1. Seamless turbulence parametrization across model resolutions
Adrian Lock and Ian Boutle Met Office
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2. Motivation
MetUM now used for many “high resolution” applications on grids of 100m to 4km
“Real world” research tool as well as for forecasting
MetUM has two distinct turbulence parametrizations (both 1st order closure)
1D non-local boundary layer parametrization = standard NWP
3D “Smagorinsky-Lilly” type, as used in the Met Office LEM
“UKV” (operational model over UK with 1.5km horizontal grid) uses a split approach:
Smagorinsky in the horizontal + 1D BL in the vertical
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3. Motivation
So, currently, the user has to choose between these and their short-comings:
3D Smagorinsky scheme
Poor when flow is un(der)-resolved, eg: stable BLs, stratocumulus (especially cloud-top entrainment), sub-cloud layer in deep convection and cold pool evolution, etc
UKV set-up
Resolved scale is 3D (eg convective storms) so missing important turbulent processes
Column-based BL diagnosis (type, parcel ascent for depth, etc) inappropriate at (partially) resolved scales
All applications will have a mixture of resolvable an unresolved scales
Ultimate aim is a scale-aware turbulence parametrization that can allow a smooth transition from unresolved to resolved scales
Initial step here is to “blend” between 3D scheme (good at high resolution) and 1D scheme (good at coarse)
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4. Pragmatic turbulence scheme blending
Both 1D and 3D schemes have the same vertical turbulent flux parametrization
So can write:
where W1D is some blending weight (=1 in unresolved turbulence)
and λblend is the W1D-weighted combination of λSmag (grid-dependent) and λbl (physical)
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5. Grey zone parametrization Fit to Honnert et al (2011)
Tends to zero
for “well resolved” limit
Fine
resolution
Coarse
resolution
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6. Initial seamless package
“Blended” 1D BL + 3D Smagorinsky turbulence
Additional technicalities for decoupled BLs and free-atmosphere
Scale-aware warm rain microphysics
Explicit representation of sub-grid variability in process rates
See Boutle, et al 2014, MWR for details
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7. Stratocumulus Case Study
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8. Impact of poorly resolved circulations in UKV vs UK4 stratocumulus
Excessive amplitude mesoscale variability in UKV (1.5km) leading to spurious gaps in cloud, not present in old 4km version
UKV control
UK 4km model
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9. Seamless physics package BL turbulence, microphysics
Seamless physics weakens resolved scale circulations and increases cloud cover
How realistic was this mesoscale variability?
UKV control
Seamless physics package
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10. COALESC = UK stratocumulus
Combined surface and aircraft measurements
UM simulations on 1km to 100m grids
1km UM domain
x = Surface observations
12 UTC
15 UTC
2nd Mar 2011
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11. 1km:seamless 333m:seamless 100m: seamless Cloud Fraction Liquid
Vertical
velocity
(ms-1)
Liquid
water path
(gm-2)
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12. Blending weight profiles
Plausible looking reduction in W1D in the boundary layer as the grid size decreases (note 100m still not very well resolved)
Relaxes to 3D Smagorinsky in the free-troposphere
4km
1km
333m
100m
Potential Temperature (K)
1D Weight
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13. COALESC
generates cloud length scales that look pretty good (5 km ~ 10 h)
50 km
12 UTC 2nd Mar 2011
Prevailing wind
5 km
Visible satellite
LWP (gm-2)
w (ms-1)
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14. COALESC spectra
Scale-aware package matches 3D Smag at 100m resolution; more like op UKV at 1km but weaker
Scale aware
3D Smag
Op UKV (1D BL/2D Smag)
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15. What about cumulus convection?
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16. Cumulus convection in a 1.5km model
For scales < 6 km, showers need to be parametrized
Can’t be resolved accurately
Generate significant precipitation (over the UK at least)
New grey zone massflux scheme
Mixing characteristics from the shallow scheme (high ε, δ)
Closure:
where here the grey-zone weight, W1D = fn(cloud-top height, grid-size)
Triggering: standard NWP moist adiabatic parcel but also require wLCL>0.05m/s
Only trigger parametrized convection where there is resolved convergence
Avoids the scheme triggering across all grid-columns in a large area
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17. Grey zone “shallow” cumulus parametrization 20th April 2012 (DYMECS)
UKV (1.5km grid)
Radar
UKV control
Seamless BL only
Including grey cu
Suppresses small showers
Some resolved, some paramd showers
Some large, some small showers
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18. Grey zone “shallow” cumulus parametrization Now 7pm
Still organises into deeper cells by evening (no parametrized convection is diagnosed over land at night)
UKV control
Grey Shallow Cu
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19. Summary Pragmatic blending appears to behave sensibly
Gets around having to choose between 3D Smagorinsky or 1D boundary layer parametrization at any given resolution
Suppresses near grid-scale motions (“noise”?)
“Grey zone” shallow cu closure also seems to work OK
Future work
Problems with spinning up small scale circulations (eg. at boundaries or in time-evolving flows)
Test some means of initiating turbulence at the resolved scale
Improve the underlying closures, eg:
A better closure than 3D Smagorinsky for resolved turbulence
Improvements to the blending weight, eg:
Mixed layer vs entrainment zone?
Stability dependence
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20. Questions
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