Concurrent Satellite and Ground-Based Lightning Observations in the Northwestern Mediterranean Region from the Optical Lightning Imaging Sensor (LIS), Low-Frequency Meteorage, and Very High-Frequency SAETTA

Starting 2021, the new space-based Lightning Imager (LI) on board the Meteosat Third Generation (MTG) geostationary satellite will improve the observation of lightning over Europe, the Mediterranean Sea, Africa and the Atlantic Ocean. In preparation of the use of the up-coming MTG-LI observations, we compare observations of the Lightning Imaging Sensor (LIS) on the International Space Station (ISS), which applies a technique similar to MTG-LI, to concurrent records of the Low Frequency (LF) ground-based network Meteorage. Data were analyzed over the north-western Mediterranean Sea from March 01, 2017 to March 20, 2018. Flashes are detected by ISS-LIS like a series of illuminated pixels, also called events, within a given (2.0 ms) frame and during successive frames. Meteorage describes flashes as a suite of Intra-Cloud/cloud-to-cloud (IC) and/or Cloud-to-Ground (CG) strokes. Both events and strokes are grouped to flashes using an in-house algorithm.

ISS-LIS detects 56.6 % of the flashes detected by Meteorage. The detection efficiency (DE) of Meteorage relative to ISS-LIS exceeds 80.0 %. Coincident matched flashes detected by the two instruments show a good spatial and temporal agreement. The mean distance between matches is significantly lower than the ISS-LIS pixel resolution (6.0 km nadir). The peak average flash timing difference is lower than the ISS-LIS integration time frame (2.0 ms). The closest events/strokes of matched flashes achieve sub-millisecond offsets. The further analysis of physical flash characteristics reveals that longer lasting and more extended flashes are more likely detected by both ISS-LIS and Meteorage than short duration and short extent flashes. ISS-LIS' relative DE is lower for daytime than nighttime as well as for CG than IC flashes.

A third network, the Very High Frequency (VHF) SAETTA Lightning Mapping Array enables to validate the lightning pairing. It also provides altitude information of the lightning discharges and adds a very detailed lightning description to the comparison for verification and better understanding of the processes. Both ISS-LIS and Meteorage flash detections feature a high degree of accordance with the SAETTA observations (without altitude information). It is found that Meteorage flashes with ISS-LIS match occur in significantly higher altitudes than unmatched flashes. Hence, ISS-LIS flash DE suffers from a lack of detection of low level flashes.