Using a Nature Run as Truth Versus a Succession of Analyses

in Observing System Simulation Experiments.

A nature run is a long, uninterrupted forecast by a model whose statistical behavior matches that of the real atmosphere. The ideal nature run would be a coupled atmosphere-ocean-cryosphere model with a fully interactive lower boundary. Meteorological science is approaching this ideal but has not yet reached it. We still supply the lower boundary conditions (SST and ice cover) appropriate for the span of time being simulated.

The advantage of a long, free-running forecast is that the simulated atmospheric system evolves continuously in a dynamically consistent way. One can extract atmospheric states at any time. Because the real atmosphere is a chaotic system governed mainly by conditions at its lower boundary, it does not matter that the nature run diverges from the real atmosphere a few weeks after the simulation begins provided that the climatological statistics of the simulation match those of the real atmosphere. The nature run should be a separate universe, ultimately independent from but parallel to the real atmosphere.

One of the challenges of an OSSE is to demonstrate that the nature run does have the same statistical behavior as the real atmosphere in every aspect relevant to the observing system under scrutiny. For example, an OSSE for a wind-finding lidar aboard a satellite requires a nature run with a realistic cloud climatology.

A succession of analyses is a collection of snapshots of the real atmosphere. Though (in the case of 4DVAR) the analyses may each be a realizable model state, they all lie on different model trajectories. Each analysis marks a discontinuity in model trajectory. Considering a succession of analyses as truth seems to be a serious compromise in the attempt to conduct a “clean” experiment.

I favor a long, free-running forecast as the basis for defining “truth” in an OSSE.

Tom Schlatter