The sequence of heavy precipitation and flash flooding of 12 and 13 September 2019 in eastern Spain. Part II: A hydro-meteorological predictability analysis based on convection-permitting ensemble strategies

Abstract

On 12 and 13 September 2019, widespread flash flooding caused devastating effects across eastern Spain. Within the framework of the HyMeX program, this study examines predictability of the long-lasting heavy precipitation episode (HPE) conducive to flash flooding. A set of short-range, convection-permitting ensemble prediction systems (EPSs) is built to cope with different sources of meteorological uncertainty. Specifically, the performance of an Ensemble Kalman Filter, tailored bred vectors and stochastic model parameterizations is compared to more standard ensemble generation techniques such as dynamical downscaling and multiple physics. Results indicate EPS focusing on sampling model uncertainties related to parameterization of subgrid process are skillful, especially when deep convection and its interaction with complex orography are important. Furthermore, representation of small-scale thermodynamical aspects is improved through data assimilation, leading to an enhanced forecasting skill as well. Nevertheless, predictability remains relatively low at the catchment scale in terms of exceedance probabilities in cumulative precipitation and peak discharge. The analysis presented herein could serve as a basis for the future implementation of real-time flash flood warning systems based on skillful meteorological EPSs over small-to-medium, semi-arid watersheds in eastern Spain and, by extension, over the flood-prone Western Mediterranean region.

Large wood debris that clogged bridges followed by a sudden release. The 2019 flash flood in Catalonia.

“…Besides bridges being swept away by the flood, the huge sediment load played an important role in that flood. It also reminds us of the 2018 flood in Mallorca, which claimed 17 lives (Lorenzo-Lacruz et al, 2019;Amengual et al, 2021). In the latter episode, a peak discharge of about 300 m 3 /s was estimated for a basin of barely 23 km 2 .…”

TRAM: A new non‐hydrostatic fully compressible numerical model suited for all kinds of regional atmospheric predictions

“…First, we would like to expand the options and complexity of available physical schemes as, currently, only one scheme is incorporated for each parametrized process. Besides initial/boundary conditions, physical parametrizations are recognized as a major source of uncertainty in meso‐ and convective‐scale numerical simulations and, thus, more options available would allow promoting predictability studies and the design of multiphysics ensemble forecasting applications (e.g., Amengual et al ., 2021). Another step towards widening the operational scope of the model, by being able to define autonomously well‐adapted initial states from observations and analyses, including ensemble members, will consist of implementing some of the different flavours of data assimilation methods (see Kalnay, 2002).…”

Probabilistic short-range forecasts of high precipitation events: optimal decision thresholds and predictability limits

“…A possible explanation is that heavy precipitation in autumn is more likely to be caused by orographically forced convection in the Mediterranean area. These events are driven by slowly evolving, highly coherent weather patterns (Amengual et al , 2021;Davolio et al, 2013;Nuissier et al, 2016;Caumont et al, 2021), with comparatively high predictability. The p opt statistic strongly depends on intensity, but its variations are smooth: for F2, it is close to 0.3 at low intensities, and it decreases to about 0.15 at 80 mm (depending on the period).…”