Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity W. Haag1, B. Kärcher1, J. Ström2, A. Minikin1, U. Lohmann3, J. Ovarlez4, and A. Stohl5,* 1Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre (IPA), Oberpfaffenhofen, Germany 2Stockholm University, Institute of Applied Environmental Research (ITM), Stockholm, Sweden 3Dalhousie University, Department of Physics and Atmospheric Science, Halifax, Nova Scotia, Canada 4Laboratoire de Météorologie Dynamique (LMD), CNRS-IPSL, École Polytechnique, Palaiseau, France 5Lehrstuhl für Bioklimatologie und Immissionsforschung, Technische Universität München (TUM), Freising, Germany *Now at: Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado/NOAA Aeronomyity, Laboratory, Boulder, Colorado, U.S.
Abstract. Factors controlling the microphysical link between distributions of relative humidity above ice
saturation in the upper troposphere and lowermost stratosphere and cirrus clouds are examined with the
help of microphysical trajectory simulations. Our findings are related to results from aircraft
measurements and global model studies. We suggest that the relative humidities at which ice crystals
form in the atmosphere can be inferred from in situ measurements of water vapor and temperature
close to, but outside of, cirrus clouds. The comparison with concomitant measurements performed inside
cirrus clouds provides a clue to freezing mechanisms active in cirrus. The analysis of field data
taken at northern and southern midlatitudes in fall 2000 reveals distinct differences in cirrus cloud freezing
thresholds. Homogeneous freezing is found to be the most likely mechanism by which cirrus form at
southern hemisphere midlatitudes. The results provide evidence for the existence of heterogeneous
freezing in cirrus in parts of the polluted northern hemisphere, but do not suggest
that cirrus clouds in this region form exclusively on heterogeneous ice nuclei, thereby emphasizing the
crucial importance of homogeneous freezing. The key features of distributions of upper tropospheric
relative humidity simulated by a global climate model are shown to be in general agreement with both,
microphysical simulations and field observations, delineating a feasible method to include and validate
ice supersaturation in other large-scale atmospheric models, in particular chemistry-transport and
weather forecast models.
Citation: Haag, W., Kärcher, B., Ström, J., Minikin, A., Lohmann, U., Ovarlez, J., and Stohl, A.: Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity, Atmos. Chem. Phys., 3, 1791-1806, doi:10.5194/acp-3-1791-2003, 2003.