The role of heterogeneous freezing in cirrus formation: new model sensitivity studies

The microphysical characteristics of cirrus clouds, which are responsible for their radiative properties, are determined principally in the cloud formation stage by the freezing mechanism. Studies by Gierens (2003) and Kärcher and Lohmann (2003) concluded that homogeneous ice nucleation of liquid aerosol particles dominates the ice cloud formation process in regions with vertical air motions stronger than 20-30 cm/s and that heterogeneous freezing initiated by solid ice nuclei (IN) plays a role only in situations with lower updrafts. Since the ice crystal concentrations produced by the homogeneous freezing process are independent of the number and chemistry of the precursor liquid aerosol particles for most atmospheric relevant conditions, it is believed that the indirect aerosol effect on cirrus clouds is only small, i.e. anthropogenic activities does not greatly influence the occurrence and radiative properties of cirrus clouds. In our modelling study, the influence of heterogeneous freezing on the ice crystal number and size of developing cirrus clouds has been newly evaluated by extensive model simulations using the detailed microphysical box model MAID (Bunz et al., 2008). MAID includes heterogeneous as well as homogeneous ice formation and, as a new feature, freezing thresholds for different types of IN.

Cirrus formation scenarios are simulated in the temperature range 185-240 K in 10 K steps. For each temperature, six different vertical velocities (~cooling rates) were assumed, ranging from 1 - 1000 cm/s. Thus, each scenario contains 36 model runs. A variety of scenarios are simulated by varying the number of heterogeneous ice nuclei (IN) between 0.001 and 0.2 cm-3 and by using high and low freezing thresholds, i.e. representative of coated soot and mineral dust particles. In addition, the simulations are performed for constant vertical velocities as well as for updrafts superimposed with temperature pertubations of 1 and 3 K, respectively.

Our simulations show that heterogeneous ice formation progressively influences the ice crystal concentrations up to updrafts of 100 cm/s and more, increasing with the IN number and with the temperature. Cirrus clouds with reduced ice concentrations form in most cases. Exceptions are situations with a high number of efficient IN in a small updraft, where the response changes and cirrus with a higher ice crystal number compared to homogeneous freezing form. Evaluation of the temperature of cirrus formation also show a dependence on the freezing mechanism: switching from homogeneous to heterogeneous freezing with a high freezing threshold increases the cirrus formation temperature by 1-2 K, while in cases of a lower freezing threshold ice is formed at 3-4 K higher temperatures.

Our findings clearly indicate an indirect aerosol effect on ice clouds: injection of IN into the regions of ice formation would lead to cirrus with microphysical properties influenced by the properties of the IN under most atmospheric conditions, and, in the case when IN efficiently nucleates ice, a higher coverage of cirrus clouds.

Bunz, H., Benz, S., Gensch, I., & Krämer, M. (2008): MAID: a model to simulate UT/LS aerosols and ice clouds, Envir. Res. Lett., 3, doi10.1088/17489326/3/3/035001.

Gierens, K. (2003): On the transition between heterogeneous and homogeneous freezing, Atmos. Chem. Phys., 3, 437-44.

Kärcher, B. & Lohmann, U. (2003): A parametrization of cirrus cloud formation: Heterogeneous freezing, J. Geophys. Res., 108,