Characterizing two types of cirrus clouds that differ in nucleation mechanism and radiative effect

Abstract

A new CALIPSO satellite retrieval for cirrus clouds has been developed over the last 1.5 years that retrieves ice particle number concentration, effective diameter, and ice water content.  It compares favorably with in situ measurements from many field campaigns around the world.  This talk would briefly describe the new method targeting single-layer cirrus clouds and focus on new findings resulting from this retrieval, relating them to climate model predictions.  These results indicate that there are two types or categories of cirrus clouds.  Type 1 cirrus appear to form through heterogeneous ice nucleation (het), have visible optical depths < 0.3, and are most abundant; they are what most people visualize as a “cirrus cloud”.  Type 2 cirrus may form through a combination of het and homogeneous ice nucleation, have visible optical depths > 0.3 (with visible extinction coefficients typically 4 times greater than type 1 cirrus), and are often associated with warm fronts, orographic gravity waves, and other lifting processes.  However, type 2 cirrus clouds constitute 76% to 88% (depending on latitude) of the estimated net cloud radiative effect of all cirrus clouds.  Based on comparisons between retrieved and predicted ice particle number concentrations and effective diameters, these type 2 cirrus clouds are poorly represented in climate models, possibly partly due to the predicted dependence of ice nucleation on layer-average pre-existing ice (not realistic near cloud top where ice nucleation occurs).  Predicted ice nuclei concentrations may also need revising.