracking ice nucleating particles to improve climate projections

• Applicant: Dr. Diego Villanueva
• Subject Area: Atmospheric Science
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 493541396

Clouds and aerosols impact the Earth’s thermostat and precipitation by scattering solar radiation and absorbing 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. If the climate warms and ice clouds are partially replaced by liquid clouds, then the cloud albedo increases in what is known as the negative cloud-phase feedback. Thus, this feedback depends on the representation of ice nucleation in climate models. In turn, 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 million million US$. As a result, it has been concluded that a better representation of ice production within the mixed-phase regime is urgently needed.Aerosols can act as ice nucleating particles 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. One type of aerosol in particular, mineral dust, frequently controls cloud glaciation. In previous studies, I have identified important biases regarding the dust-driven cloud glaciation in the ECHAM-HAM model, which are probably present in other climate models as well.To solve those gaps, I will implement ice processes relevant to dust-driven droplet freezing that are currently missing: First, I will implement the tracking of ice-nucleating particles focusing on their removal after droplet freezing. This is expected to reduce the relative overestimation in dust-driven cloud glaciation found over the Southern Ocean. Second, I will add a category for dust-INPs that are pre-activated at temperatures below -35°C. This could result in enhanced droplet freezing in mixed-phase clouds, which could explain and compensate for the overall underestimation in dust-driven droplet freezing found in the model. Third, I will implement the recycling of dust-INPs after the sublimation of ice crystals. This could also result in an enhancement in droplet freezing and compensate for the bias observed in the model. These new processes will be evaluated using space-borne observations and their impacts on the cloud-phase feedback and climate sensitivity will be estimated.As a secondary objective, we plan to evaluate the dust-driven heterogeneous ice nucleation at cirrus temperatures using the hemispheric and seasonal contrast of ice number concentration. We expect this to elucidate whether a biased dust-driven ice nucleation at cirrus conditions is contributing to the underestimation of the cloud-phase contrasts in the mixed-phase regime.

• DFG Programme: WBP Fellowship
• International Connection: Switzerland
• Host: Professorin Dr. Ulrike Lohmann