The close connection between the total lightning flash rate and storm updraft has been well recognized. In this study, we assessed the benefit of such a relationship in convective-scale data assimilation (DA) for model initialization. A lightning DA scheme to update model kinematic states was developed in the Weather Research and Forecasting Data Assimilation (WRFDA) three-dimensional variational (3DVar) system. This scheme combines total lightning observations with model-based prescribed vertical velocity profiles to retrieve kinematic information useful to DA. With the availability of space-borne lightning imagers in recent years, total lightning data observations from the Lightning Mapping Imager (LMI) on board the FY-4A geostationary satellite were assimilated in combination with radar DA. A detailed analysis of the impact of the lightning DA scheme on convective precipitation forecasting was conducted using a squall line case over Beijing on 13 July 2017. The results showed that the assimilation of LMI data further improves the analyses of dynamical conditions from assimilating radar radial winds. Although the microphysical states are identical due to the assimilation of reflectivity, updrafts directly form at lightning observation locations via lightning DA and hence improve the convective-scale dynamical balance. The quantitative verification of short-term convective forecasts indicated that the lightning DA adds value to current radar DA by improving the precipitation forecast skill. The new lightning DA scheme was further applied to a heavy rainfall case in 2018, and the results confirmed the effective and robust improvement in storm forecasting.

A rocket-triggered lightning flash containing negative–positive–negative current polarity reversal during its initial stage is analyzed using multiple synchronized observation data. The flash was triggered under a thunderstorm transition zone between the convective region and the stratiform region. Both positive leaders developing in the transition zone and negative leaders developing toward the convective region could be identified. As the negative initial continuous current (ICC) declined, a negative leader was transformed from a recoil leader which turned to break down virgin air off the preconditioned positive leader branch. As the negative leader developing forward, a reactivated breakdown leader bridging the grounding trunk channel and the initiation region of the negative leader caused the current polarity reversed from negative to positive 0.22 ms later, which is reported for the first time. The negative leader channel terminated after propagating for 71.08 ms, and the ICC reversed to be negative again owing to the propagation of another positive branch. The horizontal dipole charge structure contributed to the branching of positive leader and the initiation of negative leader, which combined to produce the upward bipolar lightning. During the positive ICC stage, both positive and negative channels simultaneously contributed to the channel-base current and several negative recoil leaders injecting negative charge to the grounding trunk channel produced a fast decrease of the current.