Article

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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    • "The cloud droplet activation in warm liquidphase clouds only happens at the cloud base of pre-existing clouds or in all levels of newly formed clouds, as represented in CAM5. In comparison, ice nucleation is allowed to happen in all levels of pre-existing cirrus clouds in CAM5 if the nucleation thresholds are met because RH i up to or even more than 120 % are frequently observed inside cirrus clouds (Krämer et al., 2009). The ice number concentration calculated from the ice nucleation parameterization, N aai , is assumed to be the maximum in-cloud ice number concentration in the current time step. "
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    ABSTRACT: In order to improve the treatment of ice nucleation in a more realistic manner in the Community Atmosphere Model version 5.3 (CAM5.3), the effects of pre-existing ice crystals on ice nucleation in cirrus clouds are considered. In addition, by considering the in-cloud variability in ice saturation ratio, homogeneous nucleation takes place spatially only in a portion of the cirrus cloud rather than in the whole area of the cirrus cloud. Compared to observations, the ice number concentrations and the probability distributions of ice number concentration are both improved with the updated treatment. The pre-existing ice crystals significantly reduce ice number concentrations in cirrus clouds, especially at mid- to high latitudes in the upper troposphere (by a factor of ~10). Furthermore, the contribution of heterogeneous ice nucleation to cirrus ice crystal number increases considerably. Besides the default ice nucleation parameterization of Liu and Penner (2005, hereafter LP) in CAM5.3, two other ice nucleation parameterizations of Barahona and Nenes (2009, hereafter BN) and Kärcher et al. (2006, hereafter KL) are implemented in CAM5.3 for the comparison. In-cloud ice crystal number concentration, percentage contribution from heterogeneous ice nucleation to total ice crystal number, and pre-existing ice effects simulated by the three ice nucleation parameterizations have similar patterns in the simulations with present-day aerosol emissions. However, the change (present-day minus pre-industrial times) in global annual mean column ice number concentration from the KL parameterization (3.24 × 106 m−2) is less than that from the LP (8.46 × 106 m−2) and BN (5.62 × 106 m−2) parameterizations. As a result, the experiment using the KL parameterization predicts a much smaller anthropogenic aerosol long-wave indirect forcing (0.24 W m−2) than that using the LP (0.46 W m−2) and BN (0.39 W m−2) parameterizations.
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    • "The dehydration of the air masses that typically occurs at the tropopause level is implemented using a temperaturedependent parametrisation for heterogeneous ice freezing (Krämer et al., 2009). Water ice is irreversibly removed if the inferred particle fall speed exceeds a prescribed threshold value (von Hobe et al., 2011). "
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    • "The dehydration of the air masses that typically occurs at the tropopause level is implemented using a temperaturedependent parametrisation for heterogeneous ice freezing (Krämer et al., 2009). Water ice is irreversibly removed if the inferred particle fall speed exceeds a prescribed threshold value (von Hobe et al., 2011). "
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