Polarimetric radar signatures before and after triggered and natural lightning

During the summers of 2011-2012, a C-band polarimetric Shared Mobile Atmospheric Research and Teaching (SMART) radar from the University of Oklahoma was deployed to Keystone Heights, FL to study the relationship between cloud structure and the propagation of triggered and natural lightning channels. A frequency agile, rapid scanning X-band dual-polarimetric radar augmented the radar observations in Florida briefly during 2012 before being deployed to Socorro, New Mexico to observe storms targeted for triggered lightning attempts there.

The radars operated in Range-Height-Indicator (RHI) volume scanning mode over a narrow azimuthal sector that provided high spatial vertical resolution every 60-90 seconds over the rocket launch facility at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL and at Langmuir Laboratory atop Mt. Baldy near Socorro, NM. In this presentation we will provide an overview of the radar observations surrounding triggered and natural lightning discharges with emphasis on changes in polarimetric fields leading up to and immediately after the lightning discharges.

In Florida during 2011, seven successful triggers (with return strokes) out of 20 attempts were sampled by the SMART radar from June to August. Preliminary analysis indicates that several triggers occurred when packets of moderate radar reflectivity (20-40 dBZ) near 1-4 km altitude were descending towards the surface in the vicinity of the ICLRT. Indeed, descending reflectivity packets was observed to occur prior to many of the trigger attempts.

In addition to the descending reflectivity packets, specific differential phase (KDP) and occasionally differential radar reflectivity factor, ZDR, showed evidence of ice crystal alignment due to strong electric fields in the upper portions of the convection over ICLRT around the time of launch attempts. Consecutive RHI sweeps over ICLRT revealed changes in KDP that suggested the building of electric fields and subsequent relaxation after a triggered flash. Not all flashes were observed to have pronounced changes.

Lightning Mapping Array sources of the triggered flash channels showed a preference for horizontal propagation just above the radar bright band associated with the melting layer. The propagation path also seemed to be related to the vertical distribution of KDP in some of the triggered flashes.

Examples of these results will be presented within the context of an overview of DARPA-sponsored research activity related to triggered lightning studies.