Observations of Lightning using a Continuous Broadband Interferometer and Cross Correlation

A digital broadband interferometer (DITF) developed at Osaka University has been deployed at Langmuir Laboratory with an upgraded streaming digitizing system during the summer of 2011 and 2012. The upgraded digitizer allows lightning to be located continuously in time over a broad range of source powers. The data are processed using a cross correlation based algorithm and further enhanced using maximum likelihood windowing functions. The resulting 2D images are highly detailed, having sub-microsecond time resolution as well as good angular resolution that will allow the development of positive and negative leaders to be examined in detail. Continuously radiating events such as K-changes and dart leaders are imaged in their entirety, and impulsive events associated with stepping can be isolated in time.

The continuous DITF acts as a very complementary instrument to the Langmuir lightning mapping array (LMA). The LMA locates lightning in 3D by measuring the arrival time of VHF lightning emissions in 10 microsecond windows at several locations around Langmuir Laboratory. The LMA and interferometer data can be combined such that the 2D interferometric image is projected onto the 3D LMA locations. The resulting 3D image improves upon the data provided by either system alone. Ambiguities in the interferometer data arising from sources moving towards or away from the interferometer are removed. The timing resolution of the LMA is improved to that of the interferometer. Continuously radiating emissions such as K-changes and dart leaders can be mapped in 3D. The combined continuous DITF and LMA system is proving to be a very powerful tool for the study of lightning electrical breakdown processes.

To demonstrate the capabilities of the continuous DITF, results from a handful of classic bilevel intra-cloud and cloud-to-ground flashes are presented.