Rethinking How Cirrus Clouds Influence Climate

Department of Hydrology & Atmospheric Sciences
Weekly Colloquium
Thursday, February 28, 2019
4:00 pm in Harvill Building Room 101- Refreshments at 3:45
James R. Campbell

Naval Research Laboratory
Monterey, CA


One of the more unheralded accomplishments of the NASA Earth Observing System satellite project has been the discovery that cirrus clouds are significantly more expansive and persistent globally than believed only a generation ago.  Prior to the launch of the NASA CALIPSO satellite lidar mission in 2006, for instance, and before the widespread deployment of ground-based lidars, global cloud budgets were estimated using passive radiometric imager datasets alone.  Total cirrus cloudiness was then believed to be just above 20% instantaneously, and global cloudiness overall near 68%. We now recognize that global cirrus cloudiness is roughly twice that number, and that total global cloudiness is nearer to 75%.  The difference in these inventories can almost entirely be explained by the prior undersampling of translucent and semi-transparent cirrus clouds, which can be extremely difficult to detect with passive radiometric techniques alone.  We now fully understand that more than half of all global cirrus clouds exhibit an optical depth less than 0.3, making “optically-thin cirrus” the most common cloud sub-type in the atmosphere.  In this presentation, we discuss the impact of this emerging understanding of cirrus clouds, our evolving understanding of how best to effectively model cirrus clouds for radiative effects on weather and climate, and how the influence of thin cirrus clouds is changing the way we look at the role of upper tropospheric ice in influencing atmospheric remote sensing.