An Eta Driven by ECMWF 32-day Ensemble Experiment: What Happens to Large Scales? Do Lateral Boundary Conditions Make a Difference?

Fedor Mesinger


Considerable number of authors presented experiments in which degradation of large scales occurred in regional climate integrations when large-scale nudging was not used. There have however been Eta model RCM experiments in which improvement in large scales is hard to dispute. This issue is revisited - by Katarina Veljović, Borivoj Rajković and the presenter - by comparing the large scale skill of the Eta RCM ensemble against that of the ECMWF 32-day ensemble used as its driver. Another issue looked into is that of the lateral boundary condition (LBC) scheme. This is addressed by running the Eta model in two versions differing in the lateral boundary scheme used. One of these is the traditional relaxation scheme and the other is the Eta model scheme in which information is used at the outermost boundary only and not all variables are prescribed at the outflow boundary.

A novelty in these experiments is the verification used. In order to test the large scale skill forecast position accuracy of the strongest winds at the jet stream level is verified. This is done by calculating bias adjusted equitable threat and bias scores for wind speeds greater than a chosen wind speed threshold, with the ECMWF analyses used as truth. 250 hPa is taken as the jet stream level. For greater confidence a traditional RMS difference between the forecast and analyzed winds at this same level is also calculated.

The results show the Eta RCM skill in forecasting large scales with no interior nudging to be just about the same and slightly higher than that of the ECMWF driver ensemble. As to the LBC impact no disadvantage compared to relaxation was seen from using the Eta LBC scheme, in spite of its requiring information from the outermost RCM boundary only.