The Dominance of Heterogeneous Versus Homogeneous Freezing in Cirrus: A Comparison of American- and European-based Midlatitude Datasets

Cirrus clouds are known to play an important role in the radiative balance of the atmosphere. The nature of this role is determined by the macrophysical and microphysical properties of a cloud. Thus, it is crucial that we have a complete and accurate understanding of properties such as the ice water content (IWC) and ice crystal concentration (Ni). However, these properties are difficult to parameterize due to their large variability and the fact that they are influenced by a number of other factors such as temperature, vertical velocity, relative humidity with respect to ice (RHice), and ice nuclei. The combination of those factors ultimately determines which pathway will lead to crystal formation, namely homogeneous or heterogeneous ice nucleation. Thus, it is also essential that the major ice nucleation pathway in the atmosphere is known since the microphysical and therefore radiative properties of cirrus clouds are so strongly influenced by the ice forming mechanism.

In the recent study from Cziczo et al. (2013), which is based on field measurements, it is stated that heterogeneous freezing might be the dominant freezing pathway. In this study, we present a statistical analysis of IWC and Ni measurements as a function of temperature and vertical velocity from the 2011 Midlatitude Airborne Cirrus Properties Experiment (MACPEX) - one of the field experiments reported on by Cziczo et al. (2013) - which confirms that the heterogeneous freezing pathway is more dominant. This was indicated by the combination of high IWC, low Ni, and low RHice inside and outside of the cirrus clouds. Furthermore, a recent model study from Kraemer et al. (2014) reaches the same conclusion concerning the connection between this particular IWC/Ni/RHice combination and heterogeneous freezing. The study goes on to demonstrate a mechanistic explanation for how this outcome is possible in cirrus clouds.

However, it is currently not clear whether this conclusion concerning the dominance of heterogeneous freezing can be applied to all midlatitude datasets. Thus, the work presented here aims to explore these results and lingering questions further through comparisons of additional datasets to MACPEX. European-based datasets from the 2003, 2004, and 2006 CIRRUS field campaigns and the recent AIRTOSS campaign in 2013 are used to demonstrate that not all midlatitude datasets provide a clear indication that heterogeneous freezing is the dominant pathway.

For example, in a comparison of the RHice (inside and outside of cirrus) data from each of these campaigns, it can be seen that the RHice values observed during the 2004 CIRRUS campaign were high enough for homogeneous freezing to make a more clear contribution than the observations from either MACPEX or AIRTOSS would suggest. Also, these results may not be surprising given the difference between the dynamic conditions responsible for producing cirrus between the United States and Europe (e.g. the dominance of convection versus frontal systems). Additional details concerning the observed IWC and Ni during each campaign will also be provided to demonstrate the similarities and differences that exist between American- and European-based midlatitude datasets. This will allow us to better assess whether parameterizations based on observations from one geographic region (i.e. the U.S. or Europe) are applicable to another geographic region.

Cziczo et al. (2013), Clarifying the Dominant Sources and Mechanisms of Cirrus Cloud Formation, Science, 340, 1320 – 1324, doi: 10.1126/science.1234145.

Kraemer et al. (2014), A Microphysics Guide to Cirrus Clouds, in preparation.