Description of the UCP

1. Implementation and Evaluation of a New Drag and Length Scale Parameterization in the Multilayer Urban Canopy Parameterization of WRF A.Simón-Moral1*, A. Martilli1, J. L. Santiago1, S. Krayenhoff2andres.simon@ciemat.esEnvironment dept, Research Centre for Energy, Environment and Technology (CIEMAT), Madrid, Spain1Department of Geography, University of British Columbia, Vancouver, Canada2

2. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Description of the UCP Wake diffusion Radiation Drag Heat Momentum Turbulence • The Building Effect Parameterization (BEP, Martilli et al. 2002) solves the effect on the flow caused by the buildings inside a city • Building drag: • Sinks for momentum • Source of TKE • At the moment Cd=0.4 (Raupach, 1992) • Turbulent transport and dissipation • Turbulent diffusion coefficients • Turbulence dissipation • Temperature and Radiation • Radiation trapping • Energy balance in each surface

3. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Motivation • From Computational Fluid Dynamics (CFD) simulations over aligned configurations of cubes new features are observed • Drag coefficient depends on the configuration of the buildings • Length scales depends on the density of buildings • A new parameterization derived from CFD results for the Cd and the length scales considering different configurations of buildings is presented

4. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Methodology • A CFD-RANS model (StarCCM+), previously validated against DNS or wind tunnel measurements is used for data collection (J. L. Santiago et al. 2008)‏ • This is a CFD-RANS model, with a k-e standard turbulent scheme • CFD-RANS results are spatially averaged over horizontal slabs to extract properties for the UCP • The Drag coefficient (Cd) is parameterized as a constant with height value in the canopy • Length scales for the turbulent transport and dissipation are parameterized • The UCP is tested in a 1D column model

5. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Drag Parameterization • Different values for the Cd have been found from different aligned configurations of cubes with different packing densities (λp). • Cd is parameterized as constant value with height, by means of the distance between buildings in the directions parallel and perpendicular to the wind, represented by: • Sheltering factor λs=Wx/H • Channelling factor λc=Wy/By

6. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Drag Parameterization CFD Values λch= 1 λch= 1.31 λch= 1,83 λch= 2 λch= 3 Parameterization λch= 1 λch= 1.31 λch= 1,83 λch= 2 λch= 3 λs Sheltering Channelling Interaction

7. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Length Scales Parameterization • The length scale is found to depend on the packing density of the configurations • According to its behaviour, three zones are observed: • Le/Ce=α1·(h-d) for z/h<1 • Le/Ce=α1·(z-d) for 1<z/h<1.5 • Le/Ce=α2·(z-d2) for z/h>1.5 Where α1= 2.19 and α2=1.2 and d2 is calculated in order to guarantee continuity • Displacement height is calculated by means of the packing density λpas: • Lk is calculated by: where Ci are model constants

8. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Column Model Test • Distinguish between different configurations with same λp. • Relative differences between configurations well reproduced • Better results are found for U field inside canopy. • TKE mean value is well reproduced, although • There is an overestimation at canopy heigth • Different shape • underestimation of Km and <u’w’> inside canopy • U, is mainly characterized by Cdrag

9. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Implementation in WRF • The parameterizations are introduced in BEP, already implemented in WRF • 3d ideal city is simulated in order to test the improvement • Simulation set-up: • Square city rounded by rural area‏ • Domain: 100x100x51 • City: 10x10, centred in the domain • dx = dy = 1000m • dt = 2s • Boundary conditions: Open • Coriolis force used

10. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Results • Temperature, wind and TKE are analysed and compared against the previous version of the parameterization • Case S1CH1, λch=1, λs=1, λp=0.25 • Cdeq(NEW)=0.71, Cd(OLD)=0.4 • Resultsanalysed in the center of thecity

11. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Old VS New Comparison • Night • Larger Cd in New version • Larger drag effect, lower wind • Larger length scales • Enhancement of the vertical mixing Wind Temperature

12. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Old VS New Comparison • Day • Larger Cd in New version • Larger drag effect, lower wind • Larger length scales up to 1.5 H • Upward propagation of the perturbation and enhancement of vertical mixing • Lower length scales over 1.5 H • Downdward propagation from high levels Temperature Wind

13. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Old VS New Comparison • Turbulent Kinetic Energy • Combination of Cd, U, dU/dz & dT/dz • Cd TKE U TKE dU/dz TKE • KmdT/dz TKE NIGHT DAY

14. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 New Parameterization Skills • New parameterization is able to distinguish between configurations λs=1 λch=1 λp=0.125 1) λp=0.25, λch=1 1) λp=0.25, λs=1 1) λs=1.83, λch=1.83 2) λp=0.125, λch=3 2) λp=0.125, λs=3 2) λs=3, λch=1 3) λp=0.167, λch=2 3) λp=0.33, λs=0,5 3) λs=1, λch=3

15. 14th Annual WRF User's Workshop 24 – 28 JUNE 2013 Conclusions • A new parameterization for drag coefficient and length scales is presented and implemented in BEP • The new drag coefficient depends on the buildings configuration • New Length scales for turbulence are parameterized as a function of the packing density λp • The new parameterization is able to distinguish between different configurations • Future work: • Find easily measurable urban parameters to implement in BEP • Generalize the parameterization for different shape buildings

16. Thank you very much for your attention! Thank you very much for your attention!

17. Turbulent kinetic energy Drag (veg. canopies) urban rural Modification of length scales