The Scientific Basis for a Radar-Based Lightning Launch Commit Criterion for Anvil Clouds

Triggered lightning poses a threat to the launch of space vehicles at the NASA Kennedy Space Center (KSC) and other launch sites. The Airborne Field Mill Project (ABFM) was conducted during June 2000 and May/June 2001 near KSC to investigate the magnitude and duration of the electric fields inside thunderstorm anvils, and how these fields and their lifetimes were related to the cloud microphysics and radar reflectivity. The overall motivation for this work was to develop improved and physically-based Lightning Launch Commit Criteria (LLCC) that would be safe but less restrictive than the current LLCC. The airborne measurements were made using University of North Dakota Citation II jet aircraft in conjunction with simultaneous radar coverage from the Patrick Air Force Base WSR74C (5 cm) radar and the Melbourne NEXRAD WSR88D (10 cm) radar. We found that when the reflectivity in anvil clouds near the aircraft was less than 5 to 10 dBZ, the magnitudes of the 3-dimensional electric field were less than 3 kV/m, a value that poses little threat of triggering lightning.

Different reflectivity parameters were examined for possible application in the LLCC. Early analysis focused on the average reflectivity over a volume that was larger than an individual radar pixel so that larger reflectivities that were nearby and that might contain higher electric fields were included. Volume averaging also provided a meaningful way to include vertical portions of the scans that were missing because of non-overlapping radar sweeps, the so called “scan gaps.” But since a thin anvil that poses little threat of triggered lightning can have the same effect on a volume average as a thicker anvil, which could indeed contain high fields and be a threat, we found that multiplying the volume average reflectivity by the thickness of the cloud provided a better proxy for the threat of triggered lightning. This parameter, the Volume Averaged, Height Integrated Radar Reflectivity (VAHIRR) was selected as the parameter to improve the LLCC for both attached and detached anvil clouds. VAHIRR is calculated over an 11 x 11 km area extending from the altitude of the 0 °C temperature level to the 0 dBZ cloud top. A statistical analysis of extreme values for the 11x11 km VAHIRR ≤ 10 dBZ km, (equivalent to an average of 10 dBZ in a 1 km thick anvil, or 5 dBZ in a 2 km thick anvil) showed that the probability of having an electric field larger than 3 kV/m. was less than 1 in 10,000.

Examples of ABFM measurements, of different reflectivity parameters including VAHIRR and scatter plots of electric field vs. VAHIRR for the entire ABFM anvil data set will be presented.