Sensitivity of Cirrus Cloud Parameterizations to Anthropogenic Impacts

Cirrus clouds are one of the biggest uncertainties in the radiative budget, crucial to the understanding of short- and long-term trends in climate. Satellite observations also suggest that ice clouds have an important impact on earth’s climate by influencing the radiation balance and hydrological cycle. Improved parameterizations of cirrus clouds in global climate models (GCMs) require good understanding of the cloud properties and the anthropogenic impacts on cirrus.

This study compares existing size distribution parameterizations in mid-latitude and in tropical cirrus derived from data sets collected at different locations, during different campaigns around the world: both, remote, without anthropogenic impacts and mid-latitude, continental. The goal is to evaluate the potential impact of inadvertent weather modification. The sensitivity to anthropogenic impacts is broadly supported by observations and some previous model studies, but is still poorly quantified.

Our findings indicate that the radiative properties of remote areas tropical cirrus versus mid-latitude continental cirrus, for the same temperature legs and for the same ice water content (IWC), are considerably different. For example, tropical cirrus from remote areas over the tropical ocean, not associated with continental anthropogenic impacts, should reflect considerably more sunlight at colder temperatures for the same IWC.

These results may also point to the different mechanisms by which convective and non-convective cirrus are generated over land with a lot of anthropogenic impact, and over the tropical ocean. The temperature dependence of their size spectra may yield important clues for unraveling the underlying physics that determine the evolution of size spectra in these different cloud systems.