Towards global constraints on short-wave radiative properties of ice clouds 

Bastiaan van Diedenhoven, Brian Cairns, Ann Fridlind and Andrew Ackerman 

It has long been known that global climate model projections are sensitive to the 
simulated properties of ice clouds, such as those of widespread cirrus. The 
fundamental radiative properties of ice clouds can be represented in terms of their 
optical thickness, effective ice crystal size, and the asymmetry parameter of the 
scattering phase function. Satellite measurements such as those from MODIS are 
currently used to retrieve cloud optical thickness and effective radius, but cannot 
effectively constrain the asymmetry parameter. Moreover, MODIS cloud products 
intended to constrain climate model simulations are sensitive to the asymmetry 
parameters of the ice crystal habits assumed in the retrievals. To date, no global 
estimates of the asymmetry parameter of ice clouds exist. While ice crystal habits 
are highly variable, it has been shown that observed variations in the asymmetry 
parameter of ice clouds can be represented in terms of the aspect ratio and surface 
roughness of simple hexagonal ice particles. Multidirectional polarization 
measurements, such as those made by the Research Scanning Polarimeter (RSP), 
AirMSPI and POLDER instruments, provide sufficient constraints on aspect ratio and 
surface roughness to allow for a robust retrieval of the asymmetry parameter. Here 
we present a remote sensing technique to simultaneously retrieve ice crystal 
asymmetry parameter from multidirectional polarization measurements and ice 
crystal effective radius and ice cloud optical thickness from near-infrared and 
visible reflectances. After outlining the technique, we present an evaluation using 
simulated measurements. Then we apply the approach to measurements from the 
airborne RSP taken during the CRYSTAL-FACE campaign, and compare results with 
those of the collocated MODIS Airborne Simulator and in situ Cloud Integrating 
Nephelometer measurements obtained within the same cloud. Finally, we explore 
the possibility to apply the technique to combined observations of the MODIS and 
POLDER instruments in the A-train constellation in order to obtain a global 
climatology of ice cloud asymmetry parameters that can be stratified by cloud 
optical thickness, cloud top temperature and particle size.