Variations in Cn2 measured by LAS scintillometery over the city of Nantes during the FluxSAP 2010 measurement campaign

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Variations in Cn2 measured by LAS scintillometery over the city of Nantes during the FluxSAP 2010

measurement campaign

Mark Rankin Irvine, Jean-Pierre Lagouarde, Sylvia Dayau, Aurore Brut, Pierre-Adrien Solignac, Jean-Louis Selves, Jean-Martial Cohard, Georges

Najjar, Céline Quentin, Fabrice Rodriguez, et al.

To cite this version:

Mark Rankin Irvine, Jean-Pierre Lagouarde, Sylvia Dayau, Aurore Brut, Pierre-Adrien Solignac, et al.. Variations in Cn2 measured by LAS scintillometery over the city of Nantes during the FluxSAP 2010 measurement campaign. 3rd Scintillometer Workshop, Apr 2011, Wageningen, Netherlands.

2011. �hal-02806505�

Vari ati ons i n Cn2 measured by LAS sci nti l l ometery over the ci ty of Nantes duri ng the Fl uxSAP 2010 measurement campai gn.

Irvi ne

M. R. , Lagouarde

J-P. , Dayau

S. , Brut

A. , Sol i gnac

P. A. , Sel ves

J-L. , Cohard

J. M. , Najjar

G. , Quenti n

C. , Rodri guez

F. , Mestayer

P.

1 INRA, UR1263 EPHYSE, F-33140 Vi l l enave d‛ Ornon 2 CESBIO, UMR5126, F-31401 Toul ouse 3 LTHE, UMR5564, F-38400 Sai nt-Marti n d‛ Hères 4 LSIIT, UMR7005, F-67412 Il l kri ch 5 IRSTV, FR CNRS 2488, F-44321 6 IFSTTAR, GER, F-44341

Fi gure 4 Cn2 for the di fferent sci nti l l ometer paths

Fi gure 6 Sensi bl e Heat Fl uxes

Fi gure 5 Rel i ef profi l es

Fi gure 2 Bui l di ng di stri buti on i n the ci ty of Nantes

Tabl e 1 Sci nti l l ometer paths

Fi gure 3 Sci nti l l ometry fl ux esti mati on Resul ts Fi gure 4 presents Cn2 for the

4 sci nti l l ometer paths (note the doubl ed path, not present here, was i denti cal provi di ng measurement confi dence).

The whol e measurement campai gn i s shown i n the top l eft wi th a zoom on a 10 day peri od. One can cl earl y see 10 day peri od. One can cl earl y see di fferences between the measurement paths, where on some days there are more i n agreement. In order to better understand these observed di fferences i t i s necessary to account for rel i ef di fferences between the sci nti l l ometers.

Fi gure 5 presents a si mpl e, fi rst, Fi gure 5 presents a si mpl e, fi rst, extracti on al ong each sci nti l l ometer path of ground l evel and bui l di ng hei ght from a topographi cal database. These rel i ef profi l es are i ntegrated i nto Fi gure 6, whi ch presents H esti mati ons.

Resul ts (cont) It can be seen that despi te thi s i ni ti al attempt to esti mate the rel i ef there i s a l arge vari ati on of the fl ux between the di fferent paths.

Thi s vari ati on for exampl e of the CESBIO sci nti l l o- meter i ndi cates l ower fl uxes on days 138-140 and hi gher fl uxes on days 144-146 compared to the LTHE scnti l l ometer. Fi gure 7 provi des wi nd di recti on scnti l l ometer. Fi gure 7 provi des wi nd di recti on i nformati on whi ch i ndi cates qui te di fferent di recti ons for the 2 before menti oned exampl es.

Method Cn2 i s di rectl y measured by the LAS vi a the recei ved ampl i tude fl uctuati ons from the i nfrared source (σ

). Temperature structure parameter (Ct2) can be determi ned from Cn2 usi ng a few meteorol ogi cal vari abl es and thereafter sensi bl e heat fl ux (H) by expl oi ti ng si mi l ari ty theory (Fi gure 3).

expl oi ti ng si mi l ari ty theory (Fi gure 3).

Concl usi on The spati al vari abi l i ty of Cn2 i s an i mportant questi on i n our anal yses of the sci nti l l ometry Fl uxSAP data as we want to be abl e to i mprove our understandi ng

of sensi bl e heat fl ux vari abi l i ty. Our i ni ti al resul ts i ndi cate a rather l arge vari abi l i ty i n a rather smal l area (< 5 km2), despi te a fi rst attempt to el i mi nate rel i ef effects. Observati ons suggest an i nfl uence from effects. Observati ons suggest an i nfl uence from wi nd di recti on, whi ch probabl y i ndi cates a ‘ footpri nt‛

factor. Thi s ‘ factor‛ may have a tri pl e i mpact: 1 the rel i ef i nfl uence i s di recti onal , 2 gi ven the bui l di ng di stri buti on (Fi gure 2) there i s i ndeed fl ux vari abi l i ty and 3 the vegetati on di stri buti on may have a i nfl uence on the fl uxes. To concl ude a ‘ footpri nt‛ /rel i ef study has to be done i n order to determi ne the true fl ux has to be done i n order to determi ne the true fl ux vari abi l i ty.

Fi gure 1 Sci nti l l ometers duri ng Fl uxSAP

Fi gure 7 Wi nd Informati on Introducti on In the framework of a l arge French research program

i nvesti gati ng sustai nabl e devel opment of ci ti es (ANR ‘ Vi l l es Durabl es‛ ), one project, VegDUD, proposed the expl orati on of the rol e of the vegetati on i n the urban envi ronment. Wi thi n thi s project an i ntensi ve experi mental campai gn, know as Fl uxSAP, was pl anned, i nvol vi ng remote sensi ng and surface fl ux measurements (further detai l s can be found on http: //www. i rstv. fr/). The ai m of Fl uxSAP i s to i mprove our on http: //www. i rstv. fr/). The ai m of Fl uxSAP i s to i mprove our characteri sati on of fl uxes over a ci ty, wi th Nantes i n the north-west of France chosen as model .

Experi mental setup The measurement stage of the Fl uxSAP ‘ vol et‛ was

undertaken duri ng the month of May 2010. As part of the set-up 5

Large Aperture Sci nti l l ometers (LAS) were i nstal l ed over a peri -urban

part of the ci ty i n fi rstl y, a tri angul ar formati on (3 LAS) wi th path-

l engths between 1. 3 and 2. 4 kms, secondl y a LAS crossi ng through the

tri angul ar formati on (pathl ength 3. 5 kms) and thi rdl y a fi fth LAS was

used to doubl e up one of the paths on the tri angul ar formati on to be

used to doubl e up one of the paths on the tri angul ar formati on to be

sure of the repeatabi l i ty of the refl ecti ve i ndex structure parameter

(Cn2) measurement by the LAS (see tabl e 1 & Fi gure 2). Over 3 weeks

of common data are avai l abl e from the LAS to study the Cn2 and

sensi bl e heat fl ux (H) vari abi l i ty.