Moving toward Subkilometer Modeling Grid Spacings: Impacts on Atmospheric and Hydrological Simulations of Extreme Flash Flood–Inducing Storms

Flash floods develop over small spatiotemporal scales, an attribute that makes their predictability a particularlychallenging task. The serious threat they pose for human lives, along with damage estimates that canexceed one billion U.S. dollars in some cases, urge toward more accurate forecasting. Recent advances in mputational science combined with state-of-the-art atmospheric models allow atmospheric simulations atvery fine (i.e., subkilometer) grid scales, an element that is deemed important for capturing the initiation andevolution of flash flood–triggering storms. This work provides some evidence on the relative gain that can beexpected from the adoption of such subkilometer model grids.Anecessary insight into the complex processesof these severe incidents is provided through the simulation of three flood-inducing heavy precipitation eventsin the Alps for a range of model grid scales (0.25, 1, and 4 km) with the Regional Atmospheric ModelingSystem–Integrated Community Limited Area Modeling System (RAMS–ICLAMS) atmospheric model. Adistributed hydrologic model [Kinematic Local Excess Model (KLEM)] is forced with the various atmospheric simulation outputs to further evaluate the relative impact of atmospheric model resolution on thehydrologic prediction. The use of a finer grid is beneficial in most cases, yet there are events where theimprovement is marginal. This underlines why the use of finer scales is a step in the right direction but not asolitary component of a successful flash flood–forecasting recipe.