Recent work with stochastic physics at the Met Office

Warren Tennant, Anne Mccabe, Glenn Shutts and Claudio Sanchez GIFS-TIGGE Workshop 12-14 June 2013, Exeter © Crown copyright Met Office

Contents

• Stochastic physics schemes at the Met Office • SST, LST & SMC perturbations • SKEB • Cyclone tracks and tropical wave performance • Adding temperature increments • Random Parameters • Convective-scale modelling experiments • Future stochastic physics schemes © Crown copyright Met Office

Stochastic physics schemes used at the Met Office

• Random Parameters (RP): • Knowledge uncertainty in values of physics parameters (entrainment rate, fallspeed, gravity-wave drag coefficient

etc

) • Fixed RP (e.g. QUMP) samples uncertainty in the model attractor (i.e. like an alternate physics scheme) • RP varying during the forecast (MOGREPS) samples uncertainty in the model evolution • Stochastic Kinetic Energy Backscatter (SKEB): • Injects vorticity proportional to the SQRT of diagnosed kinetic energy dissipation from semi-lagrangian advection and missing sources from deep convection © Crown copyright Met Office

Other Stochastic physics schemes (not operational)

• Stochastic Perturbed Physics Tendency (SPPT): • Physics tendencies are multiplied by a random number with a prescribed global pattern • (we are having difficulty to get the scheme stable and still have a significant impact) • Vorticity Confinement: • Injects vorticity around cyclones as an anti-diffusion action • (generally of use in low-resolution climate models) • Stochastic Convection Parametrization (Plant-Craig): • Stochastic representation of convective plumes in grid-box • (non-trivial task to implement and maintain a new scheme) © Crown copyright Met Office

Surface Perturbations

• Surface fields in each ensemble member had identical initial conditions, so to improve surface spread we implemented two schemes: • SST perturbations are generated with a random pattern of prescribed spatial structure • SMC perturbations are formed through a simple breeding method • see Tennant + Beare (2013)

accepted in QJRMS

• Ways of perturbing initial land-surface temperatures is also being investigated • We enhance initial temperature perturbations using orographic roughness © Crown copyright Met Office

SST + SMC perturbations ::

Helps address under-dispersive T2m © Crown copyright Met Office

--- LST Initial Perturbations ---

Inflating interpolated perturbations using orographic standard deviation

Land Surface Temp. perturbations (LAM): • Driving global surface temperatures interpolated to LAM; • Inflate perturbations using orography standard deviation ~ (0.5

o C/100m); • hilly terrain has higher inflation values © Crown copyright Met Office

--- LST Initial Perturbations ---

Inflating interpolated perturbations using orographic standard deviation

Results for period: 15-30 Dec 2010 • reasonable impact on spread at start of run (2%); • Benefit of spread increase decays with lead-time • Partly LBC flushing, but also highlights limited shelf-life of initial LST perturbations © Crown copyright Met Office

Stochastic Kinetic Energy Backscatter (SKEB)

• Cyclone tracks :: How does the SKEB scheme influence storm tracks?

• Tropical variability :: Is it the correct kind of variability?

© Crown copyright Met Office

Feature based diagnostics: TRACK

• Uses 6hrly vorticity field at 850hPa, filters between wavenumbers 5 and 42.

• Storm tracks are computed by minimizing a cost-function amongst these fields.

• A storm from the model matches a storm from the analysis if: • 60 of their points overlap in time.

• First 4 points that coincide in time have a separation distance to the analysis S < D (where D=4 degrees) © Crown copyright Met Office

Storm statistics (paired)

• Tracked storms are: • More intense with SKEB2 • Slightly better positioned without SKEB2 • SKEB2 does not scale well across resolutions.

• Default value for intensity at N320.

b R

is optimal for the representation of NH storms with the adequate • • • • • [Left]: Intensity difference for: N96 control N96 SKEB2 br=0.0275 N96 SKEB2 br=0.1

N96 SKEB2 br=0.2

N96 SKEB2 br=0.3

© Crown copyright Met Office

Tropical impacts (OLR)

© Crown copyright Met Office UM at GA3.0 (control) generates too much divergence -> more convection -> Less OLR at TOA -> More precipitation SKEB2 improves the divergence field and thus tropical representation of radiative fields, clouds and precipitation, this increase is proportional to the

b R

increase. SKEB2 accelerates North West winds over the Arabian Sea, leading to a better representation of a very dry India (but generating too much convection over West Pacific).

Outgoing Longwave Radiation (OLR) at the Top Of the Atmosphere for jja (TOA) (a) CONTROL minus CERES (b),(c) SKEB2 at different amplitudes minus Control.

SKEB2 improvements over the tropics not physically realistic: • Weaker Kelvin waves • Spurious Rossby waves at frequency of 3 days and wavenumber 5.

Tropical Impacts (variability)

Control SKEB ERA-int © Crown copyright Met Office

SKEB Temperature Increments

• SKEB adds wind increments • Is there merit in also adding temperature increments? How?

© Crown copyright Met Office

SKEB random forcing pattern and wind increments

• Power spectrum: g(n)  {5;60} • Deduced using coarse-graining methodology applied to a cloud-resolving model to give the power in a single mode as  (n) = n -1.27

• Resulting back scattered power spectrum injects energy at selected scales

SKEB T-increments :: multiply

• For each model level: • Take SKEB random pattern (modulated by dissipation) • Use global average physics │  T │ to re-scale pattern to prescribed magnitude • multiply physics temperature increment by random pattern • Stability controls: • Maintain existing Max/Min of field • Use masking function є {0;1} to taper off increments between 1 and 3 std-dev to zero © Crown copyright Met Office

SKEB T-increments :: add

• For each model level: • Take SKEB random pattern (modulated by dissipation) • Use global average physics │  magnitude T │ to re-scale pattern to prescribed • As a starting point for this experiment we used the backscatter ratio of 3% • add to existing physics temperature increment © Crown copyright Met Office

SKEB T-incr + T-mult improve spread and decrease EM RMSE of T2m © Crown copyright Met Office

SKEB Changes in ENDGame

• ENDGame dynamical core is more active than New Dynamics • SKEB energy injection had to be turned down to compensate • Energy input wave numbers: • 5-60 -> 20-60 • Numerical dissipation component halved © Crown copyright Met Office

Random Parameters Scheme

© Crown copyright Met Office

Impact of RP on MOGREPS-UK © Crown copyright Met Office

Initialisation :: Visibility Forecasts • Stochastic physics (Random Parameters) is being tested to improve ensemble spread for visibility • Mixing length – affects wind turning and BL depth Observations • Stability function – similar effect to mixing length above • Entrainment rate – mixing in dry air from above Control New Params © Crown copyright Met Office

Impact of RHCRIT parameter (single global value) on cloud field © Crown copyright Met Office

Summary and Discussion

• SST and SMC perturbations provide useful increase in spread of near-surface variables • SKEB perturbations not always physically realistic and may need tuning for use in DA • Adding temperature increments to SKEB is beneficial • Random Parameter scheme does demonstrate usefulness in cases and we plan to extend this scheme to include a spatial structure © Crown copyright Met Office

Thank-you any questions…

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