18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Monday, 30 July 2001
Grid resolution and surface flux and boundary layer parameterizations in high-resolution mesoscale models (formerly paper 3.1)
Michelle Whisenhant, NPS, Monterey, CA; and Q. Wang, S. Wang, and J. D. Doyle
In response to the fast the improvement of computation capabilities and the increasing demand of resolving mesoscale features on the order of kilometer scales, many mesoscale models have attempted to run at very high horizontal and vertical resolutions comparable to the size of boundary layer energy containing eddies. One emerging issue concerning increasingly high-resolution mesoscale models is that sub-grid scale (SGS) parameterization becomes inappropriate when the grid size becomes comparable to the length scale of the phenomenon. Essentially, the turbulent parameterizations were developed to represent the energy or transport of the ensemble turbulence eddies. In numerical models, turbulence is parameterized with respect to resolvable mean quantities. When the model grid size is large, the parameterized physical processes occur on a scale smaller than the grid size. Thus, the ensemble parameterization approach approximates the SGS processes. When the grid size is comparable to the scale of the phenomenon, the phenomenon itself is partly resolved by the model. The partial resolution of eddies creates an artificial interaction between the resolved and unresolved turbulence, complicating the interpretation of these intermediate resolution model outputs of turbulent quantities.

This study intends to answer some of the basic questions associated with the scale resolution in mesoscale models. First, at what grid-scale should a mesoscale model question the validity of the ensemble turbulence parameterization? Second, what are the sensitivities of boundary layer statistics from the mesoscale models to grid-size resolution? And third, how should we parameterize the SGS process when a portion of the large energy containing eddies are explicit resolved? To pursue these questions, we performed detailed spectral analyses to quantitatively evaluate the dependence of turbulence variances and fluxes on the scale of the integration under various boundary layer conditions. Measurements of turbulence perturbations from various field experiments are used for this purpose. To pursue the second and the third questions, we used the Navy’s Coupled Oceanic and Atmospheric Prediction System (COAMPS) to perform various sensitivity studies and tests of new proposed turbulence parameterizations. Results from both the observational study and model testing will be discussed in this presentation.

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