Formation, lifetime and climate impact of cirrus clouds using ECHAM

Introduction

Cirrus clouds are clouds in the upper troposphere/lowermost stratosphere (UTLS) region consisting purely of ice crystals. As all other cloud types cirrus clouds affect the radiation budget of the Earth-Atmosphere system via two different effects

1. Ice crystals scatter and reflect incoming solar radiation back to space, leading to a cooling of the system (albedo effect)
2. Ice crystals absorb and partly re-emit terrestrial infrared radiation, leading to a warming of the system (greenhouse effect)

Up to now the net effect of cirrus clouds on the radiation budget is uncertain; for thin cirrus clouds a net warming of the Earth-Atmosphere system is assumed but not confirmed. This uncertainty is mainly due to the complexity of microphysics in cirrus clouds, e.g. different nucleation mechanisms for ice crystals involving solution droplets and so-called ice nuclei (IN).

Cirrus clouds in the general circulation model (GCM) ECHAM

Since the global net effect of cirrus clouds is highly uncertain, the representation of cirrus clouds in the ECHAM5 GCM is a focus of our work. We are working on the impact of aerosols on cirrus clouds in the ECHAM5 GCM for a better representation of cirrus clouds. In cirrus clouds different nucleation processes can compete and change the microphysical and radiative properties of the developing cirrus cloud.

Impact of enhanced stratospheric aerosol loading on cirrus clouds in ECHAM

Stratospheric aerosol loadings can be enhanced by natural processes like volcanic eruptions or by anthropogenic intervention to counteract global warming, termed Climate-engineering. In both cases sulphate is injected into the stratosphere. Aerosols injected into these heights can sediment to lower levels into the upper troposphere, modify the aerosol composition and thus might impact cirrus clouds. Current global simulations show that the microphysical properties of cirrus clouds are indeed modified due to stratospheric sulphate emissions in both cases. The results suggest that via a rather complex chain of cause and effect including feedbacks on the dynamics cirrus clouds are impacted significantly.