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RF01/RF02: LES sensitivity studies

This study analyzes the sensitivity of the Met Office LES model for RF01 and RF02 simulations, focusing on subgrid heat flux and subgrid mixing across the inversion. Various modifications and advection schemes are tested to improve the results.

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RF01/RF02: LES sensitivity studies

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  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?

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