Uncertainties in COSMO-DE precipitation forecasts introduced by model perturbations and variation of lateral boundaries

As a first step towards a convection-permitting ensemble prediction system (EPS), this study explores the use of perturbation methods within the numerical weather prediction (NWP) model COSMO-DE. The study isolates uncertainty sources so that their impact can be separately studied. The focus is set on uncertainties in model physics and lateral boundary conditions which are represented by a multi-parameter and a multi-boundary approach. Experimental ensemble forecasts of precipitation with a lead time of 24 h are generated. Three ensemble setups are constructed: one with model perturbations, one with variations of boundaries and one with combined perturbations. The investigation period comprises 15 days in summer 2007. Deterministic verification shows that each individual member leads to quantitative precipitation forecasts (QPFs) within a reasonable quality range. Verification shows that the probabilistic precipitation forecasts of the experimental ensembles are superior to the deterministic forecasts. Measures of ensemble dispersion show that the impact of the perturbations on the forecast strongly varies with lead time, with model perturbations always dominating the first few hours and variations of lateral boundaries often dominating the following forecast hours. The study concludes that the applied perturbation methods lead to potentially useful probabilistic precipitation forecasts and should be considered as part of a future EPS design.

Research Highlights
► We examine perturbation techniques for an atmospheric ensemble prediction system.
► Model physics and lateral boundary conditions are varied.
► We focus on precipitation forecasts in a convection-permitting model.
► There is a quality gain compared to a single unperturbed forecast.
► Relative effect of perturbation types on ensemble dispersion depends on lead time.