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

© Crown copyright 2005

RF01/RF02: LES sensitivity studies

Adrian Lock and Eoin Whelan


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


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=0.7 R&D, k=0.23

RF01, k~0.5

x

Ri < 0

e

tq

( / )e

p t

kL c q


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 1st order upwind) for subsidence forcing

z=0

ziz

neutl


Impact in RF01

These ‘simple’ changes give some increase in LWP But still a factor of 2 too low


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?


Monotone advection of all variables

Monotone advection of momentum (as well as scalars) gives results almost identical to Bjorn’s fixCan get to the top of the LWP league table without having to half switch off the subgrid model!


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

Increase subgrid diffusion

0 sc x


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


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


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 ziv spectra at zi


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 momentum Monotone advection of momentum


Monotone advection of all variables

No problem with monotone advection of momentum in matching the observed w’w’


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


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


RF02, at last!

Same sensitivity as RF01


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


Questions?

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

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