The two faces of cirrus clouds
1School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, USA
2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
*now at: NASA Goddard Space Flight Center, Greenbelt, MD, USA
Abstract. Low ice crystal concentration and sustained in-cloud supersaturation, commonly found in cloud observations at low temperature, challenge our understanding of cirrus formation. Heterogeneous freezing from effloresced ammonium sulfate, glassy aerosol, dust and black carbon are proposed to cause these phenomena; low updrafts however is required for cirrus characteristics to agree with observations and is at odds with the gravity wave spectrum in the upper troposphere. Instead, background temperature fluctuations can establish a "dynamical equilibrium" between ice production and sedimentation loss that explains low temperature cirrus properties. This newly-discovered state is favored at low temperatures, does not require heterogeneous nuclei to occur, and is insensitive to their presence. Our understanding of cirrus clouds and their role in anthropogenic climate change is reshaped, as the type of dynamical forcing will set these clouds in one of two "preferred" microphysical regimes with very different susceptibility to aerosol.