Rethinking the Radiative Influence of Cirrus Clouds in Climate—Lessons Learned from CALIPSO and MPLNET

One of the more unheralded accomplishments of the NASA Earth Observing System project has been the discovery that cirrus clouds are significantly more expansive and persistent globally than previously believed. Prior to the launch of the CALIPSO satellite lidar mission, for instance, and before the widespread deployment of ground-based lidars, led by the MPLNET project, 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%. Through CALIPSO and MPLNET however, 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, how the influence of thin cirrus clouds is changing the way we look at the role of upper tropospheric ice in climate and atmospheric remote sensing and the resulting meridional daytime cirrus cloud forcing gradient that varies between ± 4 W/m².