Relationship of Graupel Shape to Differential Reflectivity: Theory and Observations

The formation of graupel requires the presence of a moderate updraft and supercooled water so that riming can occur. These conditions are also favorable for icing, which is a threat for aviation. The ability to identify graupel with dual polarization radar would help detect potential icing regions inside a storm. Graupel comes in a wide variety of shapes, density, and sizes which makes it challenging to identify with a polarimetric radar. Furthermore, graupel may be coated with water changing its dielectric properties and affecting the polarimetric response. Hydrometeor classification algorithms utilize these variables to infer the presence of graupel. The calibration of the polarimetric radar is critical to successful inference. The occurrence of conical graupel is particularly useful since it is associated with a slight negative differential reflectivity. This response distinguishes this common hydrometeor from raindrops which are widely recognized to show exclusively positive values for Zdr. Many studies simulate polarimetric radar observations of conical graupel. However, even within the category of conical graupel, many different shapes can occur. Studies show that differential reflectivity, Zdr, is highly dependent on the apex angle (or height to width ratio). Zdr varies between a few negative dB for small apex angles (approximately 20°) to slightly positive values for larger angles (90°). Simulations of this dependency will be discussed. Results will be shown from some mixed-phase precipitation events that occurred on November 11, 2011 in Indiana and April 12, 2012 in Massachusetts where conical graupel was reported and collected on the ground. The graupel produced slightly negative Zdr around the area of the reports, mainly between -0.5 and 0dB, but a few range gates had a Zdr as low as -1.5 dB. These results are consistent with the conical shapes of the observed graupel that were estimated to have apex angles of less than 60 degrees.

This work was sponsored by the Federal Aviation Administration under Air Force Contract No. FA8721-05-C-0002.  Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Government.