Optically thin ice clouds in Arctic: formation processes

Description

Arctic ice cloud formation during winter is poorly understood mainly due to the lack of observations and the remoteness of this region. Their influence on climate is of paramount importance, and the modification of their properties, linked to aerosol-cloud interaction processes, needs to be better understood. High concentration of aerosols (Na) in Arctic during winter is associated with long-range transport of anthropogenic aerosols from the midlatitudes. Observations and models show that this may lead to a significant transport of acidified aerosols. Laboratory and in situ measurements show that at cold temperature (<-30 °C), acidic coating lowers the freezing point and deactivates ice nuclei. The IN concentration is reduced in these regions, and there is less competition for the same available moisture. Large ice crystals form in relatively small concentrations. It is hypothesized that the observed low concentration of large ice crystals (NIce) in thin ice clouds (TICs) is linked to the acidification of aerosols. To check this, we are combining case studies and statistical approaches to analyze aerosol transport and cloud properties in Arctic. Extensive measurements from ground-based sites and satellite remote sensing reveal the existence of two types of extended TICs in Arctic during the polar night and early spring. The first type (TIC1) is seen only by the lidar but not the radar and is found in pristine environments, whereas the second type (TIC2) is detected by both sensors, associated with high Na, possibly anthropogenic, and characterized by low NIce that are large enough to precipitate. To investigate the interactions between TICs and aerosols, airborne and satellite measurements of specific cases observed during the ISDAC field experiments have been first analyzed. This field campaign took place in Alaska in April 2008. The airborne instruments include a complete set of dynamic, thermodynamic, radiation, aerosol, and microphysical sensors. From these observations, a first classification has been performed, and significant differences have been identified between flights. A comparison of microphysical properties of different TICs will be presented. The Lagrangian Particle Dispersion Model FLEXPART is used to study the origin of observed air masses, to be linked with pollution sources. First results from the statistical analyses of cloud properties over the ISDAC area using the database created from CloudSat and CALIPSO observations will be discussed.