Cirrus ≠ Cirrus: The Origin of Cirrus Clouds in the ECHAM-HAM Global Climate Model and CALIPSO/CloudSat Satellite Data

Cirrus clouds are the most frequent cloud type on Earth and as such contribute considerably to the planetary radiative balance and the upper tropospheric water transport. Nevertheless, their microphysical and radiative properties are poorly understood, not well constrained by observations, and represent a large challenge for climate models. Part of this challenge derives from the multiple possible formation mechanisms of cirrus clouds.

With the help of the ECHAM-HAM general circulation model and CALIPSO satellite data we distinguished two separate cirrus classes based on their formation environment:

• Liquid origin cirrus dominate at temperatures warmer than -55°C. They form in deep convective cloud anvils or by glaciation of mixed-phase clouds and are associated with extinction coefficients of up to 1 km^-1 and ice water contents of up to 0.1 g m^-3. They typically consist of high ice crystal number concentrations.
• In situ cirrus dominate at temperatures colder than -55°C. They form through homogeneous or heterogeneous ice nucleation, are optically thinner, and have a smaller ice water content. Homogeneously formed ice crystals tend to be smaller and occur in much higher number concentrations than heterogeneously formed ice crystals.

The ECHAM-HAM model further allowed us to distinguish between the two microphysically different types of in situ cirrus clouds. At elevations between 9 and 14 km altitude heterogeneous ice nucleation on dust aerosols generally dominates, in particular downwind of the main dust source regions. Homogeneous nucleation prevails only in the tropical tropopause region and over mountains.