What cut-cell Eta features additional to its intersecting the topography are responsible for its successful performance?

Despite its successful performance compared to other U.S. NWS/NMC and later NCEP/EMC operational models, the Eta model was essentially frozen in 2002. This decision was made primarily due to the problem with step topography identified by Gallus and Klemp (2000); see the remarks by DiMego cited in Mesinger and Veljovic (2017, hereafter MV17). However, after about three years of EMC efforts dedicated to its planned replacement by NMM/GSI, the results of the EMC four-plus-month “parallel” test in 2006 showed verification scores favoring the Eta system (see MV17, Fig. 4).

Even so, the Gallus–Klemp problem needed to be addressed. This was accomplished by changing from step topography to “sloping steps,” now generally referred to as cut-cells (see MV17, Fig. 7). Regardless, the Eta continued to perform well; see the results of Veljovic et al. (2010), Mesinger and Veljovic (2013), and MV17, Fig. 11, the last of which discussed further in Mesinger and Veljovic (2020). In this later experiment, the Eta ensemble members driven by ECMWF (EC) members, although having about the same resolution for the first 10 days, achieved better scores for 250 hPa winds stronger than 45 m s⁻¹ than their EC drivers.

A puzzling byproduct of that experiment was that the Eta ensemble, when switched to use sigma, still achieved these 250 hPa wind scores better than their driver members, although to a lesser extent. The reasons for these results are addressed. Unique features of the Eta contributing to this advantage and addressed include its “fairly well-posed” (McDonald 2003) lateral boundary conditions (LBCs), specification of topography without smoothing, and the introduction of finite-volume vertical and slantwise advection.

New results presented here for the impact of the Eta LBCs show an advantage of the Eta LBCs over the ubiquitously used relaxation LBCs are giving more than twice as many times more accurate verifications of the upper tropospheric winds stronger than 45 m s⁻¹ than the relaxation LBCs. And a more accurate average equitable threat score adjusted to unit bias, ETSa.

The introduction of finite-volume vertical and slantwise advection compared to previous Lorenz-Arakawa centered finite-difference schemes enabled a more accurate placement of the foehn-type warming in the lee of Andes, in a challenging zonda downslope windstorm. In that experiment very steep topography of the Andes was used as generally done in the Eta without smoothing, customary in terrain-following models.