Comment on "Use of Observing System Simulation Experiments in the U.S."

Observing System Simulation Experiments (OSSEs) for numerical weather prediction (NWP) use an entirely simulated framework to explore the impact of new observational data on analysis quality and forecast skill. The basis of an OSSE is a Nature Run (NR), usually a free forecast of a high resolution NWP model that may be multiple years in length. The NR replaces the real atmosphere (and/or ocean) for the purposes of the OSSE, and as such should be as realistic as possible. Simulated observations are drawn from the NR fields, so the NR should ideally have accurate representations of all those fields that affect real observations, including clouds, moisture, temperatures, winds, and surface characteristics. Additionally, the NR fields are used as the ‘truth’ for verification of the experiment analyses and forecasts. It is best practice for the NR to have higher resolution and more sophisticated physics than the model used for ingesting simulated observations and performing experiments in the OSSE framework. Since the experiments are often performed with operational versions of NWP models, NRs are frequently produced using the highest resolution under development. NRs and other critical issues in OSSE development and application are presented in a White Paper on the subject (Errico and Privé 2018) prepared for the World Meteorological Organization.