Advanced Water and Ice investigations in Subtropical, Mid-Latitude and Arctic Cirrus (ACIS)

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Upper troposheric observations of high supersaturations in- and outside of cirrus as well as low/high ice crystal numbers in cold/warm ice clouds initiated controversial debates on our understanding of the physical processes in ice clouds as well as intense activities to improve the accuracy of the water and ice instrumentation. The aim of ACIS is to put forward the understanding of ice clouds by investigating subtropical, mid-latitude and Arctic cirrus ice clouds using advanced water and ice instrumentation on board of HALO during the missions ML-CIRRUS, POLSTRACC and CIRRUS-RS. Further, the ability of the in-situ instrumentation to observe thin cirrus will be critically addressed by comparison of the in-situ observations with ground-based Lidar measurements. From a strict qualitiy controlled data analysis of the measured parameters, an ‘ACIS water and ice data base’ will be prepared for each mission. This ACIS data base will serve as input for comprehensive simulations with a detailed microphysical ice model. The role of heterogeneous and homogeneous freezing in ice nucleation as well as the supersaturation distribution in cirrus and its link to the ice crystals number and size will be evaluated.