Inadequate representation of the macrophysical, microphysical and radiative properties of clouds in global circulation models is one of the largest sources of uncertainties in Earth’s climate projections. In the case of radiative properties, in particular those of ice clouds are highly uncertain because of the myriad of ice crystal shapes and sizes that can form and evolve in natural ice clouds. Currently, satellite instruments provide global estimates of two of the three fundamental radiative properties of ice clouds, namely optical thickness and effective ice crystal size near cloud top. However, very little is known about the natural variation of the third fundamental radiative property of ice clouds, the asymmetry parameter, which mainly depends on ice crystal shape. Moreover, current satellite retrievals of ice cloud optical thickness and effective ice crystal size are highly uncertain because they depend on an assumed, fixed asymmetry parameter that does not vary as in nature. In this talk, I will discuss a newly developed technique to simultaneously infer all three fundamental radiative properties of ice clouds from satellite measurements.
I will first explain the relation between the radiative and microphysical properties of ice crystals. I will review the current technique to infer ice cloud optical thickness and crystal size and will show how such retrievals can be greatly enhanced by simultaneous retrievals of the asymmetry parameter using multi-directional polarization measurements. Examples will be shown of this technique applied to detailed aircraft measurements, to satellite measurements over the Tropical West Pacific, and finally to global measurements. The variation of ice crystal size, shape and asymmetry parameter with atmospheric conditions will be discussed.
Location: Physics Bldg., Room 401