On the initial conditions of the ICON-D2-EPS ensemble: An analysis in terms of spread and skill.

The COSMO-D2-EPS ensemble is running operationally at DWD at a resolution of 2.2 km. In the framework of the transition from the COSMO to the ICON model for the limited-area applications, the ICON-D2-EPS ensemble is starting its pre-operational phase. Therefore, the perturbation strategy developed for COSMO-D2-EPS is adapted to the new ensemble.
In this work, the focus is on the initial conditions, which are provided by the first 20 analyses generated by a LETKF ensemble data assimilation system (KENDA).
The KENDA analyses present the advantage of providing perturbed initial conditions to the convection-permitting ensemble, where the perturbations contain also the information on the convection-permitting scale uncertainties. On the other hand, the KENDA analyses are optimised for the purpose of data assimilation. The ensemble of analyses which is the most suitable for initialising the next data assimilation cycle may not be the same which is the most suitable for initialising the weather forecast ensemble, e.g. in terms of spread.

The analyses generated by the KENDA cycle are evaluated from the point of view of their usage for ensemble forecasting initialisation. Their spread is computed for different variables, assessing also how it varies with the spatial scale and with the weather situation. Furthermore, the spread is compared to the error of the analyses and of the forecasts, in order to assess the ability of the analyses to describe the initial condition uncertainty. 
The growth of the differences between the members during the first hours of the forecasts is studied as well, in dependence on the weather situation.

The final aim of this work is to identify possible improvements for deriving the ensemble initial conditions from the KENDA analyses.