Seamless turbulence parametrization across model resolutions Adrian Lock and Ian Boutle Met Office © Crown copyright Met Office

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 © Crown copyright Met Office

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) © Crown copyright Met Office

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) © Crown copyright Met Office

Grey zone parametrization Fit to Honnert et al (2011) Tends to zero for “well resolved” limit Fine resolution Coarse resolution © Crown copyright Met Office

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 © Crown copyright Met Office

Stratocumulus Case Study © Crown copyright Met Office

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 © Crown copyright Met Office

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 © Crown copyright Met Office

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 © Crown copyright Met Office

1km:seamless 333m:seamless 100m: seamless Cloud Fraction Liquid Vertical velocity (ms-1) Liquid water path (gm-2) © Crown copyright Met Office

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 © Crown copyright Met Office

COALESC UM @100m 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) © Crown copyright Met Office

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) © Crown copyright Met Office

What about cumulus convection? © Crown copyright Met Office

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 © Crown copyright Met Office

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 © Crown copyright Met Office

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 © Crown copyright Met Office

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 © Crown copyright Met Office

Questions © Crown copyright Met Office