Particle backscatter and relative humidity measured across cirrus clouds and comparison with microphysical cirrus modelling
- ISSN: 16807316
- DOI: 10.5194/acp-12-9135-2012
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.