An Extension of the Mellor–Yamada Model to the Terra Incognita Zone for Dry Convective Mixed Layers in the Free Convection Regime

Junshi Ito, Hiroshi Niino, Mikio Nakanishi, Chin Hoh Moeng

Research output: Contribution to journalArticlepeer-review

45 Citations (Scopus)

Abstract

The terra incognita (TI) or grey zone arises in conventional planetary boundary-layer parametrizations when the grid resolution of a numerical model is comparable to the size of the energy-containing turbulent eddies $$\sim $$∼1 km or less. Here, we investigate a simple, plausible extension of the Mellor–Yamada (MY) level-3 scheme for TI-scale grid size using a large-eddy simulation (LES) as a benchmark. Horizontal filtering of the benchmark simulation data for the dry convective mixed layer in the free convection regime yields subfilter-scale components whose statistics are then retrieved for various filter sizes. This leads to a modified MY level-3 scheme for TI-scale grid sizes. The proposed TI scheme incorporates: (1) modification of various length scales in the conventional MY scheme by an empirical function that depends on the horizontal grid size normalized by the convective boundary-layer height; (2) a new length scale for horizontal turbulent fluxes; and (3) a linear relationship between the local dissipation length and subfilter-scale turbulent kinetic energy. A posteriori tests of the proposed TI scheme show a much improved performance compared with the conventional MY level-3 scheme. The ratio of the grid-scale to the subgrid-scale turbulent intensity is comparable to that obtained from the filtered LES solutions. Sensitivity tests show that the modification of the dissipation length scales has the largest impact, while the new length scale for horizontal fluxes also proves important. A simulation that includes all of the above modifications results in the optimum performance.

Original languageEnglish
Pages (from-to)23-43
Number of pages21
JournalBoundary-Layer Meteorology
Volume157
Issue number1
DOIs
Publication statusPublished - 2015 Oct 7
Externally publishedYes

Keywords

  • Grey zone
  • Horizontal resolution
  • Mellor–Yamada model
  • Terra incognita
  • Turbulence parametrization

ASJC Scopus subject areas

  • Atmospheric Science

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