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An experiment reported in Mesinger and Veljovic (JMSJ 2020) showed anadvantage of the Eta over its driver ECMWF ensemble members in placing 250 hPa jetstream winds during a period of an upper tropospheric trough crossing the Rockies. Abyproduct of that experiment was that of the Eta ensemble switched to use sigma,Eta/sigma, also achieving 250 hPa wind speed scores better than their driver members,although to a lesser extent. Nevertheless, it follows that the Eta must include feature orfeatures additional to the eta coordinate responsible for this advantage over theECMWF.An experiment we have done strongly suggests that the van Leer type verticaladvection of the Eta, implemented in 2007, is a significant contributor to this advantage.In this experiment, having replaced a centered finite-difference Lorenz-Arakawa schemethis finite-volume scheme enabled a successful simulation of an intense downslopewindstorm in the lee of the Andes.While apparently a widespread opinion is that it is a disadvantage of terrainintersecting coordinates that “vertical resolution in the boundary layer becomes reducedat mountain tops as model grids are typically vertically stretched at higher altitudes,” avery comprehensive 2006 NCEP parallel test gave just the opposite result. Withseemingly equal ABL schemes, the Eta showed a higher surface layer accuracy overhigh topography than the NMM, using a hybrid terrain-following system (Mesinger, BLM2022).Hundreds of thousands of the Eta forecasts and experiments performeddemonstrate that the relaxation lateral boundary conditions almost universally used inregional climate modeling (RCM)–in addition to conflicting with the properties of thebasic equations used–are unnecessary. Similarly, frequently applied in RCMs so-calledlarge scale or spectral nudging, being based on an ill-founded belief, should only bedetrimental if possible numerical issues of the limited area model used are addressed.Note that this is confirmed by the results we refer to above.

Almost universally, in Regional Climate Modeling (RCM) integrations, Davies’ relaxation lateral boundary conditions are applied. They force variables in a number of rows around the boundary to conform to the driver global model values, completely at the boundary, and less and less toward the inside of the integration domain. Very often, in addition, investigators apply so-called large scale or spectral nudging inside the domain, forcing the integration variables not to depart much from those of the driver model.It is pointed out that there is no scientific basis for these two practices. So why are they used? In particular for the former of these two, it is suggested that reasons must be either a belief that this is a practice RCM should follow, or a technique to address numerical issues of the limited area model used, or a combination of the two. For the latter, a belief only.Examples are shown that, in the absence of these two stratagems, the limited area model can improve on large scales inside its domain. This demonstrates that their use, aimed to force variables inside the domain not to depart much from the driver model data, should be detrimental, if possible numerical issues of the model used were to be remedied.