Microphysical and radiative characterization of a subvisible midlevel Arctic ice cloud by airborne observations – a case study 1Alfred Wegener Institute for Polar and Marine Research, 14473 Potsdam, Germany
2Johannes Gutenberg-Universität, 55099 Mainz, Germany
3Institut für Physik der Atmosphäre, DLR Oberpfaffenhofen, 82234 Oberpfaffenhofen, Germany
4Laboratoire de Météorologie Physique UMR 6016 CNRS/Université Blaise Pascal, France.
5Laboratoire de Météorologie Physique, Institut Universitaire de Technologie de Montluçon, 03101 Montluçon Cedex, France
Received: 31 Oct 2008 – Published in Atmos. Chem. Phys. Discuss.: 08 Jan 2009 – Published: 16 Apr 2009Abstract. During the Arctic Study of Tropospheric Aerosol, Clouds and Radiation
(ASTAR) campaign, which was conducted in March and April 2007, an optically
thin ice cloud was observed south of Svalbard at around 3 km altitude. The
microphysical and radiative properties of this particular subvisible
midlevel cloud were investigated with complementary remote sensing and in
situ instruments. Collocated airborne lidar remote sensing and spectral
solar radiation measurements were performed at a flight altitude of 2300 m
below the cloud base. Under almost stationary atmospheric conditions, the
same subvisible midlevel cloud was probed with various in situ sensors
roughly 30 min later.
From individual ice crystal samples detected with the Cloud Particle Imager
and the ensemble of particles measured with the Polar Nephelometer,
microphysical properties were retrieved with a bi-modal inversion algorithm.
The best agreement with the measurements was obtained for small ice spheres
and deeply rough hexagonal ice crystals. Furthermore, the single-scattering
albedo, the scattering phase function as well as the volume extinction
coefficient and the effective diameter of the crystal population were
determined. A lidar ratio of 21(±6) sr was deduced by three
independent methods. These parameters in conjunction with the cloud optical
thickness obtained from the lidar measurements were used to compute spectral
and broadband radiances and irradiances with a radiative transfer code. The
simulated results agreed with the observed spectral downwelling radiance
within the range given by the measurement uncertainty. Furthermore, the
broadband radiative simulations estimated a net (solar plus thermal
infrared) radiative forcing of the subvisible midlevel ice cloud of
−0.4 W m−2 (−3.2 W m−2 in the solar and +2.8 W m−2 in the thermal
infrared wavelength range).
Citation: Lampert, A., Ehrlich, A., Dörnbrack, A., Jourdan, O., Gayet, J.-F., Mioche, G., Shcherbakov, V., Ritter, C., and Wendisch, M.: Microphysical and radiative characterization of a subvisible midlevel Arctic ice cloud by airborne observations – a case study, Atmos. Chem. Phys., 9, 2647-2661, doi:10.5194/acp-9-2647-2009, 2009.