Evidence of ice crystals at cloud top of Arctic boundary-layer mixed-phase clouds derived from airborne remote sensing
1Johannes Gutenberg-University Mainz, Institute for Atmospheric Physics, Mainz, Germany
2Lab. for Atmospheric and Space Physics (LASP), University of Colorado, Boulder, USA
3Lab. de Météorologie Physique (LAMP), Université Blaise Pascal, Aubière Cedex, France
4Alfred Wegener Institute for Polar and Marine Research (AWI), Potsdam, Germany
5Inst. of Atmospheric Physics, German Aerospace Center (DLR), Oberpfaffenhofen, Germany
*now at: Leipzig Institute for Meteorology (LIM), University of Leipzig, Leipzig, Germany
Abstract. The vertical distribution of ice crystals in Arctic boundary-layer mixed-phase (ABM) clouds was investigated by airborne remote sensing and in situ measurements during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign in March and April 2007. From airborne measurements of spectral solar radiation reflected by the ABM clouds information on the spectral absorption of solar radiation by ice and liquid water cloud particles is derived. It is shown by calculation of the vertical weighting function of the measurements that the observed absorption of solar radiation is dominated by the upper cloud layers (50% within 200 m from cloud top). This vertical weighting function is shifted even closer to cloud top for wavelengths where absorption by ice is dominating. On this basis an indicator of the vertical distribution of ice crystals in ABM clouds is designed.
Applying the in situ measured microphysical properties, the cloud top reflectance was calculated by radiative transfer simulations and compared to measurements. It is found that ice crystals near cloud top (mixed-phase cloud top layer) are necessary to reproduce the measurements at wavelengths where absorption by ice is dominating. The observation of backscatter glories on top of the ABM clouds generally indicating liquid water droplets does not contradict the postulated presence of ice crystals. Radiative transfer simulations reproduce the observed glories even if the cloud top layer is of mixed-phase character.