Convection-resolving precipitation forecasting and its predictability in Alpine river catchments

Predictability limitations in quantitative precipitation forecasting arising from small-scale uncertainties in the initial conditions are investigated for Alpine river catchments, with particular consideration of their implications on hydrological runoff forecasting. To this end, convection-resolving ensembles of limited-area simulations are performed using a nonhydrostatic numerical weather prediction (NWP) model, and results are analysed in terms of catchment-averaged precipitation. The applied ensemble strategy uses slightly modified initial conditions representing observational uncertainties, but identical lateral boundary conditions representing a perfectly predictable synoptic-scale forcing. A total of four case studies is carried out for different synoptic conditions leading to heavy precipitation. Ensemble integrations of 12 members are analysed for 24-h forecasting periods, with particular attention paid to precipitation in the Po basin and in its sub-catchments in the Lago Maggiore area. The simulations exhibit a large variability in the predictability of precipitation amounts, both from case to case and from catchment to catchment. It is demonstrated for an episode of thermal convection, that the predictability may be very low even in large-scale catchments of ∼50,000 km2. In more synoptically dominated cases, predictability limitations appear to be restricted to catchments smaller than ∼10,000 km2, while in one case predictability is found to be high in catchments as small as 200 km2. Overall, the simulations show that precipitation forecasts for alpine river catchments may on occasions be critically affected by predictability limitations, even though the NWP model and the synoptic-scale forcing are assumed to be prefect. It is demonstrated that a substantial fraction of the predictability limitations is due to the scattered and unpredictable occurrence of convective cells, but the presence of convective precipitation alone does not necessarily limit predictability. It is also shown that the predictability is systematically higher in mountainous catchments.