Atmos. Chem. Phys. Discuss., 12, 9553-9586, 2012
© Author(s) 2012. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Particle backscatter and relative humidity measured across cirrus clouds and comparison with state-of-the-art cirrus modelling
M. Brabec1, F. G. Wienhold1, B. Luo1, H. Vömel2, F. Immler2, P. Steiner3, and T. Peter1
1Institute for Atmospheric and Climate Science, ETH, Zurich, Switzerland
2Deutscher Wetterdienst, Meteorologisches Observatorium Lindenberg, Germany
3Federal Office of Meteorology and Climatology MeteoSwiss, Switzerland

Abstract. Advanced measurement and modelling techniques are employed to determine the partitioning of atmospheric water between the gas phase and the condensed phase in and around cirrus clouds, and thus to identify in-cloud and out-of-cloud supersaturations with respect to ice. In November 2008 the newly developed balloon-borne backscatter sonde COBALD (Compact Optical Backscatter and AerosoL Detector) was flown 14 times together with a CFH (Cryogenic Frost point Hygrometer) from Lindenberg, Germany (52° N, 14° E). The case discussed here in detail shows two cirrus layers with in-cloud relative humidities with respect to ice between 50% and 130%. Global operational analysis data of ECMWF (roughly 1° × 1° horizontal and 1 km vertical resolution, 6-hourly stored fields) fail to represent ice water contents and relative humidities. Conversely, regional COSMO-7 forecasts (6.6 km × 6.6 km, 5-min stored fields) capture the measured humidities and cloud positions remarkably well. The main difference between ECMWF and COSMO data is the resolution of small-scale vertical features responsible for cirrus formation. Nevertheless, ice water contents in COSMO-7 are still off by factors 2–10, likely reflecting limitations in COSMO's ice phase bulk scheme. Significant improvements can be achieved by comprehensive size-resolved microphysical and optical modelling along backward trajectories based on COSMO-7 wind and temperature fields, which allow accurate computation of humidities, ice particle size distributions and backscatter ratios at the COBALD wavelengths. However, only by superimposing small-scale temperature fluctuations, which remain unresolved by the NWP models, can we obtain a satisfying agreement with the observations and reconcile the measured in-cloud non-equilibrium humidities with conventional ice cloud microphysics.

Citation: Brabec, M., Wienhold, F. G., Luo, B., Vömel, H., Immler, F., Steiner, P., and Peter, T.: Particle backscatter and relative humidity measured across cirrus clouds and comparison with state-of-the-art cirrus modelling, Atmos. Chem. Phys. Discuss., 12, 9553-9586, doi:10.5194/acpd-12-9553-2012, 2012.
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