RP 3: The importance of ice nuclei types and freezing modes for the initiation of the ice phase and precipitation - Model studies based on laboratory measurements.

Dr. Karoline Diehl

Institute for Atmospheric Physics, University of Mainz

Dr. Miklós Szakáll

Institute for Atmospheric Physics, University of Mainz

 

Mitarbeit: Oliver Eppers (Masterstudent), Amelie Mayer (Bachelorstudent)

 

 

During INUIT-1, this project investigated the impact of ice nucleating particles on mixed-phase convective clouds. An air parcel model with detailed cloud microphysics which provides a direct link between aerosol particles and cloud development was used for microphysical process studies. Descriptions of heterogeneous freezing processes in immersion, contact, and deposition modes were modified or newly added. These parameterizations are based on previous and new field and laboratory measurements mainly from the INUIT research group. Figure 1 shows experimental data used for the parameterization of immersion freezing.

 

Figure 1: Parameterization of immersion freezing: Numbers of active sites per unit mass as function of temperature for various particle types based on previous and INUIT measurements. From Diehl and Mitra, ACPD, 2015, with changes.

 

 

The experimental part included laboratory experiments of immersion and contact freezing by means of the Mainz vertical wind tunnel and an acoustic drop levitator (WP1). Figure 2 gives example results for illite NX.

 

Figure 2: Surface densities of active sites as function of temperature determined from wind tunnel (WT) and acoustic levitator (AL) measurements for various particle surface areas per drop. From Diehl et al., ACP, 2014.

 

 

As a next step in INUIT-2, it is planned to switch to a more complex state-of-the-art model system. To understand the link between atmospheric aerosol particles, cloud properties, and precipitation, the most exact description of cloud microphysics is given by spectral microphysical schemes such as the one used during INUIT-1. During early years it was only possible to combine such schemes to simple models such as box or axisymmetric ones but for the last decade computational resources allow three-dimensional modeling. This will be performed with the 3D model COSMO-SPECS.


In the experimental part, experiments in the immersion mode are continued with two techniques, the Mainz vertical wind tunnel and the acoustic drop levitator. For contact freezing, new experimental methods will be designed at the wind tunnel to simulate realistic conditions. Single supercooled drops will be freely floated in the tunnel while potential contact ice nucleating particles are carried along with the air stream and collide with the drops as in real clouds. The collision rate will be calculated by numerically integrating the differential equation of particle motion in a viscous flow around a drop.

 

The objectives of INUIT Phase 2 are:

  • Development of an improved method to investigate contact freezing at the Mainz vertical wind tunnel, comparison of the results to INUIT methods in WP-L.
  • Comparison of two methods to study immersion freezing with each other and to INUIT methods in WP-L and WP-F.
  • Derivation of parameterizations of immersion, contact, and deposition freezing for new particle types and improvement of existing parameterizations on the base of INUIT results from WP-L and WP-F.
  • Extension of a state-of-the-art model by improved treatment of ice nucleation: the 3D convection-resolving model COSMO-SPECS with spectral bin microphysics.
  • Assessment of the atmospheric relevance of the ice nucleation regimes probed in INUIT WP-L and WP-F.
  • Quantifying the contribution of possible ice formation modes to atmospheric ice formation in convective clouds.
  • Studying the sensitivity of the cloud microphysics and precipitation amount to realistic variations in the ice nuclei types and distributions.