Cut-cell Eta ensemble skill vs. ECMWF: Lessons learned
- 1Serbian Academy of Sciences and Arts, Belgrade, Serbia
- 2Faculty of Physics, Univ. of Belgrade, Belgrade, Serbia
- 3National Institute for Studies (INPE), Cachoeira Paulista, SP, Brazil
- 4NCEP Environmental Modeling Center (EMC), College Park, MD, USA
An experiment reported in Mesinger and Veljovic (JMSJ 2020) showed an
advantage of the Eta over its driver ECMWF ensemble members in placing 250 hPa jet
stream winds during a period of an upper tropospheric trough crossing the Rockies. A
byproduct 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 or
features additional to the eta coordinate responsible for this advantage over the
ECMWF.
An experiment we have done strongly suggests that the van Leer type vertical
advection of the Eta, implemented in 2007, is a significant contributor to this advantage.
In this experiment, having replaced a centered finite-difference Lorenz-Arakawa scheme
this finite-volume scheme enabled a successful simulation of an intense downslope
windstorm in the lee of the Andes.
While apparently a widespread opinion is that it is a disadvantage of terrain
intersecting coordinates that “vertical resolution in the boundary layer becomes reduced
at mountain tops as model grids are typically vertically stretched at higher altitudes,” a
very comprehensive 2006 NCEP parallel test gave just the opposite result. With
seemingly equal ABL schemes, the Eta showed a higher surface layer accuracy over
high topography than the NMM, using a hybrid terrain-following system (Mesinger, BLM
2022).
Hundreds of thousands of the Eta forecasts and experiments performed
demonstrate that the relaxation lateral boundary conditions almost universally used in
regional climate modeling (RCM)–in addition to conflicting with the properties of the
basic equations used–are unnecessary. Similarly, frequently applied in RCMs so-called
large scale or spectral nudging, being based on an ill-founded belief, should only be
detrimental if possible numerical issues of the limited area model used are addressed.
Note that this is confirmed by the results we refer to above.
How to cite: Mesinger, F., Veljovic, K., Chou, S. C., Gomes, J. L., Lyra, A. A., and Jovic, D.: Cut-cell Eta ensemble skill vs. ECMWF: Lessons learned, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-17562, https://doi.org/10.5194/egusphere-egu23-17562, 2023.
