The grey zone challenge: experiences from mesoscale modelling Jeanette Onvlee, WWRP/WG-MWFR

1. The grey zone challenge: experiences from mesoscale modelling Jeanette Onvlee, WWRP/WG-MWFR WGNE meeting, Tokyo, October 2010

2. Outline • When can grey zone problems be expected; ways to deal with them • Experiences from the mesoscale • Sensitivities of convective behaviour • Proposed experimentation WGNE meeting, Tokyo, October 2010

3. More than one grey zone to deal with… • Grey zone for convection • Down to ~5km: fully parametrized convection does ok • Below this: partially resolved down to ~several 100m • In this partially resolved range: neither the assumption of fully parametrized nor that of fully resolved convection works quite well… • Grey zone for turbulence • Experiences MetOffice, others: 3D treatment of turbulence important at resolutions of ~2km or less • Evidence from energy spectra WGNE meeting, Tokyo, October 2010

4. Use Meso-NH in LES mode to see what happens at NWP-scale WGNE meeting, Tokyo, October 2010

5. 8000m 62.5m Averaging over larger boxes of original 62.5m results in Meso-NH WGNE meeting, Tokyo, October 2010

6. Subgrid Resolved DX/H WGNE meeting, Tokyo, October 2010

7. Strategies used for dealing with the convection grey zone • Try to jump over it (use stretched or variable grids) … but then what about the “next” grey zone? • On-off approach: switch off deep convection parametrization at some resolution (most models) … but is this ok to describe partially resolved convection? • Scale-sensitive convection-turbulence parametrization: gradual shut-down of parametrized deep convection (ALARO) … but can we make this work? WGNE meeting, Tokyo, October 2010

8. Experiences with mesoscale modelling Different types of convection are not equally well handled by existing mesoscale models. In the extratropics one should distinguish: • Orographically forced convection: generally very well described • Forcing by large-scale flow (fronts, convergence zones etc): generally well described if large scale flow is well captured • Open cell convection (forcing mostly from surface): problematic! WGNE meeting, Tokyo, October 2010

9. A case study of open cell convection WGNE meeting, Tokyo, October 2010

10. “Fractal” behaviour, no convergence to “resolved solution” • Observed coherent precip structures not reproducible WGNE meeting, Tokyo, October 2010

11. ALARO/AROME exp at 2, 1 and 0.5kmwith/without deep convection parametrization • Without convection parametrization: • Strong upward motions, high clouds, overestimate of precip peak WGNE meeting, Tokyo, October 2010

12. A need for 3D physics? Present paradigm: parametrize all subgrid processes only in the vertical. This may need to be revised. Indications for need of 3D-treatment for turbulence: • experiments with 3D-turbulence, SLHD • great sensitivity of convective behaviour to horizontal diffusion settings However, this is a very delicate issue numerically (Piotrovski et al 2009)! • GCSS deep convection working group case 4: • Increasing delay of rain onset with decreasing resolution • 3D turbulence reduces overshoot and difference in time of onset precipitation WGNE meeting, Tokyo, October 2010

13. Highly sensitive to details of numerical setup… WGNE meeting, Tokyo, October 2010

14. Impact of 3D-structure of clouds on radiative balance: need for 3D-treatment of radiation? WGNE meeting, Tokyo, October 2010

15. Outflow problems: grid point storms? • Outflow unrealistically strong and in all directions • Seen in several models. Occurs in evolving deep convection situations with very high upward velocities • Put brake on updraft velocities? WGNE meeting, Tokyo, October 2010

16. Other aspects affecting (open cell) convective behaviour • Great sensitivity to details of physics-dynamics interaction • Horizontal diffusion settings (if no 3D-turbulence) • Order of slow / fast physics processes wrt dynamics calculations • ...? • Microphysics • Balance between hydrometeors, fall speeds • Initialization of hydrometeors and clouds • Domain size and nesting strategy • Details of surface treatment WGNE meeting, Tokyo, October 2010

17. Relevant diagnostics • Energy spectra (effective scale of model information) • LES as “truth” • Standard scores, 1D diagnostics not appropriate; focus on detailed (3D) behaviour of clouds, timing/intensity/scale of precipitation, hydrometeors • Beware of sensitivities towards phys-dyn interface, hor diffusion, nesting/initialization strategy etc! • For verification over longer periods: • Stratify verification according to type of convection. • Use scale-sensitive verification techniques like upscaling WGNE meeting, Tokyo, October 2010

18. WGNE meeting, Tokyo, October 2010

19. Proposed experimentation (1): case studies Extra-tropics: SRNWP ET-physics proposes: • Open cell convection over sea • Tackle the most difficult problem of open cell convection • But avoid difficulties of prescribing land surface forcing • Select situation with very little large scale flow, • Choose sufficiently large model domain to allow convection to evolve within the model and not be dominated by the LBC • 1, 3D models plus LES for range of resolutions • Include mentioned sensitivities explicitly in experimentation Tropics: • Strongly forced deep convection? WGNE meeting, Tokyo, October 2010

20. Proposed experiments (2) More prolonged intercomparisons: • Ongoing in Europe: Real-time comparison of AROME-France and COSMO-DE in overlapping domain (in future maybe also MetOffice/UKV?) • Purpose: no beauty contest! Goal is to find differences in overlapping area and learn from them • Careful with effect of boundaries, different initialization etc. • Many convective situations show significant and systematic differences. Gives indications of potential areas of improvement. WGNE meeting, Tokyo, October 2010