Page 1© Crown copyright 2005 RF01/RF02: LES sensitivity studies Adrian Lock and Eoin Whelan

Page 2© Crown copyright 2005 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

Page 3© Crown copyright 2005 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 M&M, k=0.7R&D, k=0.23 RF01, k~0.5 x Ri < 0

Page 4© Crown copyright 2005 RF01: simple changes 1.MacVean (1993): reduce neutral mixing length towards the inversion to reflect geometrical constraint on eddy size 2.Use monotone scheme (rather than 1 st order upwind) for subsidence forcing z=0 zizi z

Page 5© Crown copyright 2005 Impact in RF01  These ‘simple’ changes give some increase in LWP  But still a factor of 2 too low

Page 6© Crown copyright 2005 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?

Page 7© Crown copyright 2005 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!

Page 8© Crown copyright 2005  Is the diffusion implicit with monotone advection equivalent to having a more active subgrid model?  Try standard (non-monotone) advection but with c s =0.32 (instead of 0.23, )  Some improvement in LWP but still some way short  Could increase c s further but subgrid fluxes and entrainment would increase further Increase subgrid diffusion

Page 9© Crown copyright 2005 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… Monotone advection of momentumCentred-difference advection of momentum

Page 10© Crown copyright 2005 Spectra  Monotone advection of momentum leads to reduction in energy at scales close to the grid-scale  Similar to c s =0.32 w spectra at 500m

Page 11© Crown copyright 2005 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? w spectra at z i v spectra at z i

Page 12© Crown copyright 2005 Turbulence or noise?  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) Centred-difference advection of momentumMonotone advection of momentum

Page 13© Crown copyright 2005 Monotone advection of all variables  No problem with monotone advection of momentum in matching the observed w’w’

Page 14© Crown copyright 2005 TKE budget  Total dissipation also very similar between Bjorn’s fix and monotone momentum: Total TKE dissipation = subgrid model + Advection scheme (=residual) Bjorn’s fix Monotone momentum

Page 15© Crown copyright 2005 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

Page 16© Crown copyright 2005 RF02, at last!  Same sensitivity as RF01

Page 17© Crown copyright 2005 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

Page 18© Crown copyright 2005 Questions?