A Charging Proxy Based on Storm Structure

There have been a number of studies that infer Wilson current generation based on lightning flash rates and rainfall totals. However, none of these proxies take into account the scale and precipitation structure of electrified clouds. We use high-altitude ER-2 overflights to build a retrieval algorithm that computes the 20-km electric field vector and resulting Wilson current based on the passive microwave scene below the aircraft.

An algorithm is presented for estimating the total three-dimensional electric field vector at an arbitrary point over electrified weather from 37 GHz or 85 GHz passive microwave measurements. These frequencies are sensitive to column ice, which is important for electrification. The algorithm uses the geospatial distribution of ice mass as a proxy for charge accumulation across the microwave scene and then employs Coulomb’s Law to approximate the orientation and relative strength of the electric field vector that would be felt by an observer at a specified position and altitude.

This microwave proxy is then translated into actual electric fields using a transfer function developed with coincident passive microwave and electric field measurements provided by the NASA ER-2 high-altitude aircraft. Despite the algorithm’s assumption that all electrified clouds follow a convective charge structure and limitations in the ER-2 passive microwave measurements, the algorithm is found to produce reasonable electric field estimates over individual sample cases and statistically. Key areas that remain to be addressed, however, are the charge structure in stratiform precipitation and inverted polarity storms.