Implementation and Evaluation of a New Drag and Length Scale

Parameterization in the Multilayer Urban Canopy Parameterization of

WRFA.Simón-Moral1*, A. Martilli1, J. L. Santiago1, S.

Krayenhoff2

andres.simon@ciemat.esEnvironment dept, Research Centre for Energy, Environment and

Technology (CIEMAT), Madrid, Spain1

Department of Geography, University of British Columbia, Vancouver, Canada2

Page 214th Annual WRF User's Workshop

24 – 28 JUNE 2013

Description of the UCP

• 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

VortortdD SUUCWindF )(

VortdD SUCTKEF

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Drag Wake diffusion Radiation

Page 314th Annual WRF User's Workshop

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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

Page 414th Annual WRF User's Workshop

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Methodology

Page 514th 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 (λ

p).

• 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

Page 614th Annual WRF User's Workshop

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Drag Parameterization

Sheltering Channelling Interaction

λch= 1

λch= 1,83λch= 1.31

λch= 2λch= 3

λch= 1,83λch= 1.31

λch= 2λch= 3

λch= 1

Parameterization

CFD Values

λs

Page 714th Annual WRF User's Workshop

24 – 28 JUNE 2013

• 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 λp as:

• Lk is calculated by:

where Ci are model constants

15.0/ pHd

Length Scales Parameterization

Page 814th Annual WRF User's Workshop

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• 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

Column Model Test

Page 914th Annual WRF User's Workshop

24 – 28 JUNE 2013

• 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

Implementation in WRF

Page 1014th Annual WRF User's Workshop

24 – 28 JUNE 2013

Results

Page 11

• Night– Larger Cd in New version

• Larger drag effect, lower wind– Larger length scales

• Enhancement of the vertical mixing

14th Annual WRF User's Workshop24 – 28 JUNE 2013

Old VS New Comparison

Temperature Wind

Page 12

• 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

14th Annual WRF User's Workshop24 – 28 JUNE 2013

Old VS New Comparison

Temperature Wind

Page 1314th Annual WRF User's Workshop

24 – 28 JUNE 2013

• Turbulent Kinetic Energy– Combination of C

d, U, dU/dz & dT/dz

– Cd

TKE U TKE dU/dz TKE– K

m dT/dz TKE

Old VS New Comparison

NIGHT DAY

Page 1414th Annual WRF User's Workshop

24 – 28 JUNE 2013

• New parameterization is able to distinguish between configurations

New Parameterization Skills

λp=0.125

1) λs=1.83, λch=1.832) λs=3, λch=13) λs=1, λch=3

λs=1

1) λp=0.25, λch=12) λp=0.125, λch=33) λp=0.167, λch=2

λch=1

1) λp=0.25, λs=12) λp=0.125, λs=33) λp=0.33, λs=0,5

Page 1514th Annual WRF User's Workshop

24 – 28 JUNE 2013

• 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

Conclusions

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

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