Cirrus cloud occurrence as function of ambient relative humidity: a comparison of observations obtained during the INCA experiment J. Ström1, M. Seifert1, B. Kärcher2, J. Ovarlez3, A. Minikin2, J.-F. Gayet4, R. Krejci1, A. Petzold2, F. Auriol4, W. Haag2, R. Busen2, U. Schumann2, and H. C. Hansson1 1ITM, Air Pollution Laboratory, Stockholm University, Sweden 2DLR, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany 3LMD, Ecole Polytechnique, CNRS-IPSL, Palaiseau, France 4LAMP, Université Blaise Pascal, Aubiere, France
Abstract. Based on in-situ observations performed during the Interhemispheric differences in cirrus
properties from anthropogenic emissions (INCA) experiment, we introduce and discuss the
cloud presence fraction (CPF) defined as the ratio between the number of data points
determined to represent cloud at a given ambient relative humidity over ice
(RHI) divided by the total number of data points at that value of RHI. The CPFs are measured with four
different cloud probes. Within similar ranges of detected particle sizes and concentrations, it
is shown that different cloud probes yield results that are in good agreement with each other.
The CPFs taken at Southern Hemisphere (SH) and Northern Hemisphere (NH) midlatitudes
differ from each other. Above ice saturation, clouds occurred more frequently during the NH
campaign. Local minima in the CPF as a function of RHI are interpreted as a systematic
underestimation of cloud presence when cloud particles become invisible to cloud probes.
Based on this interpretation, we find that clouds during the SH campaign formed
preferentially at RHIs between 140 and 155%, whereas clouds in the NH campaign formed at
RHIs somewhat below 130%. The data show that interstitial aerosol and ice particles coexist
down to RHIs of 70-90%, demonstrating that the ability to distinguish between different
particle types in cirrus conditions depends on the sensors used to probe the
aerosol/cirrus system. Observed distributions of cloud water content differ only slightly between the NH
and SH campaigns and seem to be only weakly, if at all, affected by the freezing aerosols.
Citation: Ström, J., Seifert, M., Kärcher, B., Ovarlez, J., Minikin, A., Gayet, J.-F., Krejci, R., Petzold, A., Auriol, F., Haag, W., Busen, R., Schumann, U., and Hansson, H. C.: Cirrus cloud occurrence as function of ambient relative humidity: a comparison of observations obtained during the INCA experiment, Atmos. Chem. Phys., 3, 1807-1816, doi:10.5194/acp-3-1807-2003, 2003.