Publication Date:
2020-09-23
Description:
Proper behavior of physics parameterizations in numerical models at grid sizes of order 1 km is a topic of current research. Modifications to parameterization schemes to accommodate varying grid sizes are termed “scale aware”. The general problem of grids on which a physical process is partially resolved is called the “grey zone” or “terra incognita.” Here we examine features of the Mellor-Yamada-Nakanishi-Niino (MYNN) boundary layer scheme with Eddy Diffusivity and Mass Flux (EDMF) that were intended to provide scale awareness, as implemented in WRF version 4.1. Scale awareness is provided by reducing the intensity of non-local components of the vertical mixing in the scheme as the grid size decreases. However, we find that the scale-aware features cause poorer performance in our tests on a 600 m grid. The resolved circulations on the 600 m grid have different temporal and spatial scales than are found in large-eddy simulations of the same cases, for reasons that are well understood theoretically and described in the literature. The circulations (model convectively induced secondary circulations, or M-CISCs) depend on the grid size and on details of the model numerics. We conclude that scale awareness should be based on effective resolution, not on grid size, and that the grey zone problem for boundary layer turbulence and shallow cumulus cannot be solved simply by reducing the intensity of the parameterization. Parameterizations with different characteristics may lead to different conclusions.
Print ISSN:
0027-0644
Electronic ISSN:
1520-0493
Topics:
Geography
,
Geosciences
,
Physics
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