Abstract
Abstract. The knowledge of properties of ice crystals such as size, shape, concavity and roughness is critical in the context of radiative properties of ice and mixed-phase clouds. Limitations of current cloud probes to measure these properties can be circumvented by acquiring two-dimensional light-scattering patterns instead of particle images. Such patterns were obtained in situ for the first time using the Small Ice Detector 3 (SID-3) probe during several flights in a variety of mid-latitude mixed-phase and cirrus clouds. The patterns are analysed using several measures of pattern texture, selected to reveal the magnitude of particle roughness or complexity. The retrieved roughness is compared to values obtained from a range of well-characterized test particles in the laboratory. It is found that typical in situ roughness corresponds to that found in the rougher subset of the test particles, and sometimes even extends beyond the most extreme values found in the laboratory. In this study we do not differentiate between small-scale, fine surface roughness and large-scale crystal complexity. Instead, we argue that both can have similar manifestations in terms of light-scattering properties and also similar causes. Overall, the in situ data are consistent, with ice particles with highly irregular or rough surfaces being dominant. Similar magnitudes of roughness were found in growth and sublimation zones of cirrus. The roughness was found to be negatively correlated with the halo ratio, but not with other thermodynamic or microphysical properties found in situ. Slightly higher roughness was observed in cirrus forming in clean oceanic air masses than in a continental, polluted one. Overall, the roughness and complexity are expected to lead to increased shortwave cloud reflectivity, in comparison with cirrus composed of more regular, smooth ice crystal shapes. These findings put into question suggestions that climate could be modified through aerosol seeding to reduce cirrus cover and optical depth, as the seeding may result in decreased shortwave reflectivity.
Highlights
Cloud feedbacks remain the largest source of uncertainty in climate models
Roughening during sublimation could explain the apparent roughness of ice particles we found in the sublimation zone of cirrus, even at low humidities (Fig. 8)
The Small Ice Detector 3 (SID-3) probe was flown on the FAAM BaE-146 aircraft in mid-latitude clouds
Summary
Cloud feedbacks remain the largest source of uncertainty in climate models. In particular, uncertainties exist concerning the radiative forcing of clouds containing ice crystals, most notably cirrus. Whether cirrus clouds warm or cool the Earth surface depends critically on ice crystal morphology, among other factors. Reducing this uncertainty requires detailed in situ characterization of cloud particles, so that the scattering properties of the clouds can be correctly represented in models. One of the main barriers to achieving these goals is the inability of cloud probes to determine the contribution of small ice crystals (that is crystals smaller than about 50 μm) to the total distribution. This is due to crystal breakup on the inlets of these probes (Field et al, 2006) and their inability to resolve precisely the size and shape of small ice crystals because of the conflicting demands of high optical resolution and large sample volume (Ulanowski et al, 2004; Connolly et al, 2007; Kaye et al, 2008; Bailey and Hallett, 2009)
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