An urban canopy model for Australian regional climate and air quality modelling

1. An urban canopy model for Australian regional climate and air quality modelling www.cawcr.gov.au Marcus Thatcher

2. Overview • Motivation and applications • Basic features of Urban Canopy Models (UCM) • Design of our UCM module (ateb.f90) • Basic verification • Future work The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

3. Motivation • There is currently growing interest in the dynamical modelling of urban climates (TEB, UHSM, LUMPS, CM-BEM, VUCM, FVM, etc) • Urban climate issues of interest include: • Response of urban climates under global warming scenarios • Modelling of aerosols and atmospheric chemistry in urban environments • Implications for public health • Implications for energy security • To support the anticipated demand, we have upgraded our urban modelling capabilities for regional climate modelling and air quality simulations The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

4. Energy security - GENERSYS 2005 Load Duration Curve (SA) • GENERSYS National Electricity Market simulator (Grozev, et al). Electricity demand from (Thatcher, 2007) based on Mk3.0 downscaled by CCAM climate datasets +1oC climate change scenario (SA) (no adaptation) The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

5. Air quality and health • Study by Martin Cope, et al, using the Chemical Transport Model (CTM), after dynamically downscaling from CSIRO Mk3.0 using CCAM and TAPM. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

6. Estimated effect of climate change on Melbourne ozone(Sean Walsh, 2009 – EPA Victoria) The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

7. Properties of Urban Canopy Models • There are a few different approaches to modelling the ‘building-averaged’ urban canopy within mesoscale atmospheric models • Slab models (e.g., Oke 1988) • 2D canyon models (e.g., Masson, 2000) • 3D building array models (e.g., Kanda et al, 2005) • Multi-level canopy models (e.g., Martilli et al, 2002) • These models also attempt to solve for the urban energy budget and adapt MOST to the urban environment The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

8. Properties of Urban Canopy Models • There are many differences between urban and rural environments, e.g., • Urban areas have increased roughness lengths (e.g., 1m) • The ratio of z0m/z0h is much larger in urban areas (e.g., ~1000 or greater) • Moisture availability is reduced due to impervious surfaces (high Bowen ratio) • Storage heat flux is significantly higher in urban areas (Q = Net – H – LE) • Anthropogenic contribution to heat flux in urban areas • Reduced emissivity and albedo • Increased thermal inertia compared to rural areas • Canyon shadowing effects • Buff bodies instead of a porous canopy The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

9. Properties of Urban Canopy Models TAPM slab • Coutts et al 2007 used the urban slab model in TAPM V3 to model the partitioning of fluxes at different sites in Melbourne • After tunning the model, Coutts et al were able to obtain a reasonable partitioning of energy for January 2003 Obs The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

10. Properties of Urban Canopy Models TAPM slab • However, the partitioning of the fluxes in July 2003 is incorrect (note the heat storage term) Obs The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

11. Properties of TEB • As part of the upgrading of our urban modelling capabilities for regional climate simulations, we have coded a urban module based on an idealised 2D canyon following Town Energy Budget (TEB) approach • Note that the new urban model is designed around research requirements, not operational requirements. Specifically, many parts of the model are designed for future extensions. • Before considering the new urban model, we first briefly review some of the properties of TEB models The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

12. Properties of TEB • In TEB, the urban energy budget is separated into three energy budgets for roofs, roads and walls. The canyon is rotated through all possible orientations and so a single wall energy budget is used. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

13. Properties of TEB • The TEB scheme recommends using three layers when representing roofs, walls and roads. Soil moisture and in-canyon vegetation is usually neglected (see later) • Internal building heat source (modifies behaviour of ‘heat storage’) • A single layer snow scheme and a simple model of surface water is included for roof and road surfaces (max 1 kg/m2) The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

14. Properties of TEB • The inclusion of a surface water reservoir is to account for short term changes to the fluxes (i.e., less than daily time scales) Masson 2000 The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

15. Properties of TEB • TEB assumes isotropic reflections in shortwave and long wave radiation. Usually an infinite number of reflections is assumed for shortwave and 1st order reflections are modelled for longwave The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

16. Properties of TEB • The canyon morphology (H/W) has a significant influence on how radiation is absorbed within the canyon (i.e., shadowing effects) • Higher canyon walls relative to the canyon width results in less radiation being reflected by the canyon system • The albedo also changes as a function of solar zenith angle due to differences in the albedo between walls and roads The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

17. Properties of TEB • TEB relies on MOST to describe the turbulent heat exchange with the atmosphere • The urban sensible heat flux includes contributions from roofs, roads, walls and (potentially) snow • Time dependent traffic fluxes are included in the canyon sensible heat flux budget • ‘Industrial’ sensible heat is added to the total urban sensible heat flux The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

18. Properties of TEB • Fairly simple relationships are used by TEB for parameterising in-canyon aerodynamical resistances The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

19. ateb.f90 • To improve the urban modelling capability for regional climate modelling and air pollution, we have written an urban module called ateb.f90 • Although the new module has been coded from scratch, much of the science is based on the TEB / 2D canyon approach • However, we have taken the opportunity to structure ateb.f90 to allow for extensions to various components including • Use of Harman et al (2004) scheme for in-canyon aerodynamical resistances • Prognostic equations for a second canyon wall • Vegetation and AC contributions to in-canyon fluxes • Use of Luhar et al (2008) parameterisation of turbulence for low winds • Implicit numerical techniques are used to ensure stability with large time steps (e.g., dt=20mins in CCAM 60 km resolution simulations) The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

20. ateb.f90 • ateb.f90 uses separate prognostic equations for two canyon walls (i.e., the canyon is rotated through 180o instead of the usual single wall rotated through 360o) road easterly wall westerly wall buildings buildings canyon The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

21. ateb.f90 • In-canyon aerodynamical resistances are based on Harman et al (2004) • This formulation accounts for recirculation and venting regimes in the canyon • Some of the parameters have been modified where necessary • Parameterisations for aerodynamical resistances used by Masson (2000) and Kusaka et al (2001) are also available The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

22. ateb.f90 • ateb.f90 includes an in-canyon big-leaf vegetation scheme based on Kowalczyk (1994), assuming f=1 for simplicity and therefore no multi-layer soil model under vegetation (see also Lee and Park 2008) • We have added a simple AC heat flux into canyon (see also Ohashi et al, 2007) • Time dependent traffic fluxes are based on Coutts et al (2007) • Canyon air and vegetation canopy temperatures are solved simultaneously with the Newton-Raphson method AC traffic Schematic representation of the aerodynamic resistances. Note snow and water have been omitted in this diagram The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

23. ateb.f90 • The stability functions have been modified for low winds following Luhar et al (2008) • However, there are complications with the aerodynamical resistance between the canyon and the atmosphere AC traffic Schematic representation of the aerodynamic resistances. Note snow and water have been omitted in this diagram The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

24. ateb.f90 • Based on the results of Kanda et al (2008), we have experimented with changes to the roughness length for heat when calculating the sensible heat flux between the atmosphere and the canyon air temperature • Basically zh depends on the definition of the ‘urban’ temperature Kanda et al 2008 The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

25. ateb.f90 • Note that zoh for the urban tile is still much smaller than zom • z0h=10-3z0m is fed back to the host atmospheric model for diagnosing screen level variables (ateb.f90 also supports the Brutsaert (1982) parameterisation) zh=z0m Solve for z0h (z0h <<z0m) approximated by TS=(L/)1/4 Kanda et al 2008 ateb.f90 The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

26. ateb.f90 • We have included a single layer snow scheme based on Douville et al (1995) • Note that changing snow albedo can effect the albedo of the canyon system • Therefore, we have simply expanded the number of reflections to Nth order for both longwave and shortwave (3rd order by default), rather than solve for an infinite number of reflections (slightly complicated by the 2nd canyon wall) • The time dependent urban albedo is fed back to the radiation scheme in the host atmospheric model The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

27. ateb.f90 • ateb.f90 is a single layer model for consistency with the vegetation canopy scheme (e.g., CABLE) • For regional climate modelling, this is acceptable since the results of Kusaka et al (2001) suggest that the fluxes calculated by a single level model are close to that obtained from multi-level models • Nevertheless, air pollution modelling would probably benefit from a multi-level model and this may be a future extension The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

28. Our study is based on Melbourne after downscaling from 1o resolution NCEP analyses to 3 km resolution using a multiple nesting technique TAPM is run for 1 year (2003) The results presented here are based on the Alphington site (ALP) Observations are from the Environmental Protection Authority (EPA) monitoring stations and from Coutts et al (2007) Basic evaluation of ateb.f90 The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

29. Simulated temperature climatology • ‘No urban’ shows a cold bias due to absence of urban heat island • ‘aTEB urban’ predicts low temperatures better than the SLAB approach • The correlation between modelled and observed temperatures is also improved with aTEB No urban SLAB urban aTEB urban The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

30. Simulated wind climatology • aTEB and SLAB predict similar wind speed climatologies, but both are better than for ‘No urban’ • Wind direction errors are essentially the same for all three approaches No urban SLAB urban aTEB urban The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

31. March energy partitioning • aTEB shows improved partitioning of energy (particularly ‘storage’). Note the slight lag in fluxes Coutts et al 2007 The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

32. May energy partitioning • aTEB also partially captures changes in energy partitioning between different months Coutts et al 2007 The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

33. Nesting in CCAM Nested in NCEP analyses • Urban temperature climatology is still retained when the correlation for time-paired temperatures is (intentionally) degraded Nested in CCAM (forced by NCEP analyses) The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

34. Nesting in CCAM Nested in NCEP analyses • The wind climatology is also preserved when TAPM is nested in CCAM Nested in CCAM (forced by NCEP analyses) The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

35. ateb.f90 and CABLE • ateb.f90 has also been used with a mosaic/tiled version of CABLE in CCAM • In the Oklahoma city JUL2003 study we dynamically downscale to 1 km resolution • Urban prognostic variables are initialised from an urban tile in the CCAM 60 km resolution simulation that has been spun-up Oklahoma city, JU2003 study – Screen temperature Urban heat island The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

36. Future work • Parameter tunning • The simulated climatology is independent of may parameters (e.g., industrial and traffic heat fluxes seem to have a minor effect) • Canyon geometry is estimated from Coutts et al (2008) study for suburban Melbourne • Thermal properties of building materials is taken from the ECOCLIM dataset • Issues with heat storage in roofs and methods for predicting screen level observations within the canyon • More realistic treatment of turbulent heat fluxes between canyon and atmosphere • Planned experiments include • Simulation of the 20thC Australian urban climate downscaled from various GCMs • Simulation of 2050 and 2070 Australian urban climate • Application to air quality and energy models The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

37. Thank you The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Acknowledgements to Peter Hurley Marcus Thatcher Research Scientist Phone: 03 9239 3530 Email: marcus.thatcher@csiro.au Web: www.cawcr.gov.au Thank you www.cawcr.gov.au

38. March energy partitioning The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

39. May energy partitioning The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology