urban modelling 1 03/2003 © crown copyright urban scale nwp with the met office's unified...

Urban Modelling 1 03/2003 © Crown copyright

Urban Scale NWP with the Met Office's Unified Model

Peter Clark

Mesoscale Modelling Group

Met Office

Joint Centre for Mesoscale Meteorology

University of Reading


Why Mesoscale Models? Mesoscale variation of boundary layer structure due to:

– Changes in surface characteristics.

» Urban/rural

» Land/sea

» Soil moisture/surface temperature

– Orography.

Mesoscale flows induced by above.– Land/sea breeze.

– Drainage flows.

Mesoscale structure in synoptic meteorology– Fronts

– Convective storms

AirQuality

WeatherForecasting


Why the Unified Model (UM)? Non-hydrostatic, fully compressible, deep atmosphere.

Suitable for use at very high spatial resolution. The UM surface exchange scheme treats urban areas and is being

improved.

Data assimilation is a powerful tool; makes UM data a major resource.

UM availability and improvements.– Available to and adopted by NERC community.

– Portable UM 5 released VERY soon.

– SLICE: A Semi-Lagrangian Inherently Conserving and Efficient scheme for transport problems

Operational Plans: – UK ~4 km 2005.

– 1 km ~2008-10 for v short range.


Example results from High Resolution Trial Model

Visibility (m)

12 km 4 km 1 km


Blending Height

Surface

UM Tile surface exchange Treats heterogeneous

surfaces using ‘blending height’ techniques.

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.


s Ts4g Tg

4

H E s Ts4

G

RN

Urban Tiles 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 (Reading) to improve representaion, especially of radiative effects.


12 km

4 km

1 km

Model ConfigurationMet Office non-hydrostatic semi-implicit, semi-Lagrangian Unified Model, 38 levels on stretched height based terrain following grid.

One-way nested

–Global (~60km) 20 min timestep

–~12 km (146X182) 5 min timestep

–~4 km (300x300) 2 min timestep

–~1 km (300x300) 0.5 min timestep 12 km down to 1 km run from operational 3D VAR mesoscale analysis at 12Z 10th May 2001

Tiled land surface scheme using CEH 25 m Landsat based land-use including ‘urban’ fraction.

Model ignores man-made heat sources


Example 1 km domain

Orography Urban Fractionin Land UseGrass Fractionin Land Use


Formation of the night timeurban heat island

Light Wind

00Z 11/05/2001

Urban-No-Urban Near surface temperature difference


Urban fraction

Point 1: 1 %

Point 2: 97 %

Point 3: 50 %

Point 4: 1 %

Urban Fraction and contours of Urban Impact on 20 m Temperature

1

23

4

0.5

1.0

1.5

00Z 11/05/2001


Point 2

Point 3

Point 1

Point 4

Point 1: Upstream

Point 2: Central London

Point 3: Downstream Suburbs

Point 4: Downstream Rural

Evolution of vertical structure over London


Impact on vertical mixing.

Urban Area produces 200 mnear neutral boundary layer

Central London


Tracer release broadly reflecting smoothed emissions

Afternoon deep convection

brings down clean air

Night time stabilization

Blocking of flow by North Downs

Arbitrary Units

Predictable?

Unpredictable!


Regional tropospheric data assimilation of

tracers/chemistry? Ambitious, but shouldn’t we be?

3D VAR stratospheric chemistry, tropospheric 'aerosol' already a reality.

DA discipline forces objective analysis of model error, observation error and representativity/covariance.

4D VAR of tracers has potential to improve model transport as well as provide concentration fields.

Tracer assimilation straightforward using model dynamics. Conceivable using alternative dispersion if incorporated into UM.

DARC


Towards a Modelling Strategy Based on the Unified Model

Provision of higher resolution UM output to drive offline transport/dispersion (NAME + Others).

Use of UM in NERC community to validate/improve meteorology.

– Benefits of using operational analyses including sub-surface. Closer coupling of transport/chemistry with UM.

– Using UM transport OR alternative (NAME, parcel, Eulerian)– Consistent physics– Shorter updating interval for winds

Start thinking in a data assimilation framework– Model error covariances– Observation representativity defined with respect to model.

Eventual implementation of multiscale DA of quasi-conserved species.

urban modelling 1 03/2003 © crown copyright urban scale nwp with the met office's unified...

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