Small-scale spatial rainfall variability during the extreme convective rain event of June 11th, 2018, over the city of Lausanne

Localized and Intense Rainfall Events (LIREs) can cause significant societal and economic damages. Typically developing over very small spatial and temporal scales, the accurate characterization and forecasting of such events remains, however, particularly challenging. By collecting distributed space-time observations, weather radars can provide useful information for the analysis of such events. In this study we take advantage of the experimental high-resolution radar data from the MeteoSwiss operational radar network available at 83 m radial resolution, every 5 minutes, over 20 different elevations to analyze the small-scale spatial variability associated with the extreme Lausanne LIRE of June 11^th, 2018, leading to the largest ever recorded 10-min rain gauge accumulation in Switzerland (41 mm). First, investigating the large-scale processes associated with this extreme event, a synoptic and dynamic analysis was conducted. This revealed the presence of a moderately unstable maritime tropical airmass which aided in the formation of a multicell thunderstorm which produced a wet microburst right over the city of Lausanne pouring an enormous quantity of water over very small spatial and temporal scales and leading to considerable localized flood and wind damage. Then, relying on the high-resolution radar data, the variability at small scale was measured by comparing rain rate values derived at different resolutions. More specifically, starting from the 83 m radar data, different existing hydrometeor-specific Z-R / Z-S relationships were used to compute an equivalent rain rate value at the gate level. Those were then compared against the corresponding rain rate values integrated at coarser radial resolutions of 500 m and 1000 m, and the difference across resolutions was derived as an indicator of small-scale spatial variability. With 1.5%, 0.41% and 0.18% of the total extracted and pre-processed gate volume showing differences larger than 25, 50 and 75 mm/hr between the 83 m and the 500 m data, a few but extreme small-scale rainfall variability peaks were observed, mostly associated with intensity peaks. Although most of these peaks were located above or within the melting layer, several of them were still observed below the melting layer, at proximity to the ground, and potentially decisive for hydrological applications. Converting this 3D information into 2D maps of sub-grid variability, a significant variability at the 5 min / 1km² resolution was observed highlighting not only the highly dynamic evolution of this event but also and the added value of high-resolution radar data to capture small-scale peaks associated with this extreme LIRE. By providing complementary insights on rainfall variability peaks, the retrieved sub-grid information can help improve the characterization of LIRE and enrich existing rainfall products.