Stateof the art of urbanmesoscale modelling and possible use of the Helsinki testbed data

Transcription

1 Stateof the art of urbanmesoscale modellng and possble use of the Helsnk testbed data Alberto Martll CIEMAT, Madrd, Span

2 Snce the orgn of mesoscale meteorologcal modelng, urban areas have been nvestgated. Bornsten 1975 However, cty was represented n a crude way (strong roughness and heat flux)

3 Snce then, urban mesoscale modelng evolved due to Increase of understandng of the behavor of the urban atmosphere Increase of CPU power avalable New requrements to mesoscale models (ar polluton, clmatology, etc.) New urban parametrzatons for mesoscale models

4 The most mportant urban effects are Drag Wake dffuson Radaton, anthropogenc heat, buldng materals Momentum Turbulence Heat Dynamcal effects Thermal effects

5 Thermal effects Sem-emprcal approach Objectve Hysteress Model (OHM Grmmond et al., 1991). Reasonable expectaton that Q S (storage) s a fracton of R (net all-wave radaton). A daly plot of Q S vs R results n a hysteress loop Q s H. Taha (1999) mplemented OHM n a mesoscale model Q s = = 1, n a 1 R + a 2 R t + a 3 R Changng the coeffcents, t can work for any surface. Problem: a long seres of data s needed to fnd the parameters a 1,a 2,a 3.

6 Physcally based approaches Wegthed average of fluxes from dfferent urban surfaces (road, wall, roof) (Masson 2000, Kusaka et al. 2001, Martll et al.2002). H H r, w, s = C heat ( T T ) r, w, s r, w, s ar r, w, s r=roof, = flux s=street w=wall C r,w,s s a coeffcent functon of wnd speed and surface roughness. Surface temperatures are estmated through an energy budget.

7 Radaton s composed by short (solar), and long (nfrared) waves. For walls and street radaton trappng must be consdered. Solar Short wave radaton reachng the surface R W1 R W2 Basc dea R R R W S 1 W 2 = R = R = R WD SD 1 WD 2 + α R S + α W S + α S Ψ Ψ Ψ WS SW SW R R W 1 S R S + α + α W + α W W Ψ Ψ Ψ WW WW WS R R W W R 2 2 W 1 Isotropc reflecton 3 equatons 3 unknonwn R, R, R WD 1 WD 2 α W,α S = ΨSW, ΨWS, Ψ SD= Incdent radaton at walls and street functon of the solar zenth angle and street orentaton. Albedo of wall and street WW = Vew factors street-to-wall,wall-to-street,wall-to-wall. Vew factor from surface A to surface B, s defned as the fracton of radatve energy leavngsurface A that reaches surface B

8 Valdatons Masson 2000 over Marselle (from Lemonsu et al. 2004)

9 Moreover there are addtonal anthropogenc sources of heat. From Ichnose et al for Tokyo In lmted areas, they can reach peaks of hundreds of W/m 2. Of the same order of the the solar radaton. Injected as a source term n the atmosphere

10 A step forward to evaluate energy fluxes n urban areas. Account for Buldng Energy. An example, nspred to Kkegawa et al. (2003). Ar condtonng (heatng) Ar condtonng (coolng) Heat conducton through walls Indoor heat sources (occupants, equpments) ventlaton Solar radaton through wndows Important n estmates of energy savngs for UHI mtgaton strateges.

11 Dynamcal effects Tradtonal method. Logarthmc profle U u z * ( z) = ln k z o Roughness length (z0)~of 1-3 m,. Based on smlarty theorythat assumes that turbulent fluxes are constant wth heght n the surface layer.

12 However... Turbulent fluxesare not constant wth heght (Rotach 1993) n the Urban Roughness Sublayer (1-3 tmes mean buldng hegth). The smlarty theory cannot be appled. Zurch, Swtzerland Rotach, 1993

13 Approaches to parameterze momentum drag are mutuated from vegetaton canopy modellng. Small dfferences between the approaches. =1,2, e. g. the drag force s horzontal Uno et al., 1989 ~ p = ρcdηa( z) x u ( ) 2 2 u + u 1/ a(z)=buldng surface area densty, η fracton of buldng area, C d =0.1 Severs, 1990 ~ p = ρc x d w f u r u w f wall area densty, C d =0.2 Brown and Wllams, 1998 ~ p = ρfroof Cd a( z) u u x Martll et al. 2002, ~ p x ~ p x = ρc = ρc d d λ S V f w ar ( 1 β ) u u ort Coceal and Belcher 2004 r u u ort f roof =horzontal fracton of model grd covered by buldngs, a(z)buldng surface area densty S w wall surface n the cell, V ar =ar volume of the cell, u ort = wnd component ortogonal street drecton, C d =0.4 λf total frontal area per unt ground area, (1-β)=fractonal volume of the cell occuped by ar, C d =1.

14 Usually a TKE budget s solved to estmate the turbulent exchange coeffcents. To do ths, an extra term must be added n the TKE eqn. ρ ε E t + = D ρ U x E Uno et al., 1989 E ρ K Brown and Wllams, 1998 ρ ew z 3 ρc d ηa( z) u1 + u z U z x 2 + U z y u w' K z 2 ε = C g θ o = K = C l ε k z k E l ε U z E 3/ 2 1/2 ρ K z θ z D = ρf D E E roof = ρc C Martll et al. 2002, drag d U a( z) 3 ort IU V IU u 3 V IU S V IUbuld Length scales are modfed to account for eddes generated by buldngs

15 Reynolds Stress From Martll et al TKE

16 How to mprove? Use street canyon CFD models to derve propertes of the mean flow and parameterzatons for mesoscale models. Buldngs are explctly resolved. Smulaton at hgh resoluton, but for very small doman. CFD models valdated aganst wnd tunnel data.

17 CFD smulaton wth model FLUENT of flow over an array of obstacles (made by Jose Lus Santago). Reproducton of wnd tunnel experment of M. Brown at U.S. EPA. Spatal average of the results over thn slces of buldng-canyon unts, and over the whole array. These s the closest to the average needed for mesoscale models.

18 u w Reynolds stress z/h u ~ w~ Dspersve stress z/h countergradent Dspersve stress n the canopy s comparable, n magntude, and opposte n sgn to the Reynolds stress. More mportant at cty boundares, less nsde.

19 Athens Nght Urban Urban-rural

20 Athens Day Urban Urban-rural

21 Athens Ozone Urban Urban-rural

22 Possble use of the Helsnk testbed data Urban schemes have never been tested for nordc clmate. Interactons between sea breeze and urban areas. Impact of cty on urban boundary layer (n partcular for stable stratfcaton) Urban vegetaton (parks) Influence of cty on weather (fog, precptaton, etc.)