Ice supersaturations and cirrus cloud crystal numbers

ATMOSPHERIC CHEMISTRY AND PHYSICS (Impact Factor: 5.3). 01/2009; 8(6). DOI: 10.5194/acpd-8-21089-2008
Source: DOAJ

ABSTRACT Upper tropospheric observations outside and inside of cirrus clouds indicate water vapour mixing ratios sometimes exceeding water saturation. Relative humidities over ice (RHice) of up to and more than 200% have been reported from aircraft and balloon measurements in recent years. From these observations a lively discussion continues on whether there is a lack of understanding of ice cloud microphysics or whether the water measurements are tainted with large uncertainties or flaws. Here, RHice in clear air and in ice clouds is investigated. Strict quality-checked aircraft in situ observations of RHice were performed during 28 flights in tropical, mid-latitude and Arctic field experiments in the temperature range 183–240 K. In our field measurements, no supersaturations above water saturation are found. Nevertheless, super- or subsaturations inside of cirrus are frequently observed at low temperatures (ice deviating from saturation, we analysed the number densities of ice crystals recorded during 20 flights. From the combined analysis – using conventional microphysics – of supersaturations and ice crystal numbers, we show that the high, persistent supersaturations observed inside of cirrus can possibly be explained by unexpected, frequent very low ice crystal numbers that could scarcely be caused by homogeneous ice nucleation. Heterogeneous ice formation or the suppression of freezing might better explain the observed ice crystal numbers. Thus, our lack of understanding of the high supersaturations, with implications for the microphysical and radiative properties of cirrus, the vertical redistribution of water and climate, is traced back to the understanding of the freezing process at low temperatures.

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    ABSTRACT: CALIOP is the first lidar to monitor the Earth from space autonomously and continuously. Since its laser first fired in April 2006, it has been docu- menting atmospheric particles: aero- sols, liquid cloud droplets, ice crystals. In this article, we describe the scientific discoveries CALIOP helped make possible about atmospheric crystals: vertical struc- ture of ice clouds, global maps, micro- physical properties (shape and orientation), the link between ice crystals and water vapour in the Tropics, and crystals in the polar stratosphere...
    06/2012; 8(77):41. DOI:10.4267/2042/47374
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    ABSTRACT: Ice supersaturation in the upper troposphere is a complex and important issue for the understanding of cirrus cloud formation. On one hand, infrared sounders have the ability to provide cloud properties and atmospheric profiles of temperature and humidity. On the other hand, they suffer from coarse vertical resolution, especially in the upper tropo-sphere and therefore are unable to detect shallow ice super-saturated layers. We have used data from the Measurements of OZone and water vapour by AIrbus in-service airCraft ex-periment (MOZAIC) in combination with Atmospheric In-fraRed Sounder (AIRS) relative humidity measurements and cloud properties to develop a calibration method for an esti-mation of occurrence frequencies of ice supersaturation. This method first determines the occurrence probability of ice su-persaturation, detected by MOZAIC, as a function of the rel-ative humidity determined by AIRS. The occurrence proba-bility function is then applied to AIRS data, independently of the MOZAIC data, to provide a global climatology of upper-tropospheric ice supersaturation occurrence. Our cli-matology is then compared to ice supersaturation occurrence statistics from MOZAIC alone and related to high cloud oc-currence from the Cloud-Aerosol Lidar with Orthogonal Po-larization (CALIOP). As an example of application it is com-pared to model climatologies of ice supersaturation from the Integrated Forecast System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF) and from the European Centre HAmburg Model (ECHAM4). This study highlights the benefits of multi-instrumental synergies for the investigation of upper tropospheric ice supersaturation.
    Atmospheric Chemistry and Physics 01/2012; 12(1). DOI:10.5194/acp-12-381-2012 · 5.51 Impact Factor
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