Seminar

Abstract

Cirrus clouds still represent one of the largest uncertainties in the prediction of the Earth's climate (IPCC, 2013) since their microphysical and radiative properties are not yet well known. Our study aims to provide a guide to cirrus microphysics, which is compiled from an extensive set of model simulations covering the broad range of atmospheric conditions for cirrus formation and evolution. The model results are portrayed in the same parameter space as for field measurements, i.e. in the temperature - ice water content (IWC) parameter space. We validate this cirrus analysis approach by evaluating cirrus data sets from fifteen aircraft campaigns, conducted in the last fifteen years, spending about 60h in cirrus over Europe, Australia as well as Southern and Northern America. It can be shown that the field observations indeed show the characteristics expected from the cirrus guide. For example, high/low IWCs are found together with high/low icw crystal number. As a result, it is now possible to track, to a certain degree, the history and the formation mechanism of observed cirrus clouds only from the measurement of IWC and the relative humidity. Important findings from our study are that (i) a substantial part of thick cirrus originates from mixed phase clouds, i.e. via freezing of liquid droplets, while thin cirrus are mainly formed directly as ice, and (ii) a significant fraction the observed cirrus are formed by heterogeneous ice nucleation.