Met Office Unified Model NIMROD Nowcasting - LCRS

1 Rachel Capon 04/2004 © Crown copyright 

Met Office Unified Model 

NIMROD Nowcasting 

Rachel Capon 

Met Office JCMM

Unified Model 5.+, 6.+ 

– New Dynamics Core 

2 Rachel Capon COST 722 Paris 25/06/2004 © Crown copyright 

Outline 

– Physical Parametrisations 

– Limited Area Configuration 

Single Column Unified Model 

Site Specific Forecast Model 

Nimrod Nowcasting System


New Dynamical Core 

OLD formulation 

Eulerian 4 th order advection 

Split-explicit time integration 

Horizontal B grid 

Vertical staggering – Lorenz 

Sigma pressure coordinate 

Quasi-hydrostatic formulation 

Unified Model 5.x 

operational since Aug 2002 

NEW formulation 

Semi-Lagrangian advection 

Semi-implicit time integration 

Horizontal C grid 

Vertical – Charney-Phillips 

Hybrid height coordinate 

Non-hydrostatic formulation

Fully-compressible deep atmosphere 

equns 

Du r uvtanφ 1 ∂p 

⎛uw ⎞ 

− −2Ω sinφv+ 

= − ⎜ + 2Ωcosφw⎟ F 

Dt r ρrcosφ λ ⎝ r 

⎠ + 

∂ 

2 

D r v u tan φ 

1 ∂ p ⎛ vw ⎞ 

+ + 2Ω sin φ u + = − ⎜ ⎟ + F v 

Dt r ρ r ∂ φ ⎝ r ⎠ 

{ D w} 

1 ∂p∂Φ Dt ρ ∂ r { ∂ r 

r 

+ + = 

Dr 

ρ 

+ ρ∇⋅ 

r u= 

0 

Dt 

≈ 


g 

2 2 ( u + v ) 

r 

r D Fθ 

+ 2Ωcosφu + F 

θ 

= p = ρRT 

Dt 

w 

u 

( ) ( 2 

vcosφ 

∂ r w) 

D ∂ u ∂ v ∂ ∂ 1 ⎛∂u ∂ ⎞ 1 

= + + + w , ∇ ⋅ u= 

+ + 

D cosφ λ φ cosφ⎝ 

λ φ ⎠ 

r 

r 

2 

t ∂t r ∂ r∂ ∂r r 

⎜ 

∂ ∂ 

⎟ 

r ∂r


Unified Model Dynamics 

What’s ‘New’ 

Fully compressible, non-hydrostatic, deep atmosphere. 

– Universal application (Climate to microscale) 

Semi-Lagrangian, Semi Implicit solution. 

Du t+ dt t t+ dt t 

= Forcing → u = ud + αForcing + (1 −α) 

Forcing d 

Dt 

ud is value at departure point, found by high 

order interpolation. 

No stability limit on timestep. No additional 

diffusion required.


Unified Model Dynamics 

What’s ‘New’ 

Terrain following height based vertical coordinate 

– r(i,j,k) specified 

Arwakawa C Grid in horizontal (not B) 

– No averaging of pressure gradient 

– No grid decoupling 

– Better geostrophic adjustment for short waves 

Charney-Philips Grid in vertical 

– No computational modes 

– More consistent with thermal wind balance 

w,,q 

u v 

,⌫(p) 

– Can have complications in coupling with boundary layer parametrisation 

y 

x


Physical Parametrisations 

Edwards-Slingo Radiation 

– (Edwards & Slingo 1996) 

Mixed phase precipitation 

– (Wilson & Ballard 1999) 

– Extending to prognostic cloud fraction (Wilson, Bushell) 

– Extending to prognostic cloud water, rain water, ice, snow, graupel (Forbes) 

Met Office Surface Exchange Scheme (MOSES I and II) 

– (Cox, Essery, Betts) 

Non-local Boundary Layer 

– (Lock et al 2000) 

New GWD scheme + GLOBE orography, smoothed (Raymond 

filter) 

Mass flux convection scheme with CAPE closure, downdraft and 

momentum transport, separate shallow cumulus 

– (Gregory and Rowntree, Kershaw, Grant)


Microphysical parametrisation 

Complexity vs. Efficiency 

Operational Unified Model 

Wilson and Ballard (1999) 

“Cloud Resolving” Models 

Enhanced Microphysics

Treats heterogeneous surfaces 

using ‘blending height’ techniques 

and tiles. 

Surface exchange from each surface 

type treated separately 

Nine surface types, 

– Broad Leaf Trees 

– Needle Leaf Trees 

– C3 Grass 

– C4 Grass 

– Shrub 

– Urban 

– Water 

– Soil 

– Ice 

Each tile has fixed characteristics. 

4 layer soil temperature and 

moisture. 


MOSES II 

Blending Height 

Surface


Tiles surface exchange 

Each tile has a full surface 

energy balance. 

This includes a radiatively 

coupled ‘canopy’. In the 

urban case this has high 

thermal inertia to simulate 

wall effects. 

Work in progress 

(Harman, Belcher, Best) 

to improve urban 

representaion. 

R N 

H λE ε s σ T s 4 

ε g σ T g 4 

G 

ε s σ T s 4


Boundary Layer Scheme 

First order, moist 

Allows for non-local mixing in unstable regimes (top 

down/bottom up) 

Scheme diagnoses 6 different mixing regimes in 

order to represent stable, well mixed and cumulus 

and stratocumulus processes 

Scheme includes boundary layer top entrainment 

parametrisation. Especially well suited for stratocumulus. 

Improved interaction with the convection scheme 

New non-local stable scheme


NWP Model Domains 

Research Configuration 

Horizontal Vertical (lowest km)


NWP Model Orography 

12 km 4 km 1 km


Single Column Unified Model 

1D column version with full physics 

Choice of forcing 

– Observational 

– Statistical 

– Fixed


Site Specific Forecast Model 

1D (semi-)automated model based on 

UM “physics” - full column 

Greatly increased resolution in BL & soil 

“Dynamics”=“Forcing data”: grad p, 

advection, etc. 

Simple forcing correction for orography 

MOSES with tile surface exchange for 

separate treatment of land use types 

Radiative canopy coupled to surface 

exchange 

Upwind satellite derived land-use 

determines drag & surface fluxes of 

heat, moisture 

Surface landuse weighting via a stability 

dependent Source Area Model

16 Rachel Capon 04/2004 © Crown copyright 

NIMROD Nowcasting System 

Rod Brown, Stephen Moseley 

Input 

– radar + satellite data 

– SYNOPS 

– Mesoscale model forecasts 

– Sferics 

Output includes 

Visibility, T, Td, total water, liquid water 

temp, fog probability (200m, 1km, 5km), 

relative humidity

Model T 

and T d 

Model Aerosol 

concentration 


Visibility Analysis 

Visibility 

Analysis 

Satellite data 

Synops


Visibility Forecast 

Initial analysis from satellite data and SYNOPs 

Trends in liquid water temperature and total mixing 

ratio from the Mesoscale model are applied to the 

analysis to produce an extrapolation forecast 

Forecast values are merged with the model and 

persistence values 

Visibility is diagnosed using the model aerosol 

concentration


Visibility Analysis

T+3 Forecast 


Analysis

10 Appendix: Figures 

21 Rachel Capon COST 722 Paris 25/06/2004 © Crown copyright

Temperature and Dew Point Forecasts 


Probabilistic Visibility Forecast 

qt median 


qt 

qt threshold 

The probability of the 

visibility being less than 

200 m, 1 km, 5 km is 

also forecast 

A triangular distribution 

of qt is assumed about 

the forecast (median) 

value

T+1 F/C Probability of Visibility < 5 km

Met Office Unified Model NIMROD Nowcasting - LCRS

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