Polarimetric Radar Characteristics of Weakly Electrified Clouds over the Eastern Range

Weakly electrified clouds have received less attention than thunderstorms because they typically do not produce severe weather and are not associated with natural lightning. Knowledge of electrification mechanisms and associated cloud processes in weakly electrified clouds is incomplete. Methods to detect remotely the processes that lead to weakly electrified clouds are limited. A challenge is the variety of cloud types that exhibit weak electrification. Nonetheless, weakly electrified clouds can be a significant triggered lightning risk for commercial aviation and space launch activities.

The Eastern Range is a Spaceport that supports rocket launches from Kennedy Space Center (KSC) and Cape Canaveral Space Force Station (CCSFS) in Florida. The 45th Weather Squadron utilizes a set of safety rules called the Lightning Launch Commit Criteria (LLCC) to help ensure safe launch operations at the Eastern Range. To identify cloud conditions potentially associated with the natural or triggered lightning threat, the LLCC utilize various observations (e.g., lightning, surface electric field, radar reflectivity) and cloud types (e.g., cumulus, attached and detached anvil, debris clouds, disturbed weather, thick cloud layers) relative to temperature levels and cloud history thought to be associated with cloud electrification.

To fill knowledge gaps with the long-term goal of increasing launch availability while maintaining launch safety, we present the polarimetric radar characteristics of weakly electrified clouds over the Eastern Range using the Melbourne (KMLB) Next Generation Weather Radar (NEXRAD), which is about 48 km from the center of KSC-CCSFS. Weakly electrified clouds are defined as clouds with a vertical component of the electrostatic field at the surface > 1 kV m-1 and no lightning. The surface electric field is measured by the Launch Pad Lightning Warning System (LPLWS), which is a network of 31 field mills over KSC-CCSFS. The Mesoscale Eastern Range Lightning Information Network (MERLIN), which uses VHF interferometry and LF magnetic direction finding/time-of-arrival techniques, detects and locates both intra-cloud and cloud-to-ground lightning.

Fourteen weakly electrified cloud cases are identified between January 2018 and February 2022. All but one of the cases occurred during the cool season between November and March. The cases are associated with a variety of cloud types, including weak and shallow cumulus, disturbed weather around fronts, stratiform clouds and thick cloud layers. The vertical cloud structure and evolution are analyzed using KMLB NEXRAD over nearby LPLWS field mills in order to infer processes that affect the surface electric field. Radar reflectivity, differential reflectivity, specific differential phase and the correlation coefficient are analyzed in tandem and input into a fuzzy-logic based hydrometeor identification algorithm. The evolution of the ice-phase over field mills indicating weakly electrified clouds is of particular interest. If universal polarimetric characteristics exist between the weakly electrified cases despite the variety of cloud types, then the potential exists to simplify future LLCC and increase launch availability at the Eastern Range. Preliminary results suggest that all thick cloud layer cases (5) and all disturbed weather cases with stratiform precipitation (3) are associated with ice processes. For the weak/shallow cumulus cases (6), the preliminary results are mixed with one warm rain case, three cases with ice processes and two cases with possible ice processes.