DETERMINATION OF WIND SPEED PROFILE PARAMETERS IN THE SURFACE LAYER USING A MINI-SODAR

Transcription

1 DETERMINATION OF WIND SPEED PROFILE PARAMETERS IN THE SURFACE LAYER USING A MINI-SODAR A. Coppalle, M. Talbaut and F. Corbn UMR 6614 CORIA, Sant Etenne du Rouvray, France INTRODUCTION Recent mprovements on acoustc wnd proflers allow to carry out wnd measurements wth a vertcal spatal resoluton equal to a few meters (typcally 5 m). Ths s due to the use of hgher frequences (45 hz n the present study) whch favor smaller errors n the wnd velocty. However, the vertcal range of the mn- s reduced snce hgher acoustc frequences are attenuated much more n the atmosphere than lower ones. Ths not a restrcton n our work snce the man objectve has been to study the wnd speed profles U(z) n the surface layer above an urban area. Ths lower part of the urban boundary layer has a great nfluence on the dsperson of pollutants emtted close to the ground; Rotach (1997, 1999). Thus, wth the mn-, we measured the vertcal profle of wnd speed U and drecton. Measurements obtaned n the Marselle cty (France) wll be presented. They have been carred out n the framework of the UBL/CLU-Escompte experment, whch amed at documentng the structure of the urban boundary layer (UBL) n connecton wth the urban canopy therynamcs durng a summer perod wth low wnd and breeze, from June 5 to July 15, 1, Mestayer (). In order to check the performances of the profler n a smple atmospherc flow case, a seres of observatons was carred out on a flat terran over a rural zone. Comparson wth anemometers set up on hgh mast wll be shown. DESCRIPTION OF THE TWO OBSERVATION SITES The urban staton was located at the south of Marselle cty (1.5 Mllon of nhabtants) at 4 km from the cty centre, nsde an nsttute. The closest dstance to the sea s.5 km n the West drecton. So see breeze effects are perceptble on the wnd profles. The mn- was put on a flat roof of a buldng (5 meter heght). The emsson power was 15 W wth a frequency equal to 45hz, the vertcal resoluton beng 5m (see Pcture 1). A 3 meter heght mast was nstalled on the ground and close to the mn- buldng, wth a anemometer at the top. In order to check the performances of the profler n a smpler atmospherc flow case, a seres of observatons was carred out on a flat and clear terran over a rural zone (see Pcture ). Pcture 1. Vew of the mn- n the urban staton (Marselle, France Pcture. Vew of the mn- and the mast n the rural staton

2 Comparsons wth the results of one anemometer set up on a 39 meter heght mast wll be shown. For both stes the maxmum heght of detecton was generally about 15 m (not m as announced by the manufacturer), except durng rany days, for whch the qualty of the data was very poor. In the urban ste case, one expected the background nose emtted by the urban zone could lower the S/N rato. However, the qualty of the echo sgnal was suffcent to perform measurements up to 15 meter heght. Ths s manly due to the dfference between the mn emsson frequency (45 Hz) and the urban background nose whch s n the 1-15 Hz frequency range. COMPARISONS WITH A SONIC ANEMOMETER RESULTS Fgures 1- show the comparson between mn- and wnd speed measurements (U n the horzontal drecton) durng one week of each expermental campagn. (June 1 for the urban staton, Aprl for the rural staton). In the rural case (Fgure ), there s a good agreement, the statstcs of comparson are for the correlaton coeffcent r=.815, for the bas B=.93 m/s and for the root mean square dfference C=.733 m/s, whch s smlar to other studes (Crescent, 1997). Fgure 1 shows the same agreement, r=.967 B=.538 and C=1.769, ndcatng the mn- was runnng correctly, t provded accurate results despte the nosy background emtted by the cty, n partcular for low wnd speed, the values reported n the above fgures are n the range -6 m/s. In the rural staton, much hgher wnd speed values were observed and the agreement s better. However, we have to say the best agreement for the urban case s obtaned f we plot the mn- ffth level values (5 m above the flat roof of the 5 m heght buldng) wth the anemometer results (at 3 m above ground). Fgures 3-4 show the comparson for the r.m.s. values of the vertcal wnd speed fluctuatons (rmsw) for the same perods as n Fgures 1-. The overall agreement s found to be smlar n the rural and n the urban cases, excepted for low values, whch are overerestmated by the mn. No explanatons are found at ths tme, but we thnk the mn- results have to be analysed carefully snce low rmsw values occurs at low wnd speed, wth low echo ntensty. Further works are needed to analyse ths pont. DETERMINATION OF WIND SPEED PROFILE PARAMETERS IN THE SURFACE LAYER The pattern of ar flows over a regon may be complcated. For example n the case of Marselle cty, there s a conjuncton, n sprng and summer, of see breezes (n the West-East drecton) and the Mstral wnd blowng from the north. So, n the daytme, large varatons of wnd drecton can be observed and strong shears exst n the boundary layer. In such cases, t was not possble to detect well establshed vertcal profles of the horzontal wnd speed U(z) as llustrated n Fgure 5. However, durng days wth stable wnd drectons, we observed well defned wnd speed profles U(z) (wth U values ncreasng contnuously wth heght z) as shown n Fgure 6. For such vertcal profles U(z), we have determned both the frcton velocty u* and the roughness length z usng a least square mnmsaton ft of the error functon I( u *, z) = ( U U where U obs ) s the value gven by the logarthmc law at level of the mn

3 U u * ( z = Ln k d) z and d s the dsplacement heght. Each mnmsaton ft was appled for several consecutve wnd profles. So the parameters u* and z have been optmsed from data observed durng one hour and wth fxed d value. Result are shown n Fgures 7-8 for one specfc hour. In the rural case, we found u* =.6 m/s and the average value gven by the anemometer s u* =,9 m/s (at 39 m heght above ground). In the urban case, the values are u* =.91 and u* =.79 m/s (at 3m heght above ground). The agreement s better n the urban case. The comparson between the two mn- results shows the urban value s greater, whch s not the case for the anemometer results. If we look at Fgures 7-8, we see the speed profle vares more strongly n the urban case than n the rural case, whch ndcate a greater u* values close to the ground for the urban case. It s hard to conclude wth only one pont of comparson and other data wll have to be used next. The optmsed roughness length z are.8 and.5 m respectvely for the urban and rural cases. These values are physcally plausble f we look at other works, as for example Grmmond (Grmmond, 1998). The effect of the dsplacement heght seems not to be mportant for the case presented n Fgure 7. There s no large dfferences f we take d=h, h beng the buldng heght, or d=h-3m. We have to say that the mnmsaton ft s much more dffcult f we try to determne smultaneously the three parameters u*, z and d Fgure 1. Mean wnd speed, comparson (ffth level)/ at the urban staton rms W (m/s) Fgure. Mean wnd speed, comparson / at the rural staton. 1,5 1, rmsw (m/s).5, Fgure 3. rms value of the vertcal wnd speed W, comparson (ffth level)/ at the urban staton.,,5 1 1,5 Fgure 4. rms value of the vertcal wnd speed W, comparson / at the rural staton

4 5 5 Heght above the mean roof level (m H 16 H H Fgure 5. Vertcal profles of the horzontal wnd speed observed at the urban staton durng one day wth large wnd drecton varatons. Heght above the mean roof level H 1 H 16 H Fgure 6. Vertcal profles of the horzontal wnd speed observed at the urban staton durng one day wth stable drecton. heght above roof Ln Law wth d=h obs. 15h 15h15 15h3 15h45 Ln Law wth d=h-1m Ln Law wth d=h-3m wnd speed U (m/s) Fgure 7. urban ste: Ln law profles obtaned usng the optmsed parameters u*=.91 m/s and z=.8 m, compared to observed values. Three dsplacement heghts have been used d=h, h-1m and h-3m, h beng the buldng heght. 5 Ln Law 15h h e g ht ab ove r h1 15h3 15:5 15h 15h4 5 wnd speed U (m/s) Fgure 8. rural ste: Ln law profles obtaned usng the optmsed parameters u*=;6 m/s and z=.5 m, compared to observed values. The dsplacement heght d= m - 6 -

5 CONCLUSION The mn- results compare well wth anemometer observatons, even n the background nose of an urban envronment. In our study, there was no corrupton of the data and wth our system t was possble to perform measurements up to 15m. We have looked at the potentalty of the mn- to determne the frcton velocty and the roughness length. The frst results are encouragng, however other analyses wll have to be carred out next. REFERENCES Crescent G.H., 1997 : A look back on two decades of doppler comparson studes, Bulletn of the A.M.S., 78, Grmmond C., Kng T., Roth M. and Oke T., 1998: Aerodynamc roughness of urban areas derved from wnd observaton, Boundary Layer Meteorology, 89, 1-4 Mestayer P. and Durand P., : The UBL/CLU-Escompte experment : descrpton and frst results, the AMS Fourth Symposum on Urban Envronment, May, Norfolks, Vrgna Rotach M. and De Haan P., 1997 : On the urban aspect of the copenhagen data set, Int. J. Env. Poll, 8,