Ground Based Remote Sensing of Small Ice Crystal Concentrations in Arctic Cirrus Clouds
| Subhashree Mishra | Desert Research Institute |
| David Mitchell | Desert Research Institute |
| Daniel DeSlover | University of Wisconsin |
| Greg McFarquhar | University of Illinois |
Category: Cloud Properties
Example of how AERI data can be used to improve parameterizations of the ice particle size distribution (PSD). Plotted are absorption optical depth ratios for the 10.1/11.2 µm AERI microwindow channels and for the 9.1/11.2 µm channels as predicted from a mid-latitude PSD scheme based on SPEC, Inc. PSD data. The green curves were predicted from PSD that do not contain the distinct small ice crystal mode while the blue curves represent all the PSD measurements (which indicate a small crystal mode with D < 60 µm). These early results suggest the presence of a small mode but not as large as the measurements indicate. This basic appoach will be applied using the 11 and 12 µm AERI microwindows (for a better assessment of small crystals) to "correct" our PSD scheme developed for Arctic ice clouds.
Measurement of small ice crystals (D<60 µm) remains an unsolved and controversial issue in the cloud physics community. Concentrations of small ice crystals are hard to measure due to shattering of crystals at probe inlets. However, these small ice crystals alter cirrus cloud radiative properties and may affect the cirrus cloud feedback in global climate models. 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. That is, data from the Mixed-Phase Arctic Cloud Experiment (M-PACE) field project on the north slope of Alaska (winter 2004) has been used to develop an arctic ice particle size distribution (PSD) scheme that serves as the framework of the retrieval algorithm.
M-PACE data for glaciated ice clouds is used to develop a size distribution scheme based on ice water content (IWC) and temperature of ice phase cirrus clouds. Size distribution data from the forward scattering spectrometer probe, one-dimensional cloud probe, two-dimensional cloud probe, and high-volume precipitation sampler probe are used for this analysis. Measurements made by the ground-based Atmospheric Emitted Radiance Interferometer (AERI) are used to indicate the amplitude of the small mode relative to the large mode in a bimodal PSD scheme. A new parameterization of the cirrus bimodal PSD and the modified anomalous diffraction approximation (MADA) (treating ice cloud optical properties) comprises the framework of the remote sensing algorithm and uses the theory of light absorption by photon tunneling. Photon tunneling is especially sensitive to small ice crystals and determines the emissivity difference between 12 and 11 μm wavelengths. Based on the cloud temperature and a given IWC, the large particle mode of the PSD is calculated from the PSD scheme. When the predicted difference between 12 and 11 μm emissivity based on the large ice crystal mode is less than that measured, amplitude of the small mode is incrementally increased until convergence is attained. When the predicted difference is greater than the measured difference, the mean dimension of the large mode is incremented to achieve convergence. In this way, the PSD and ice particle number concentration are retrieved for a given IWC and temperature. This new retrieval procedure can also be used to estimate the ice water path and effective size of ice crystals to a cloud optical depth less than 2.5, at which point saturation of the AERI constrains the retrieval.
http://www.dri.edu/Projects/Mitchell/
This poster will be displayed at ARM Science Team Meeting.
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