Using Factor Separation to Elucidate the Respective Contributions of Desert Dust and Urban Pollution to the 4 January 2020 Tel Aviv Lightning and Flash Flood Disaster

A numerical investigation was conducted to assess the possible impact of desert dust and urban aerosols on a very intense and deadly thunderstorm that occurred within the Tel Aviv metropolitan area on 4 January 2020. For these purposes, the cloud drop nucleation scheme within the spectral (bin) microphysics scheme (SBM) in the Weather Research and Forecasting Model (WRF) was coupled with the GoCart (desert) dust model and with hourly surface observations of urban aerosols. Using SBM integrated mass contents and simulated vertical velocities, the dynamic lightning scheme was able to forecast lightning rates within thunderclouds developing in an environment of marine or marine/continental aerosols with dust and/or urban aerosol forcings. Of these aerosol forcings (or factors), the urban aerosols (pollution) intensified lightning rates in convective clouds forming at and just off the Israeli coastline (referred to as the Urban Aerosol Lightning Enhancement Effect; ULE). The inclusion of continental aerosols with dust and urban aerosol sources resulted in the simulation exhibiting the most intense lightning and precipitation storm (referred to as Large Aerosol Small Aerosol Invigoration Effect; LASI). Yet, a comparison of model results and observations suggests that it was the combination of desert dust and urban aerosols alone that focused storm development over western Tel Aviv. The result was that high lightning rates initially focused over western Tel Aviv were accompanied by and then succeeded by a period of heavy rain that led to deadly flooding. Plain Language Summary The possible role of urban pollution on lightning and rainfall was investigated using a sophisticated cloud model embedded in a weather forecast model. The cloud model takes into account the impact of different sources of dust, including desert dust on cloud formation, and intensification into thunderstorm producing clouds. The day chosen for study was a day with lots of lightning and even deadly flooding in the city of Tel Aviv, Israel. The results showed that urban pollution intensifies lightning rates and precipitation amounts over the city. When large raindrops produced on large dust particles interacted with small drops produced from urban pollution, intense lightning and rainfall was forecast to occur in the area of deadly flooding. Hence, to forecast the intensity and location of severe storms, forecasts should take into account different sources of dust and pollution. Key Points Ur