1. 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

2. Urban Modelling 2 03/2003 © Crown copyright 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 Air Quality Weather Forecasting

3. Urban Modelling 3 03/2003 © Crown copyright 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.

4. Urban Modelling 4 03/2003 © Crown copyright Example results from High Resolution Trial Model Visibility (m) 12 km 4 km1 km

5. Urban Modelling 5 03/2003 © Crown copyright 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.

6. Urban Modelling 6 03/2003 © Crown copyright s T s 4 g T g 4 H E s T s 4 G RNRN 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.

7. Urban Modelling 7 03/2003 © Crown copyright 12 km 4 km 1 km Model Configuration Met 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

8. Urban Modelling 8 03/2003 © Crown copyright Example 1 km domain Orography Urban Fraction in Land Use Grass Fraction in Land Use

9. Urban Modelling 9 03/2003 © Crown copyright Formation of the night time urban heat island Light Wind 00Z 11/05/2001 Urban-No-Urban Near surface temperature difference

10. Urban Modelling 10 03/2003 © Crown copyright 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 2 3 4 0.5 1.0 1.5 00Z 11/05/2001

11. Urban Modelling 11 03/2003 © Crown copyright 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

12. Urban Modelling 12 03/2003 © Crown copyright Impact on vertical mixing. Urban Area produces 200 m near neutral boundary layer Central London

13. Urban Modelling 13 03/2003 © Crown copyright 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!

14. Urban Modelling 14 03/2003 © Crown copyright Regional tropospheric data assimilation of tracers/chemistry? Ambitious, but shouldnt 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

15. Urban Modelling 15 03/2003 © Crown copyright 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.