First Ever Satellite Remote Sensing of Small Ice Crystals in Cirrus Clouds:
Summary

1. The absorption of radiation by photon tunneling affects the small mode of the PSD but not the large mode, making thermal emission due to tunneling an ideal signal for detecting small ice crystals.
2. The tunneling signal is the emissivity difference between 12 and 11 μm wavelength. This was not previously understood.
3. A satellite retrieval methodology has been developed whereby it is possible to retrieve the ice particle size distribution (PSD) small-to-large mode ice crystal concentration ratio, and the ice crystal concentration for a given IWC, as well as the IWP and D_e. Note that the retrieved PSD can be bimodal.
4. Retrievals based on β_eff reported in the literature indicate the small mode (as parameterized here) contributes little to the PSD. PSD retrievals and new 2DS probe measurements from an anvil cirrus case study indicate the same.
5. The retrieved dependence of D_e and N on cloud temperature appear to support the homogeneous freezing ice nucleation mechanism.

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