RF01/RF02: LES sensitivity studies

1. RF01/RF02: LES sensitivity studies Adrian Lock and Eoin Whelan

2. Starting point: RF01 intercomparison • Met Office LES was low down the RF01 LWP league table! • And at the top for subgrid heat flux across the inversion

3. Starting point: RF01 intercomparison • Disappointing as the Met Office LES subgrid model should be stable for RF01: • Smagorinsky type + MacVean and Mason Ri • For RF01, M&M should give Ri>0 implying little subgrid mixing across the inversion Ri < 0 x RF01, k~0.5 M&M, k=0.7 R&D, k=0.23

4. RF01: simple changes • MacVean (1993): reduce neutral mixing length towards the inversion to reflect geometrical constraint on eddy size • Use monotone scheme (rather than 1st order upwind) for subsidence forcing z zi z=0

5. Impact in RF01 • These ‘simple’ changes give some increase in LWP • But still a factor of 2 too low

6. Bjorn’s fix • Switch off subgrid model for scalars (above 750m) • Crude but effective – same as Bjorn • Switch off subgrid model completely (above 750m) • Disaster - ?same as Bjorn?

7. Monotone advection of all variables • Monotone advection of momentum (as well as scalars) gives results almost identical to Bjorn’s fix • Can get to the top of the LWP league table without having to half switch off the subgrid model!

8. Increase subgrid diffusion • Is the diffusion implicit with monotone advection equivalent to having a more active subgrid model? • Try standard (non-monotone) advection but with cs=0.32 (instead of 0.23, ) • Some improvement in LWP but still some way short • Could increase cs further but subgrid fluxes and entrainment would increase further

9. Turbulence or noise? • Bjorn’s fix needs the subgrid model on for momentum – why? • We (all?) use monotone advection schemes for scalars, why not for momentum? • W field doesn’t look too noisy • Monotone advection gives loss of energy at smaller scales… Centred-difference advection of momentum Monotone advection of momentum

10. Spectra • Monotone advection of momentum leads to reduction in energy at scales close to the grid-scale • Similar to cs=0.32 w spectra at 500m

11. Spectra at entrainment flux level • Very different spectra between monotone and non-monotone advection of momentum just below the inversion • Is the extra energy near the grid scale with non-monotone advection just numerical noise? v spectra at zi w spectra at zi

12. Turbulence or noise? Centred-difference advection of momentum Monotone advection of momentum • Horizontal momentum field certainly looks noisy at the inversion • Why should we believe this noise any more than the noise we don’t like in the scalar fields? • Spurious noise in the momentum fields would reduce Ri(>0) and so increase subgrid scalar mixing across the stably stratified inversion (except with Bjorn’s fix)

13. Monotone advection of all variables • No problem with monotone advection of momentum in matching the observed w’w’

14. TKE budget Total TKE dissipation Advection scheme (=residual) = subgrid model + • Total dissipation also very similar between Bjorn’s fix and monotone momentum: Bjorn’s fix Monotone momentum

15. Resolution sensitivity • Run with: coarse (Dz=10m,Dx=35m), standard (Dz=5m,Dx=35m), fine (Dz=2.5m, Dx=17.5m) • Monotone advection ~ centred differences at doubled resolution • Not really converged • Higher resolution gives reduced entrainment and thence increased LWP • Only monotone fine resolution has cloud base ~ constant, as observed

16. RF02, at last! • Same sensitivity as RF01

17. Conclusions • Spurious noise in momentum fields close to the inversion can enhance subgrid fluxes there leading to excessive entrainment at standard resolutions (Dz=5m, Dx=35-50m) • Met Office LES gives apparently realistic results when monotone advection is used on all variables • Results are equivalent to a centred-difference scheme with double the resolution • Avoids the need for Bjorn’s fix • But simulations have not converged by Dz=2.5m, Dx=17.5m

18. Questions?