Incidence of rough and irregular atmospheric ice particles from Small Ice Detector 3 measurements 1Centre for Atmospheric and Instrumentation Research, University of Hertfordshire, Hatfield AL10 9AB, UK
12 Feb 2014
2Met Office, FitzRoy Road, Exeter EX1 3PB, UK
Received: 02 Sep 2013 – Published in Atmos. Chem. Phys. Discuss.: 25 Sep 2013 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.
Revised: 17 Dec 2013 – Accepted: 08 Jan 2014 – Published: 12 Feb 2014
Citation: Ulanowski, Z., Kaye, P. H., Hirst, E., Greenaway, R. S., Cotton, R. J., Hesse, E., and Collier, C. T.: Incidence of rough and irregular atmospheric ice particles from Small Ice Detector 3 measurements, Atmos. Chem. Phys., 14, 1649-1662, doi:10.5194/acp-14-1649-2014, 2014.