Abstract
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. Information on the spectral absorption of solar radiation by ice and liquid water cloud particles is derived from airborne measurements of solar spectral radiation reflected by these clouds. 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 dominates. On this basis an indicator of the vertical distribution of ice crystals in ABM clouds is designed. Applying in situ measured microphysical properties, the cloud-top reflectivity was calculated by radiative transfer simulations and compared to the 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 dominates. The observation of backscatter glories on the cloud top 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.
Highlights
Because of low temperatures Arctic boundary-layer mixedphase (ABM) clouds consisting of both supercooled liquid water droplets and solid ice crystals simultaneously are common
A simplified scheme of ABM clouds is presented by Harrington et al (1999) in which the coexistence of ice and liquid water relies on the balance between the nucleation rate of liquid water droplets and ice crystals, ice crystal growth rate, and removal of ice nuclei by precipitating ice crystals
From ground-based remote-sensing instruments it could be shown that even though the cloud top of ABM clouds is dominated by liquid water, ice crystals exist throughout the clouds with a maximum in lower cloud layers (Shupe et al, 2006, 2008a,b; de Boer et al, 2009)
Summary
Because of low temperatures Arctic boundary-layer mixedphase (ABM) clouds consisting of both supercooled liquid water droplets and solid ice crystals simultaneously are common. Ground-based remote sensing of Arctic clouds and in situ measurements of ice crystal microphysical properties such as ice crystal size, number concentration and shape have been conducted for many years. From ground-based remote-sensing instruments it could be shown that even though the cloud top of ABM clouds is dominated by liquid water, ice crystals exist throughout the clouds with a maximum in lower cloud layers (Shupe et al, 2006, 2008a,b; de Boer et al, 2009). This was confirmed by McFarquhar et al (2007) who investigated the vertical distribution of ice crystals by in situ measurements.
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