1. Performance of a new urban land-surface scheme in an operational mesoscale model for flow and dispersion
Ashok Luhar, Marcus Thatcher, Peter Hurley
Centre for Australian Weather and Climate Research
CSIRO Marine and Atmospheric Research, Aspendale, Australia
8 May 2013

2. Introduction
Urban surfaces need to be represented properly in mesoscale models as they influence mean meteorology, atmospheric stability and turbulence, and hence atmospheric dispersion
A surface scheme needs to be fast enough to be used in an operational mesoscale model
This puts a constraint on the level of detail that can be included in such as a scheme
We have coupled a new surface scheme to better represent urban surfaces in a mesoscale model
No substantial increase in computational burden

3. Mesoscale model
CSIRO’s TAPM is a coupled prognostic meteorological and air pollution model (Hurley et al., 2005, EMS, 20, 737–752;
Turbulence closure uses prognostic equations for E and 
Lagrangian (PARTPUFF) and Eulerian options for dispersion – former used
Current urban scheme
A simple slab approach (e.g., Oke, 1988)
Urban, vegetation and soil tiles, with weighting according to the fractions of the area covered by them
Includes an anthropogenic heat flux component
Widely used in Australia and NZ for air quality management and as a research tool

4. A new urban scheme in TAPM
Fully described by Thatcher and Hurley (2012, BLM,142,149–175)
A building-averaged canyon model based on the town energy balance (TEB) approach (Masson, 2000). It resolves energy balances for walls, roads and roofs
At a given diurnal time, the two wall energy budgets are derived by averaging the canyon fluxes over all possible canyon orientations (i.e. 360).
Aerodynamic resistance network considers recirculation and venting of air within the canyon (Harman et al., 2004)
Modified to include in-canyon vegetation, and air conditioning for energy conservation
Building-averaged fluxes are passed to the first model level
Big leaf vegetation
CSIRO

5. Example of in-canyon sensible heat fluxes during the diurnal cycle
CSIRO

6. (Photo: www.unibas.ch/geo/mcr/Projects/BUBBLE)
Evaluation data
We use 1-month’s sonic data from the 32-m urban tower (Ue1) of the 2002 Basel UrBan Boundary Layer Experiment (BUBBLE) conducted in Basel, Switzerland, 10 June – 10 July (Rotach et al., 2005)
Six levels: 3.6, 11.3, 14.7, 17.9, 22.4 & 31.7 m
Local building height 14 m
Fairly homogeneous building blocks
Dispersion experiments were also conducted
One year’s data, but we use IOP’s one month data, urban area shown
(Photo:

7. Dispersion data (Rotach et al. , 2004; Gryning et al
Dispersion data (Rotach et al., 2004; Gryning et al., 2005) : SF6 tracer was released during the day at near roof-level at two locations (R1, R2)
19 samplers:13 typically positioned above the roof level, and 6 street level
Date
Source
Release period
(CET)
Release Rate
(g/s)
26 June R1
12:00-16:00
0.0503
4 July R2
14:40-18:00
0.0499
7 July
13:10-17:00
0.3008
8 July
14:00-18:00
0.1319
( textpages/ov_frameset.en.htm)

8. Model results
500 m resolution for met., 50 m resolution for dispersion
Level 17.9-m data used – first level clearly above the building height, maximum fluxes (they decrease above and below this height)
Sensible heat flux (Hs)
Many occurrences of positive /near-zero fluxes at night which the new scheme is able to reproduce. A weak stability at night is a feature of urban meteorology.

9. Current scheme yields too many occurrences of negative Hs between 0 and – 50 W m-2 when the observations suggest values between 0 and 80 W m-2.
We have tested the model for latent and momentum fluxes too.

10. Mode results - temperature
Some outliers, but some of the minimum temperatures are better predicted by the new scheme
Some of the minimum temperatures are better predicted by the new scheme

11. Mode results - dispersion
Dispersion evaluation
Scheme d
NMSE FB
Fa2
Current
0.73
4.65
-0.52
22.9%
New
0.79
2.48
-0.07
33.3%
Current – assim.
0.78
2.40
-0.72
37.9%
New – assim.
0.82
1.51
43.8%
Wind data assimilation leads to significant improvements
No nighttime data

12. Quantile-quantile plots
Higher-end concentrations are better predicted with the new scheme

13. Higher differences after 18:00 h – no data available
Average of modelled concentrations at all receptors, as a function of time
Higher differences after 18:00 h – no data available
Exp. period
CSIRO.

14. Conclusions
The new urban scheme describes the surface exchanges much more realistically
Considerable improvement in heat flux predictions
Near-zero value of observed sensible heat flux is reproduced – this impacts on dispersion
The dispersion fields are better predicted using the new urban scheme – greater differences at nighttime
Building-averaged model – vertical structure within canyon is not resolved
Coupling surface fluxes to the atmosphere via Monin-Obukhov Similarity Theory requires more work - the roughness sublayer needs to be included explicitly

15. Acknowledgements
The BUBBLE Program (

16. Thank you Centre for Australian Weather and Climate Research
CSIRO Marine and Atmospheric Research
Dr Ashok Luhar
Principal Research Scientist
Phone:
Web:
Thank you
Contact Us
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CSIRO.