Vertical Resolution of the Surface Layer versus Finite-volume and Topography Issues

• Published in: Boundary-layer meteorology Vol. 187; no. 1-2; pp. 95 - 104
• Main Author: Mesinger, Fedor
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01.05.2023; Springer; Springer Nature B.V
• Subjects: Advection; Atmospheric models; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric Sciences; byproducts; Earth and Environmental Science; Earth Sciences; Horizontal cells; Mathematical models; Meteorology; Modelling; Numerical models; Numerical schemes; prediction; Research Article; Resolution; Surface boundary layer; Surface layers; Temperature; Terrain following; Topography; Wind speed; United States > US; Surface-layer resolution; Zilitinkevich constant; Topography; Finite-volume schemes; Vertical coordinate
• ISSN: 0006-8314; 1573-1472
• DOI: 10.1007/s10546-022-00745-2

With probably no exception, in atmospheric numerical models, a high vertical resolution is used close to the surface, with gradually reduced resolution higher up. This seems an obvious choice given the importance and complexity of processes close to the ground, and the cost of using a high near-surface resolution throughout the model atmosphere. But there are disadvantages involved that deserve attention. One is that the performance of numerical schemes is generally better for uniform resolution, in particular when the finite-volume approach is used. Another is that with the usual terrain-following vertical coordinate, horizontal flow across high topography will be subject to severe resolution changes encountering the topography. An unintended experiment of the impact of these disadvantages is a by-product of the so-called “parallel” run of two models at the U.S. National Centers for Environmental Prediction in 2006, when the operational Eta model was compared against its intended replacement, the NMM model. In that four+ month experiment the Eta model more accurately forecast 10-m wind speed and 2-m temperatures over the mostly high topography of the western United States than the NMM, despite its much poorer vertical resolution over that area and not too different physical parametrizations. It is suggested that the severe NMM grid cell resolution change of horizontal flow encountering high topography with terrain-following coordinates is the main cause of this result.