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

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

Mesoscale model CSIRO’s TAPM is a coupled prognostic meteorological and air pollution model (Hurley et al., 2005, EMS, 20, 737–752; http://www.cmar.csiro.au/research/tapm) 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

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

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

(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: www.unibas.ch/geo/mcr/Projects/BUBBLE)

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 (http://pages.unibas.ch/geo/mcr/Projects/BUBBLE/ textpages/ov_frameset.en.htm)

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.

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.

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

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

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

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.

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

Acknowledgements The BUBBLE Program (www.unibas.ch/geo/mcr/Projects/BUBBLE)

Thank you Centre for Australian Weather and Climate Research CSIRO Marine and Atmospheric Research Dr Ashok Luhar Principal Research Scientist Phone: 03 9239 4400 Email: Ashok.Luhar@csiro.au Web: www.cmar.csiro.au, www.cawcr.gov.au Thank you Contact Us Phone: 1300 363 400 or +61 3 9545 2176 Email: enquiries@csiro.au Web: www.csiro.au CSIRO.