From Atmosphere to Inundation: Coupled WRF–WRFDA and LISFLOOD‑FP Modelling of the July 2021 Flood in Luxembourg

Compared to alluvial floods, flash or pluvial floods are difficult to predict because they result from intense and brief periods of extreme precipitation. Luxembourg has a history of being impacted by floods, with notable occurrences in January 2011, May 2016, December 2017, January 2018, February 2019, and February 2020. However, July 2021 stands out as the most severe flood year on record in the region. In this study we aim to develop, a high-resolution numerical weather prediction (NWP) model for effective local heavy rainfall prediction in a nowcasting scenario and provide real-time for flood simulation. The modeling relies on the Weather Research and Forecasting (WRF) model, which incorporates local Global Navigation Satellite System (GNSS) zenith total delays and precipitation observations to simulate small-scale, high-intensity convective precipitation. As part of this, we will also test run the LISFlood flood model in an operational inundation forecast mode, meaning that the flood model will be run with the WRF precipitation forecasts as inputs.

The WRF model was configured for the Greater Region and Luxembourg using a three‑nested domain setup with horizontal grid resolutions of 12 km, 4 km, and 1.3 km, incorporating high‑resolution static datasets. Meteorological forcing for the period 20 June–20 July 2021 was obtained from the Global Forecast System (GFS) and the ERA5 reanalysis (ECMWF), which were used as initial and boundary conditions. Zenith Total Delay (ZTD) observations from 245 GNSS stations across greater region were assimilated into the model, together with additional observational datasets including Surface Synoptic Observations (SYNOP), upper‑air measurements, radiosonde profiles (TEMP), and Tropospheric Airborne Meteorological Data Reporting (TAMDAR). Following data assimilation, a sensitivity analysis of key meteorological variables—such as precipitation and 2‑m air temperature (T2)—was conducted.

 

The WRF output from the innermost domain (NWPLux, 1.3 km resolution) was subsequently used as rainfall input for LISFLOOD‑FP to simulate the flood event. For comparison, simulations were also performed using ECMWF rainfall. All flood simulations were run over a 48‑hour period, from 14 July 2021 00:00 to 16 July 2021 00:00.

Keywords: NWP, WRF, Flash flood, LISFlood, Weather forecast, High-Resolution, GNSS, ZTD