Ground Based Retrievals of Cloud Properties for Liquid, Glaciated and Mixed Phase Conditions

Cirrus cloud microphysical data from recent field programs using new instruments tend to minimize or remove the problem of ice particle shattering. These measurements suggest that in most instances, the anomalously high concentrations of small ice crystals reported in earlier in situ measurements are absent. These earlier measurements of small crystals indicated an abrupt increase in concentration for ice particle lengths around 60 μm and smaller, resulting in a "small particle mode." In addition, a new methodology we developed for satellite and ground-based remote sensing indicates that this small mode is either absent or lower in amplitude than earlier aircraft measurements have indicated. Remote sensing results presented on our website (http://www.dri.edu/Projects/Mitchell/) address both anvil and in situ synoptic cirrus in tropical and mid-latitude regions. This leads us to hypothesize that, in general, ice particle size distributions (PSD) are monomodal. This study applies this hypothesis to mixed phase clouds to estimate the ice water path (IWP) and liquid water path (LWP). When our remote sensing method indicates the cloud PSD as bimodal, the small mode is attributed to liquid water while the large mode is attributed to ice particles. Data from Mixed-Phase Arctic Cloud Experiment (M-PACE), conducted at the north slope of Alaska (winter 2004), have been used to test this new method for retrieving the LWP and IWP. The framework of the retrieval algorithm consists of the modified anomalous diffraction approximation (MADA) for mixed phase cloud optical properties, a radar reflectivity-ice microphysics relationship and a temperature-dependent ice 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) in combination with the ice PSD slope are used to derive the IWP. This provides one estimate of the LWP/TWP fraction. Another estimate is obtained by applying the principle of photon tunneling or wave resonance to radiances at 11 and 12 μm, which allows us to infer the magnitude or absence of a small mode for the retrieved PSD. Combining this small mode information with the PSD scheme describing the larger ice particle concentrations yields the retrieved PSD. While this is a work in progress, the anticipated products from this AERI-radar retrieval scheme are the IWP/IWC, LWP/TWP fraction, and LWP for mixed phase clouds and IWP/IWC, PSD slope and effective diameter for cirrus clouds. This information can be extremely useful in characterizing cloud microphysical propertied in Global Climate Models (GCMs). For example, Arctic clouds are often mixed phase and play a major role in Arctic climate, and need to be characterized in GCMs. Moreover, the PSD slope can be used to estimate ice sedimentation rates, a critical parameter affecting cirrus cloud lifetime, IWP and coverage. The fact that (1) IWP retrieved from the MMCR radar was generally consistent with the AERI TWP in glaciated regions and that (2) these retrievals were independent and based on very different physics appears promising for using this approach to approximate IWP and LWP in mixed phase clouds. This technique appears particularly useful for characterization of liquid and mixed phase clouds having TWP < 100 gm-2, a range where traditional methods have exhibited insufficient accuracy.