In Praise of Thin Cirrus Clouds

As continuous ground and satellite-based lidar atmospheric monitoring activities have matured over the last fifteen years, the community has come to recognize that cirrus clouds are far more expansive and persistent globally than previously believed.  Cloud inventories based on passive radiometric observations collected through the 80s and 90s, for instance, underestimated overall cirrus cloud presence by roughly a factor of two.  Projects like CALIPSO, MPLNET, and others, have led to a throough reevaluation, and we now generally place instantaneous cirrus cloud occurrence somewhere between 40 and 60% globally.  Equally significant, however, is that lidar measurements over that time span have helped reveal how cirrus skew overwhelmingly toward translucent and semi-transparent clouds, the very trait that makes them so difficult to detect with passive radiometers in the first place.  More than half of all cirrus clouds exhibit an optical depth less than 0.3, making “thin cirrus” the most common cloud sub-type in the atmosphere.

This talk summarizes our current understanding of thin cirrus clouds, highlighting the contributions made by CALIPSO and MPLNET observations in developing this insight.  We describe baseline occurrence frequencies and distribution, including regional and inter-annual variability, for all cirrus, but specifically distinguish those properties exhibited by thin clouds.  We further describe outstanding uncertainties in the microphysical properties of cirrus clouds in general, thin cirrus included, that have to date made them difficult to parameterize in weather and radiative models.  We close by describing pending research designed to help reconcile a more thorough picture of the thin cirrus impact on climate, and specifically their role in regulating atmospheric energy balance.  The result is a synopsis of where we stand in recognizing the physical attributes of the most tenuous and elusive cloud genus found in the troposphere.