5B.1 THE T-REX VALLEY WIND INTERCOMPARISON PROJECT

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1 5B.1 THE T-REX VALLEY WIND INTERCOMPARISON PROJECT Jerg Schmidli, B.J. Billings, R. Brton, F.K. Chow, S.F.J. De Wekker, J.D. Doyle, V. Grbisic, T.R. Holt, Q. Jiang, K.A. Lndqist, A.N. Ross, P. Sheridan, S. Vosper, C.D. Whiteman, A.A. Wyszogrodzki, G. Zaengl, S. Zhong NCAR, Bolder, Colorado 1 INTRODUCTION An accrate simlation of the eoltion of the atmospheric bondary layer is ery important, as the eoltion of the bondary layer sets the stage for many weather phenomena, sch as deep conection. Oer montain areas the eoltion of the bondary layer is particlarly complex, de to the nonlinear interaction between the bondary layer trblence and thermally-indced mesoscale wind systems, sch as the slope and alley winds. As the horizontal resoltion of operational forecasts progresses to finer and finer resoltion, more and more of the thermallyindced mesoscale wind systems can be explicitly resoled, and it is ery timely to docent the crrent state-of-the-art of mesoscale models at simlating the copled eoltion of the montain bondary layer and the alley wind system. In this paper we present an intercomparison of alley wind simlations for an idealized alley-plain configration sing 7?? state-of-the-art mesoscale models with a grid spacing of 1 km. Different sets of three-dimensional simlations are sed to explore the effects of arying model dynamical cores and physical parameterizations. This intercomparison project was condcted as part of the Terrain-indced Rotor Experiment (T-REX; Grbisic et al., ). EXPERIMENTAL DESIGN Two sets of simlations were carried ot in order to assess the relatie role of the model dynamical cores, trblence mixing parameterizations, radiation transfer and land srface schemes. A set of ncopled simlations sed a prescribed srface heat flx, and a set of copled simlations sed comprehensie model physics (radiation transfer scheme and land srface model). Althogh the setp for both sets of simlations is described below, this paper will focs on the reslts from the copled simlations..1 General setp The intercomparison is based on the idealized alleyplain system shown in Fig. 1. This setp is similar to configrations sed in preios stdies (Li and Atkinson, 1999; Rampanelli et al., ). The topography was chosen to satisfy the criteria listed in Rampanelli et al. (): (1) A horizontal alley floor, so that the along-alley wind has no pslope contribtion, () a long alley, so that the along-alley flow can deelop nhindered from nerical bondary conditions in the along-alley direction, (3) moderately steep alley slopes which can be adeqately represented by crrent mesoscale models. In contrast to preios stdies, a large comational domain and periodic lateral bondary conditions were chosen in order to minimize the inflence of the lateral bondaries on the simlated flow. The analytical expression for the topography sed is gien by z = h(x,y) = h ph x(x)h y(y) (1) where x V x 1 >< 1 cos π x Vx S x V x < x < X h x(x) = 1 X x X 3 1 >: + 1 cos π x X 3 S x X 3 < x < X x X () and >< 1 y P y 1 h y(y) = >: + 1 cos π y Py S y P y < y < Y y Y, (3) with alley deh h p = km, V x = km (alley floor half width), S x = 9 km (sloping sidewall width), P x = 1 km (platea width in cross-alley direction), P y = 1 km (platea half width in alongalley direction), S y = 9 km, and X = V x + S x, X 3 = V x+s x+p x, X = V x+s x+p x, Y = P y+s y. The simlations are started from an atmosphere at rest. The initial condition for the atmosphere is gien by the potential temperatre distribtion θ(z) = θ s + Γz + θ[1 exp( βz)] () where θ s = K, Γ = 3. K km 1, θ = 5 K, and β =. m 1, the srface pressre p s = 1 hpa, and a constant relatie hidity of %. The intial

2 temperatre profile describes an atmosphere with a constant stratification of N.11 s 1 with a sperimposed near-srface inersion. The comational domain of the simlations is km in the y (along alley) direction and 1 km in the x (across alley) direction. The grid spacing is 1 km in both directions. In the ertical, the comational domain extends to at least. km and the grid spacing aries from m near the grond to a maxim of m aboe km. The lateral bondary conditions are periodic, nless that oion was not aailable for a particlar model (see table... ). As top bondary conditions a Rayleigh sponge was specified. All simlations are rn with the Coriolis force trned off. The models were integrated for 1 hors from snrise ( local ) to snset (1 LT).. Uncopled simlations For the ncopled simlations the srface sensible heat flx is gien by Q(t) = Q sin(ωt) (5) where Q = W m, ω = π/ h 1, and the time t denotes hors since snrise. The ncopled simlations se free slip lower bondary conditions (i.e. zero moment and latent heat flx)..3 Copled simlations In the copled simlations the srface sensible heat flx is determined by the model physics, that is by the copled land srface-atmosphere system. Additional specifications inclde Radiatie forcing determined by the location (3 N, E) and time of year 1 March 7. Uniform land srface characteristics: soil type is sandy loam; egetation is semidesert with LAI, egetation fraction.1, roghness length.1 m. Albedo is set to.7, if it is not determined implicitly by the land srface model. Soil initialization: soil temperatre is set eqal to atmospheric srface temperatre, soil moistre satration ratio is %.. Valley wind theory Why does the aboe setp prodce a alley wind? It is well known, that thermally-indced alley winds are generated by locally deeloped along-alley pressre gradients prodced hydrostatically from temperatre differences along the alley s axis and between the alley and the adjacent plains (Whiteman, ). The along-alley and alley-plain temperatre differences can be prodced by at least two factors: differences in the srface sensible heat flx, and by the alley ole effect (Wagner, 193; Steinacker, 19). The first factor is exclded for the ncopled simlations, bt both factors may be important for the copled simlations..5 Models A brief descriion of the setp for the mesoscale models is gien in Table 1. TODO: Briefly smarize main common featres and differences.. Intercomparison procedre To make the reslts easily comparable, all ariables from each of the models were interpolated to destaggered grid in a common NetCDF file format. 3 RESULTS COMMENT: The reslts for UM will not be discssed as the simlations are being reised. COAMPS: nrealistic alley wind dring first few integration hors, initialization problem? Expecting new simlations for RAMS and maybe EULAG. ARPS will be remoed from plots. In this section the reslts for the copled simlations are presented. 3.1 Forcing and mean temperatre response Figre compares time eoltion of the srface sensible heat flx and the reslting potential temperatre eoltion for a alley cross section located km palley from the alley entrance, to the corresponding eoltion oer the plains. The area-aeraged srface sensible heat flx in the alley is consistently larger than the corresponding heat flx oer the plains. The maxim srface sensible heat flx oer the plain is only 7 % to % of the corresponding alley ale. It can be expected that this alley-plain difference in srface sensible heat flx will contribte to the strength of the alley wind, in addition to the alley ole effect, and to some of the differences between the models.

3 Table 1: Model Descriion. The following abbreiations are sed: h for horizontal, for ertical, SISL for semi-implicit semi-lagrangian, LOC for local, NL for non-local, and SLI for incoming radiation as a fnction of slope inclination. Model Time Differencing dt Spatial Differenc- Trblence Mixing Comational Mixing SLI and ing and Accracy Scheme Accracy ARPS Leapfrog 1 h: th order order TKE, NL h: nd order,.3 s 1 yes nd order : nd order : none COAMPS3 Leapfrog?? h+: nd order order TKE h: th order,.5 s 1 no nd order : none COAMPS Leapfrog 3 h+: nd order order TKE h: th order,.5 s 1 no nd order : none MM5 Leapfrog 3 h+: nd order 1st order PBL, NL h: th order,.1 s 1 no nd order Blackadar : none RAMS?? yes UM SISL 1 SISL 1st order PBL h: none no LOC and NL : none WRF-TKE Rnge-Ktta 1 h: 5th order order TKE h+: TKE scheme no 3rd order : 3rd order WRF-YSU Rnge-Ktta 3rd order 1 h: 5th order : 3rd order 1st order YSU K profile & contergrad. h: Smagorinsky : none no 3. Local bondary layer eoltion Figre 3 compares the bondary layer eoltion at for sites in terms of profiles of potential temperatre at 1 LT. A typical conectie bondary layer (CBL) has deeloped oer the plains, with mixed layer (ML) dehs ranging from 75 m for ARPS to 1 m for WRF-YSU. The ML is abot 1- K cooler for ARPS than for the other models. A well mixed CBL has also deeloped oer the montain ridge, with ML dehs ranging from m to 1 m. A more complex temperatre strctre has deeloped at the two alley sites. The alley center site shows the typical three-layer strctre fond in many montain alleys (Brehm, 19; Weigel et al., ), with a well mixed layer near the grond separated by a stable layer from a second nearly netral layer aloft. There are large differences in the simlated temperatre strctre between the models. The deh of the ML ranges from m (COAMPS) to 13 m (WRF- YSU); the height of the lower bondary of the pper netral layer aries between 1 m (COAMPS3) and 17 m (WRF-YSU). The sondings oer the alley slope exhibit a similar strctre to the alley center site, bt with a less marked stable layer, and somewhat less ariation between the models. 3.3 Cross-alley circlation Figre depcits ertical profiles of the cross-alley wind speed oer the western slope at x = km at 9, 1, and 15 LT. The differences between the models are relatiely small in the morning, bt qite large in the afternoon. The deh of the pslope wind layer growths from abot m at 9 LT, to 3-5 m at 1 LT, to oer 5 m in some models in the afternoon. Note also the onset of the plain-to-basin wind abot 5 m aboe ridge height, which is clearly isible in the 1 LT sonding, and the large ariability between the models in the simlated strength and strctre of this wind, especially in the afternoon. The time eoltion of the pslope wind on the west slope at x = km is shown in Figre 5. The wind starts soon after snrise and increases in strength ntil noon/early afternoon. At 1 m aboe grond, the simlated eoltion is initially qite similar, with the differences between the models increasing dring the day. And in the afternoon, the models differ by more than two hors in the timing of the reersal to downslope flow. Closer to the grond srface, the differences between the models are mch larger. The maxim pslope wind speed at 1 m aboe grond aries between less than m s 1 for MM5 to more than m s 1 for ARPS. 3. Plain-to-basin circlation The plain-to-basin circlation is clearly isible in Figre, which depcits ertical profiles of the crossridge wind oer the western ridge. By 1 LT, there is a plain-to-basin flow in the lowest few hndred meters for all models exce ARPS, and a retrn basinto-plain flow in a layer extending from abot 5 m to 1 m aboe grond. The retarded onset of the plain-to-basin circlation for ARPS oer the western ridge is de to the fact that ARPS is the only model where the incoming solar radiation is dependent on

4 the inclination of the srface. This leads to strong pslope flows on the east-facing slopes and a mch delayed eoltion on the west-facing slopes. In contrast, the other models prodce a symmetric eoltion of the slope winds oer east and west-facing slopes. By 15, there is a plain-to-basin circlation in all models, bt with large differences in its ertical strctre. 3.5 Along-alley flow Figre 7 shows ertical profiles of the along-alley wind at 1 and 15 LT oer the alley center and the western slope. At 1 LT there is good agreement in the simlation of the along alley wind between the models, the simlated maxim wind speed is abot 5 m s 1. Some differences exist as to the deh of the along-alley wind layer which aries between 1 m and 15 m. At 15 LT there are sbstantial differences in the ertical strctre of the simlated along alley wind. The deh of the alongalley wind layer aries from 1 m for COAMPS to oer m for WRF-YSU. The time eoltion of the along-alley wind for the alley center and the western slope is shown in Figre. Initially there is good agreement between the models which decreases rapidly, howeer, after noon. In the late afternoon, the simlated wind speeds ary by more than 3 m s 1. A similar decrease of agreement between the models is seen in Figre 9, which shows the along-alley ariation of the along alley wind speed at 1 LT and 15. As can be seen from Figre 1, which shows the along alley wind for the ncopled simlations, identical forcing of the models throgh the prescribed srface sensible heat flx does not redce the ariability between the models. In contrast, the differences are een larger, likely de to the free-slip lower bondary condition sed for the ncopled simlations. Figre 11 illstrates the similarities and differences in the spatial strctre of the simlated along alley wind together with the cross-alley circlation. The figre is for 1 LT, that is when the differences between the models are still relatiely small. Neertheless, the diersity of reslts is qite large. The deh of the along-alley flow layer in the alley center is qite shallow for the two COAMPS models, bt it extends almost to the height of the montain ridges for many of the other models. Also the along-alley flow is adected by the cross-alley circlation aboe the montain ridges for some models (COAMPS3, UM), bt not for others (ARPS, WRF-YSU). CONCLUSIONS We hae presented a detailed intercomparison of X mesoscale models that are applied to simlate the eoltion of the copled bondary layer - alley wind system for an idealized alley-plain configration. From the initial analysis of the reslts the following tentatie conclsions can be drawn: There is good agreement between the models in their simlation of the initial deelopment of the alley wind, between snrise and noon. The agreement is particlarly high in the lowest 5 m aboe grond leel. The strength and spatial strctre of the simlated along alley wind becomes more dierse after midday, as the along alley wind interacts with the cross-alley circlation and the trblent bondary layers. There is less agreement between the models with respect to the simlated eoltion of the local bondary layers. In the morning hors, the differences are likely related to differences in the simlated srface sensible heat flx, for instance de to specifics of the radiation schemes, and to differences in the intensity of ertical mixing indced by the PBL trblence schemes. In the afternoon hors, the differences between the models are more likely the reslt of the complex interactions between parameterized trblence, comational mixing and the arios thermally-indced mesoscale flows. ACKNOWLEDGEMENTS The spport of the National Science Fondation (grant???) for the first athor is grateflly acknowledged. The National Center for Atmospheric Research, fnded by NSF, proided the coming time sed in this research. REFERENCES Brehm, M., 19: Experimentelle nd nerische Unterschngen der Hangwindschicht nd ihrer Rolle bei der Erwärmng on Tälern. Ph.D. dissertation, Meteorologisches Institt, Uniersität München, 15 pp. [Wiss. Mitt. Nr. 5]. Grbisic, V., J. D. Doyle, J. Kettner, S. Mobbs, R. B. Smith, C. D. Whiteman, R. Dirks, S. Czycyk, S. A. Cohn, S. Vosper, M. Weissmann, S. Haimo, S. De Wekker, L. Pan, and F. K. Chow, : The Terrain-indced Rotor Experiment: An oeriew of

5 the field campaign and some highlights of special obserations. Bll. Amer. Meteor. Soc., in press. Li, J. G., and B. W. Atkinson, 1999: Transition regimes in alley airflows. Bond.-Layer Meteor., 91, Rampanelli, G., D. Zardi, and R. Rotnno, : Mechanisms of p-alley winds. J. Atmos. Sci., 1, Steinacker, R., 19: Area-height distribtion of a alley and its relation to the alley wind. Contrib. Atmos. Phys., 57, 71. Wagner, A., 193: Theorie nd Beobachtng der periodischen Gebirgswinde. Gerl. Beitr. Geophys., 5, 9. Weigel, A. P., F. K. Chow, M. W. Rotach, R. L. Street, and M. Xe, : High-resoltion largeeddy simlations of flow in a steep Alpine alley. Part II: Flow strctre and heat bdgets. J. Appl. Meteor. Climatol., 5, Whiteman, C. D., : Montain Meteorology: Fndamentals and Applications. Oxford Uniersity Press, 355 pp y [km] x [km] x [km] Figre 1: (a) Contor plot of the three-dimensional alley-plain topography (contor ales are 5, 75, and 15 m) and comational domain adoed; only the sothern half of the domain is shown. (b) Cross section of topography and initial potential temperatre distribtion.

6 Valley (y = km) Plain (y = km) shsflx 5 3 _ shsflx 5 3 _ _ 9 9 _ Figre : Time series of srface sensible heat flx (aeraged from x ± 1 km) and potential temperatre (aeraged from x ± 1 km and from the srface to 15 m). 3.5 Plain 3.5 Ridge 3.5 Valley center 3.5 Slope 3..5 _ _ Figre 3: Profiles of potential temperatre at 1 LT at for different locations. The horizontal line indicates the height of the montain ridge.

7 9 LT 1 LT 15 LT _... Figre : Profiles of cross-alley wind speed oer the western slope (x = km, y = km). The horizontal line indicates the height of the montain 1 1 m _ _ Figre 5: Time series of cross-alley wind speed oer the western slope (x = km, y = km).

8 9 LT 1 LT 15 LT _... Figre : Profiles of cross-alley wind speed oer the western ridge (x = 1 km, y = km)..5 Valley 1 LT.5 Valley 15 LT.5 Slope 1 LT.5 Slope 15 LT.... Figre 7: Profiles of along-alley wind at the alley center (x = km) and oer the western slope (x = km). The along-alley position is y = km. Valley (x = km) Slope (x = km) Figre : Time series of along-alley wind speed at 1 m AGL. The along-alley position is y = km.

9 1 1 LT 1 15 LT 1 y [km] 1 y [km] Figre 9: Along-alley ariation of along-alley wind speed at 1 m AGL. 1 1 LT 1 15 LT _c _c 1 y [km] 1 y [km] Figre 1: As in Figre 9, bt for the ncopled simlations.

10 ARPS COAMPS3 COAMPS MM5 UM (dry) RAMS WRF-TKE WRF-YSU Figre 11: Snap-shot of the flow at 1 LT in the west-east cross section at y = km. Along-alley wind component (thick bold lines), potential temperatre (thin lines), cross-alley circlation (ectors), and ertical diffsion coefficient (shading).