Global importance of different IN for initiating freezing in mixed-phase clouds

Mixed-phase clouds are clouds that occur at temperatures between 0 and -40ºC. They consist of cloud droplets and ice crystals. Empirically the cloud top temperature should be colder than -10ºC before the first ice crystals form. They form due to heterogeneous freezing, which means that freezing is aided by solid aerosol particles, such as mineral dust, biological particles or maybe soot. Particles that initiate freezing are called ice nuclei (IN). Heterogeneous freezing can be initiated from an IN that is immersed with a cloud droplet (immersion freezing), by collision of an IN with a supercooled cloud droplet (contact freezing) or by deposition of water vapor directly only the IN (deposition freezing). The first two mechanisms are thought to be the most important ones for initiating freezing in a supercooled cloud.

In the general circulation model ECHAM5, we solve prognostic equations for the number and mass of cloud droplets and ice crystals and for the number and mass of the major aerosol species, sulfate, sea salt, mineral dust, organic and black carbon. Recently we added prognostic equations for the number of aerosol particles inside cloud droplets and ice crystals (Hoose et al., 2008).

Having this information will allow us to more realistically account for contact and immersion freezing in ECHAM5-HAM.

Biological aerosols, like bacteria, have been shown in laboratory studies to be very efficient IN, while fungal spores and pollen were shown to act as cloud condensation nuclei. However, bioaerosols have so far not been considered in climate models. By adding bioaerosol modes into the M7 module of ECHAM5-HAM, we are investigating their effects on microphysical processes inside mixed-phase cloud. 

Bacteria emitted from vegetation surfaces can serve as ice or cloud condensation nuclei in mixed-phase clouds, thus changing properties of clouds and precipitation, and get removed from the atmosphere by dry or wet deposition.