A New Approach to Electromagnetic Scattering from Ice Crystal Aggregates for Improving Quantitative Radar Measurements of Clouds and Precipitation

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The goal of this research is to improve the qualitative and quantitative interpretation of radar measurements of clouds and precipitation composed of ice crystals and their aggregates. Aggregates are the dominant ice phase contributors to the area fraction of precipitation across the globe. Satellite radar systems and a growing number of ground-based research radars in the U.S. and around the world operate at millimeter wave frequencies and are focused on the qualitative and quantitative characterization of clouds. Recent studies have suggested errors in excess of an order of magnitude in the millimeter-wave radar cross sections of aggregates. A new approach to modeling aggregates provides a pathway to efficiently and accurately determine radar cross sections from irregular shaped particles. An improved accuracy of radar cross section calculations of ice particles is necessary to enhance our understanding of millimeter wave radar observations of stratiform clouds, melting layer, and ice precipitation. This research will be accomplished through a new modeling approach for studying the electromagnetic scattering characteristics of ice crystal aggregates. Advances in computer technology and computational algorithms have made it possible to evaluate millimeter wave scattering from pristine crystals with well defined shapes and composition. Irregularly shaped hydrometeors such as ice crystal aggregates (snowflakes) have been modeled as homogeneous particles with spherical or spheroidal shapes through an effective medium approximation of their dielectric constants. It has recently been shown that at millimeter wavelengths this "bulk" model approach for ice crystal aggregates leads to significant underestimation of their radar cross sections. This study models the complex shape and composition of aggregates using pristine crystal types that appear in nature (e.g., needles, columnar crystals, dendrites, bullet rosettes). The pristine crystals that makeup the aggregates are constructed using tiny ice spheres. An established computational algorithm is used to evaluate the electromagnetic scattering characteristics of the aggregates constructed in this manner.Intellectual merit: This research brings a new and efficient approach to accurately characterizing electromagnetic scattering from ice hydrometeors, which will be used for simulating millimeter wave radar observations of clouds and developing algorithms for the interpretation radar measurements.Broader impacts: Results from this study will find application in surface and space-based remote sensing of clouds and precipitation, satellite communication link design, and weather research. A graduate student and a postdoctoral fellow will participate in these research activities. They will be trained in both theoretical and experimental methodologies and techniques necessary for conducting the research. The generated electromagnetic scattering data will be made available to the research community through a database maintained on the web.