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Polarimetric weather radars are capable of detecting tornadic debris signatures (TDSs), which result from debris being lofted to the level of the radar beam and can be modulated by centrifuging and debris fallout. TDSs have been used in promising applications, such as enhanced tornado detection, improved warning and assessment of a potential tornado threat, and estimating tornado damage potential and intensity. Regions with negative differential reflectivity have been found in TDS observations but a physical explanation is yet to be determined. Some hypotheses suggest a common alignment of debris or non-Rayleigh scattering to be the cause. However, because it is inherently difficult and extremely dangerous to verify this, a simulated environment can aid in this context to reveal information that would otherwise be impossible to retrieve in practice. Under the simulation environment, the true construct of the debris is known, wherefrom the bulk distributions of position and orientation data can be extracted for statistical analysis. The primary focus of this work is to investigate the cause of nonzero mean values of in TDSs with simulated data from SimRadar, which is a polarimetric radar time series simulator developed for tornadic debris studies. The 6-degrees-of-freedom (DOF) model shows that for both small and large platelike debris, the debris face tends to have some common degree of alignment normal to the wind direction, which may be a plausible cause for the occurrence of negative in real polarimetric radar observations. Potential explanations for other hypotheses regarding tornado and debris dynamics are also briefly discussed.

Continuing advances and the availability of relatively inexpensive commercial off-theshelf UAV systems allow the development of in-situ antenna measurement systems for a wide variety of operating frequencies and applications. This paper presents a simulation framework and error analysis that provides a guideline for in-situ far-field (FF) unmanned aerial vehicle (UAV)-based antenna measurements of operational and research antennas for polarimetric weather radar systems. The analysis includes system design trade-offs which allow the evaluation of antenna measurement errors due to UAV position, gimbal orientation, and extraneous error sources that can be present in a UAV-based field-testing scenario. Results analyzed for all cases provide error bounds and limitations for two example characterization schemes. The UAV-based antenna measurement system simulation and analysis suggests and supports the feasibility of such a system for antenna characterization and polarimetric calibration of antennas in the near future. INDEX TERMS Antenna characterization, errors, in-situ measurements, UAV, antenna test.