GROUND BASED RETRIEVALS OF SMALL ICE CRYSTALS AND WATER PHASE IN ARCTIC CIRRUS

Abstract

The microphysical properties of cirrus clouds are uncertain due to the problem of ice particles shattering at the probe inlet upon sampling. To facilitate better estimation of small ice crystal concentrations in cirrus clouds, a new ground‐based remote sensing technique has been used in combination with in situ aircraft measurements. Data from the Mixed‐Phase Arctic Cloud Experiment (M‐PACE), conducted at the north slope of Alaska (winter 2004), have been used to test a new method for retrieving the liquid water path (LWP) and ice water path (IWP) in mixed phase clouds. The framework of the retrieval algorithm consists of the modified anomalous diffraction approximation or MADA (for mixed phase cloud optical properties), a radar reflectivity‐ice microphysics relationship and a temperature‐dependent ice particle size distribution (PSD) scheme. Cloud thermal emission measurements made by the ground‐based Atmospheric Emitted Radiance Interferometer (AERI) yield information on the total water path (TWP) while reflectivity measurements from the Millimeter Cloud Radar (MMCR) are used to derive the IWP. The AERI is also used to indicate the concentration of small ice crystals (D<50 μm) relative to the larger ice particles. Combining this small crystal information with the PSD scheme describing the larger particle concentrations yields the retrieved PSD. Small ice crystals are evaluated using the absorption properties of photon tunneling or wave resonance while the liquid water fraction is evaluated using classical Beer’s law absorption. While this is still a work in progress, the anticipated products from this AERI‐radar retrieval scheme are the IWP, LWP, small‐to‐large ice crystal number concentration ratio and effective diameter for cirrus, as well as the ice particle number concentration for a given ice water content (IWC).