7.3 Convective-Scale Perturbation Growth Across a Spectrum of Convective Cases

Tuesday, 25 July 2017: 2:00 PM
Coral Reef Harbor (Crowne Plaza San Diego)
David L. A. Flack, University of Reading, Reading, United Kingdom; and S. L. Gray, R. S. Plant, H. W. Lean, and G. C. Craig

Convection-permitting ensembles have led to greater understanding of the predictability of convective-scale forecasts. However, convective-scale predictability is not fully understood with respect to different convective regimes. In this presentation the convective regimes are diagnosed based on a convective timescale which identifies whether cases are in or out of equilibrium with the large-scale forcing. Five convective cases, with varying timescales, are examined in a convection-permitting ensemble. The ensemble members were generated using Gaussian buoyancy perturbations added in the boundary layer, which can also be viewed as representing turbulent fluctuations close to the gridscale. The resulting perturbation growth is shown to occur on different spatial scales with an order of magnitude difference between the regimes (O(1 km) for non-equilibrium convection and O(10km) for equilibrium convection). This scale difference is consistent with the notion of forecasts for the position of equilibrium events becoming close to chance after the first 12 hours of the forecasts. Further understanding of perturbation growth within the different regimes could lead to a better understanding of where ensemble design improvements can be made beyond increasing the model resolution and lead to improved interpretation of forecasts.
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