Using Multi-Radar Multi-Sensor (MRMS) Vertically Integrated Ice (VII) to assess cloud-to-ground lightning initiation potential for use in IDSS

The intent of this study is to investigate the use of multi-radar/multi-sensor (MRMS) vertically integrated ice (VII) as a lightning nowcasting tool, with the goal of providing forecasters with greater confidence and skill in assessing the potential for cloud-to-ground (CG) lightning strikes. This may aid forecasters in impact-based decision support services (IDSS) as it relates to CG lightning initiation. The use of MRMS VII may be useful for identifying storm initiation and the onset of electrical activity. Prior to MRMS, VII was not available operationally, but now VII can be used as a lightning nowcasting tool. VII is derived from a 3D reflectivity cube by vertically integrating reflectivity from -10 °C to -40 °C using the Rapid Refresh (RAP) model and converting to mass/area. Several hundred lightning and non-lightning producing cells were examined across the National Weather Service (NWS) Caribou, Maine area of forecast responsibility. The probability of detection (POD), critical success index (CSI), and false alarm rate (FAR) were calculated for various isothermal reflectivity levels and VII thresholds. This study showed that the greatest lead time to CG lightning initiation was associated with the lowest values of VII, which at the time of the study was 1.0 kg m-2.