A stochastic scale-adaptive subgrid transport scheme based on cloud size distributions with enhanced communication across the filter scale
Abstract
In a not too distant future the resolution of operational forecast models will approach values typical of Large-Eddy Simulation (LES). This transition from hydrostatic coarse grids to non-hydrostatic fine grids is not without problems. Perhaps the most significant hurdle is the treatment of cumulus convection, which is partially resolved throughout this grey-zone range of resolutions. The commonly adopted top-down approach is to start with an existing convection scheme of a GCM and to introduce scale-adaptive and stochastic elements. The alternative bottom-up approach is to start with a diffusive subgrid scheme for LES and add non-local elements. A benefit of the bottom-up approach is that i) the subgrid scheme at the foundation is simple, and ii) the scheme can be tested in an idealized but fully non-hydrostatic setting, being embedded in an LES grid. This presentation discusses results with a new scale-adaptive Eddy-Diffusivity multiple Mass-Flux scheme formulated in terms of discretized cloud size densities. Scale-awareness is implicit, while scale-adaptivity is introduced by means of size filtering. Stochastic effects on cloud number due to subsampling of the cloud population are introduced, using recently discovered scaling relations. Communication between the supergrid and subgrid scales is enhanced by making the stochastic module sensitive to the presence of resolved TKE structures that live on the grid. The ED(MF)n scheme is implemented into the Dutch Atmospheric LES model (DALES) as a subgrid transport scheme, and tested for a well known subtropical cumulus case across a range of resolutions that spans the grey zone. The presentation of the results focuses on the impact of the stochastic module on the model state, and the role played by the communication across the filter scale.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A13K2605N
- Keywords:
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- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSESDE: 3360 Remote sensing;
- ATMOSPHERIC PROCESSESDE: 3365 Subgrid-scale (SGS) parameterization;
- ATMOSPHERIC PROCESSESDE: 3394 Instruments and techniques;
- ATMOSPHERIC PROCESSES