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Particle backscatter and relative humidity measured across cirrus clouds and comparison with microphysical cirrus modelling

by M. Brabec, F. G. Wienhold, B. P. Luo, H. VÃmel, F. Immler, P. Steiner, E. Hausammann, U. Weers, T. Peter show all authors
Atmospheric Chemistry and Physics ()

Abstract

Advanced measurement and modelling techniques are employed to estimate\nthe partitioning of atmospheric water between the gas phase and the\ncondensed phase in and around cirrus clouds, and thus to identify\nin-cloud and out-of-cloud supersaturations with respect to ice. In\nNovember 2008 the newly developed balloon-borne backscatter sonde\nCOBALD (Compact Optical Backscatter and AerosoL Detector) was flown\n14 times together with a CFH (Cryogenic Frost point Hygrometer) from\nLindenberg, Germany (52� N, 14� E). The case discussed here in detail\nshows two cirrus layers with in-cloud relative humidities with respect\nto ice between 50% and 130%. Global operational analysis data of\nECMWF (roughly 1� � 1� horizontal and 1 km vertical resolution, 6-hourly\nstored fields) fail to represent ice water contents and relative\nhumidities. Conversely, regional COSMO-7 forecasts (6.6 km � 6.6\nkm, 5-min stored fields) capture the measured humidities and cloud\npositions remarkably well. The main difference between ECMWF and\nCOSMO data is the resolution of small-scale vertical features responsible\nfor cirrus formation. Nevertheless, ice water contents in COSMO-7\nare still off by factors 2�10, likely reflecting limitations in COSMO's\nice phase bulk scheme. Significant improvements can be achieved by\ncomprehensive size-resolved microphysical and optical modelling along\nbackward trajectories based on COSMO-7 wind and temperature fields,\nwhich allow accurate computation of humidities, homogeneous ice nucleation,\nresulting ice particle size distributions and backscatter ratios\nat the COBALD wavelengths. However, only by superimposing small-scale\ntemperature fluctuations, which remain unresolved by the numerical\nweather prediction models, can we obtain a satisfying agreement with\nthe observations and reconcile the measured in-cloud non-equilibrium\nhumidities with conventional ice cloud microphysics. Conversely,\nthe model-data comparison provides no evidence that additional changes\nto ice-cloud microphysics � such as heterogeneous nucleation or changing\nthe water vapour accommodation coefficient on ice � are required.

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