J6.7 AN EXAMINATION OF RELATIONSHIPS BETWEEN URBAN AND RURAL MICROMETEOROLOGY USING SEMI-EMPIRICAL AND COMPREHENSIVE MODELS. Riverside, CA, 92521

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1 J6.7 AN EXAMINATION OF ELATIONSHIPS BETWEEN BAN AND AL MICOMETEOOLOGY SING SEMI-EMPIICAL AND COMPEHENSIVE MODELS Sang-Mi Lee, Ashok Lhar, and Akla Venkatra Departent of Mechanical Engineering, niversity of California iverside iverside, CA, 95 CSIO Marine and Atospheric esearch, Aspendale, Vic 395, Astralia. INTODCTION ecent stdies (Venkatra et al, 5) indicate that siple dispersion odels can be sed to estiate grond-level concentrations in an rban area if eteorological inforation at the site is sed to constrct odel inpts. Becase sch inforation is sally not available, there is a need for ethods that can estiate rban variables fro ore rotinely available rral easreents. This paper exaines two ethods, one based on a two-diensional internal bondary layer odel, and the other on a 3-D prognostic eteorological odel. The perforance of these two ethods is evalated with data fro a bondary layer field stdy condcted in Basel, Switzerland, in. Data fro Joint rban 3 (J3; Allwine et al, 4) were sed for the evalation, as well. We first provide a brief description of the stdies and present reslts fro a preliinary analysis of the data sing siilarity ethods.. EXPEIMENTAL DATA We sed data collected dring a field capaign, BBBLE (Basel rban Bondary Layer Experient; Christen and Vogt, 4a; otach et al, 4) and J3. BBBLE, condcted in Basel, Switzerland dring the period of Jne th to Jly th,, provides extensive easreents of ean and trblence velocities and flxes and radiation variables in a rral as well as in an rban bilt-p area. The ain rban easreents tower, Basel- Sperrstrasse (or ), was 3 high and located inside a street canyon in an area with dense, fairly hoogeneos, residential bilding blocks. The local roof-top level was 4 above the grond level (AGL), the ean bilding height of the area was 4.6 AGL, and the plan aspect ratio of the bildings was.54. The tower easred eteorological paraeters sing sonic aneoeters at six levels, viz. 3.6,.3, 4.7, 7.9,.4 and 3.7 AGL, to characterize the vertical strctre of the ean flow and trblence Corresponding athor address: Dr. Akla Venkatra Departent of Mechanical Engineering, niversity of California iverside, CA, 95 e-ail: venky@engr.cr.ed within the rban roghness sb-layer (i.e., the layer directly inflenced by individal roghness eleents). The rral site, Village Nef (or ), located abot 6.5 k NNW of the site, easred flow and trblence at 3.3 AGL over bare soil in an agricltral area. J3 was condcted dring the period Jne 8 th to Jly 3 st, 3 in Oklahoa City, Oklahoa. The J3 easreents were ade sing a nber of instrents at ltiple sites. Aong the, we focsed on flow and trblence easreents taken in typical indstrial or warehose rban areas, and in its pstrea sbrban and rral areas (Hynh et al, 5). Measreents taken at towers # and #5 operated by.s. Ary esearch Laboratory were considered as indstrial and warehose rban vales and tower #3 was located in a rral area (Hynh et al, 5). The ean bilding height for the rban area of Oklahoa City is 5-5 (Landqist et al, 4), while the Central Bsiness District (CBD) has an average bilding height of 5 and an aspect ratio (the height-to-width ratio) of abot (Brown et al, 4). 3. SIMILAITY ELATIONSHIPS According to Monin-Obkhov (M-O) siilarity, the ean wind profile (z) in the diabatic srface layer is given by (e.g. van lden and Holtslag, 985) z d (z) = ( ζ ) + ( ζ ), () κ z where z is the height above the srface, κ is the von- Karan constant (=.4), z is the srface roghness length, d is the displaceent height, is the friction velocity, ζ = ( z d) / L, ζ = z / L and the fnction are + x + x π + tan ( ) + = x () [ exp(. )] = 7 9ζ, (3)

2 for L < and L > respectively. L is the Obkhov / 4 length, and x = ( 6ζ ). The siilarity relation () copled with () in nstable conditions can be applied to heights greater than L, even thogh, strictly speaking, they are valid for saller heights. Siilarly, Expression (3) for stable conditions, in addition to being applicable for z < L, can also be sed for z > L with good accracy (van lden and Holtslag, 985). Figre copares the wind speeds estiated sing the M-O siilarity with the observed wind speed at six heights over the rban area. At heights of 3.7 and.4, M-O srface siilarity theory is able to describe the data satisfactorily, bt for the lower levels there is an increasing overestiation of the observations. Sch departre fro the siilarity behavior is expected as these easreent levels lie very close or within the height of the roghness eleent for which siilarity theory is not applicable. Even the levels 3.7 and.4 are inflenced by the local roghness eleent and, therefore, probably lie within the roghness sblayer. Conseqently, M-O siilarity ay only be valid approxiately for these two levels, althogh siilarity reslts are in reasonable agreeent with the data. 4. -D BAN INTENAL BONDAY LAYE MODEL 4.. Internal bondary layer height over rban area There are a nber of forlae of varying coplexity to estiate the growth of the IBL (see Garratt, 99; Savelyev and Taylor, 5). One sch forla based on Miyake s diffsion analogy and discssed by Savelyev and Taylor (5) is dh ( h) = A σ w, (4) dx where h is the height of the IBL, x is the downwind distance fro the roghness change, (h) is the wind speed at height h, and A ( ) is a constant. We consider the vales of (h) and σ w to be those of the odified flow (i.e. over the rban srface). We asse that Monin-Obkhov srface siilarity theory is valid within the IBL over an rban area, which is a reasonable assption, given the coparisons presented earlier. We sbstitte for fro Eqations (), and σ w fro σ w 5. ( z d) 3 L / 3 = for L< (5) σ w =. 5, for L. (6), at z = h into Eqation (4). Note that cancels ot; however, L for the rban area needs to be specified. We asse that nder nstable rral conditions, the rban L is the sae as the rral vale, and nder stable rral conditions the rban stability is netral. Eqation (4) is solved nerically for the growth of h with x sing a 4 th order nge-ktta schee with z and d for the rban area as inpts copled with the above assption abot L. There ay be significant ean vertical velocities cased by local horizontal gradients of the ean wind speed after a step change in srface conditions in order to satisfy continity constraints. Sch a velocity ay need to be inclded in Eqation (4) (Savelyev and Taylor, 5), bt we neglect sch effects here. 4.. Calclation of the rban friction velocity We asse that the flow over the rban area above the height h is the sae as that above the rral area. Therefore, by eqating the rral and rban wind speeds at z = h, we obtain the following expression for the rban friction velocity, =, z, z, L L z, + L z, + L, where the sbscripts and represent rral and rban conditions. However, Eqation (7) cannot be sed as it is becase L, which itself is a fnction of, is not known. We asse that nder the daytie convective ixing (i.e. nstable) conditions, L is the sae as L. This is a tentative assption, bt can be jstified on the gronds that the sensible heat flx fro an rban srface is sally greater than that fro a rral area (not shown) de to the characteristics of the rban srface and the presence of an anthropogenic heat flx, and this increase in the sensible heat flx tends to copensate for the increase in the friction velocity over the rban srface in the calclation of L. Therefore, when L <, =, z, z, L L z, + L.(8) z, + L For typical rral conditions, the ters d and z / L ) in Eqation (8) can be neglected. (, (7) When rral conditions are stable in the nighttie (i.e. L > ), it is reasonable to asse that the

3 bondary layer is netral (e.g. Britter and Hanna, 3) as the air ass travels fro an area where the heat flx is negative to the center of the ch rogher rban area where the heat flx can be slightly positive. Hence, Eqation (7) can be written as:, =, z, z, + L L z,, (9) Hence, we can now calclate the friction velocity and the wind speed profile within the IBL over the rban area given the rral eteorology (above the IBL, the wind profile is taken to be the sae as the rral one). The IBL height at a distance of x = 5 k fro the rban bondary was sed in the above calclations for BBBLE case. In the following, we copare rban vales observed at.4 with the estiated vales. An analysis of the BBBLE data by Christen and otach (4b) shows that the data at.4 AGL represent the overall srface easreent conditions better than the data at any other height eslts fro the analytical schee Figre a presents a scatter plot of the observed vales over the rban area fro BBBLE vs. the estiates fro the analytical schee for nstable conditions (i.e. when L < ). Althogh there is a significant scatter, it is clear that the proposed schee predicts rban friction velocities that are in reasonable agreeent with the observations. 85% of the odel estiates lie within a factor of two of the observations. Figre b is the sae as Figre a, except for stable conditions. The saple size is only 4 copared to 4 for nstable conditions. There is soe overestiation of the observed : approxiately 7% of the predictions are within a factor of D MESOSCALE MODEL The esoscale odel TAPM is a threediensional, prognostic eteorological and air polltion odel, and is sed here to stdy the flow transition fro rral to rban areas. Details of the odel are given in Hrley () and Hrley et al. (5). Hrley et al. (3) and Lhar and Hrley (3) describe odel validation stdies condcted sing TAPM. 5.. Model Configration - BBBLE TAPM (version 3.) was rn for the period Jne Jly. We sed for nested grid doains at, 7.5,,.5 k resoltion (35 35 grid points), all centered at the location (7 36 E, N), which is eqivalent to 6.44 east and k north in the CH93 coordinate syste, and is alost the location of the (Bspr) rban onitoring site. The lowest ten of the 5 vertical levels were, 5, 5,, 5,, 5, 3, 4 and 5, with the highest odel level at 8. In view the objective of the present stdy, i.e. to estiate srface-layer eteorology over an rban area given pwind rral eteorology, we assiilated wind speeds and wind directions observed at 3.3 at the rral site in TAPM while allowing the odel to adjst these winds over the rban area, the latter can then be copared with the observations ade at the rban site. In this way, the TAPM setp is also consistent with the analytical techniqe sed earlier in which the observed rral eteorology is sed. 5.. Model Configration J3 The period of Jly 3 Jly 3 was selected for TAPM silations. For nested doains of which horizontal grid spacings are 3, 8,, and.5 k. The nbers of horizontal grid of all the doains is 4 by 4 in easting and northing, respectively, and is centered at ( k, k) of TM zone 4. The selected vertical coptational layers were identical to those of the BBBLE case. Data assiilation was condcted with observed wind speeds and directions easred at AGL of Tower # Modeling eslts The TAPM reslts were copared with easreents taken at AGL of Tower #5, which was located in a warehose rban area. The predicted wind speeds are in reasonable agreeent with observations (Figre 3). As evident fro Figre 3b, the prevailing wind direction in the Oklahoa City dring the field capaign was sotherly, which was well silated. The estiated friction velocities also copare well with observations. The scatter plot in Figre 4a copares TAPM predicted with the observations at the rral site (the total nber of observations is 45, doinated by nstable conditions). The odel perforance is very good, which is partly de to the fact that the observed winds at have been assiilated in the odel. Figre 4b shows that the odel is able to silate at the rban site (r =.48), and that the odel vales at are higher than those at, which is consistent with observed behavior. 6. CONCLSIONS This paper sed two ethods to estiate rban icroeteorology fro rral srface easreents: the first based on a two-diensional internal bondary layer odel that ses Monin-Obkhov srface siilarity theory and rral variables as pwind inpts, and the second ses a 3D prognostic odel, CSIO s TAPM, in which pwind rral observations are assiilated. eslts fro these ethods were 3

4 copared with high qality data collected fro field capaigns. rban srface friction velocities estiated fro both ethods copared well with corresponding observations. TAPM perforance was slightly better than that of the internal bondary layer odel. The reslts indicate that the internal bondary layer odel ight be sitable for rotine dispersion applications involving odels sch as AEMOD. Acknowledgeent: The research sarized in this paper was spported by grants fro the California Energy Coission (MAQ-4-6) and the National Science Fondation (NSF-FDP-ATM ). EFEENCES: Allwine, K.J., M.J. Leach, L.W. Stockha, J.S. Shinn,.P. Hosker, J.F. Bowers and J.C. Pace, 4, Overview of Joint rban 3 An atospheric dispersion stdy in Oklahoa city, Preprints. Syposi on Planning, Nowcasting, and Forecasting in the rban Zone, 5 Janary Seattle, WA, Aer. Meteor. Soc. Brown, M.J. and others, 4, Joint rban 3 street canyon experient, Preprints. Syposi on Planning, Nowcasting, and Forecasting in the rban Zone, 5 Janary, Seattle, WA, Aer. Meteor. Soc. Christen, A., and. Vogt, 4a, Energy and radiation balance of a central Eropean city, Int. J. Cliatol., 4, Christen, A., otach, M.W., 4b, Estiating wind speed at an rban reference height, Preprints. Fifth AMS Syposi on the rban Environent, 3 7 Agst, Vancover, BC, Canada. Garratt, J.., 99: The internal bondary layer r review. Bondary-Layer Meteorology 5, 7 3. Hynh, G., S. Chang, C. Klipp, C. Willason, D. Garvey, and Y. Wang, 5, spatial variability of trblence characteristics observed dring Joint rban 3. Preprints. Atospheric Sciences and Air Qality Conference, 6 9 Agst, San Francisco, CA. Hrley, P., : The Air Polltion Model (TAPM) Version. Part : Technical description. Astralia. CSIO Atospheric esearch. Technical Paper No pp. Hrley, P., Manins, P., Lee, S., Boyle,., Ng, Y., Dewndege, P., 3: Year-long, highresoltion, rban airshed odeling: verification of TAPM predictions of sog and particles in Melborne, Astralia. Atospheric Environent 37, Hrley, P. J., Physick, W. L., Lhar, A. K., 5: TAPM: A practical approach to prognostic eteorological and air polltion odeling. Environental Modelling and Software, Lindqist, J.K., J.H. Shin, and F. Goveia, 4, Observations of trblent kinetic energy dissipation rate in the rban environent, Syposi on Planning, Nowcasting, and Forecasting in the rban Zone, 5 Janary Seattle, WA, Aer. Meteor. Soc. Lhar, A. K., and Hrley, P. J., 3: Evalation of TAPM, a prognostic eteorological and air polltion odel, sing rban and rral pointsorce data. Atospheric Environent 37, otach, M. W., S.-E. Gryning, E. Batchvarova, A. Christen, and. Vogt, 4, Polltant dispersion close to an rban srface the BBBLE tracer experient, Meteorol. Atos. Phys., 87, Savelyev, S.A., Taylor, P.A., 5: Internal bondary layers: I. Height forlae for netral and diabatic flows. Bondary-Layer Meteorology 5, 5. Van lden, A.P., Holtslag, A.A.M., 985: Estiation of atospheric bondary layer paraeters for diffsion applications. Jornal of Cliate and Applied Meteorology 4, Venkatra, A., Isakov, V., Pankratz, D., Yan, J., 5: elating ple spread to eteorology and rban areas. Atospheric Environent 39,

5 Figre : Scatter plots of wind speeds estiated sing M-O siilarity verss observed wind speeds at six heights over the rban area () of BBBLE. 5

6 Figre. Scatter plot of observed at.4 of verss internal bondary layer odel estiates for (a) nstable case (i.e. when the rral Obkhov length is < ), and (b) stable case (rral Obhkov length is >). 6

7 Estiated ( s - ) 6 4 (a) Wind Speed 4 6 Observed ( s - ) (c) Friction Velocity Estiated (degree) 3 (b) Wind Direction 3 Observed (degree) Estiated ( s - ).5.5 Observed ( s - ) Figre 3. Scatter plot of TAPM-predicted verss observed (a) wind speed, (b) wind direction, and (c) Friction velocity at S AL Tower 5. Figre 4. Coparison of TAPM-predicted with observed vales at (a) rral site, and (b) rban site. 7