ESTIMATION AND ANALYSIS OF SPECTRAL SOLAR RADIATION OVER CAIRO. M.M. Abdel Wahab x International Centre for Theoretical Physics, Trieste, Italy.

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1 A 7 Internatnal Atmc Energy Agency and Unted Natns Educatnal Scentfc and Cultural Organzatn INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS INTERNAL REPORT (Lmted strbutn) ESTIMATION AN ANALYSIS OF SPECTRAL SOLAR RAIATION OVER CAIRO M.M. Abdel Wahab x Internatnal Centre fr Theretcal Physcs, Treste, Italy and M. Omran Egyptan Meterlgcal Authrty, Car, Egypt. ABSTRACT Ths wrk presents a methdlgy t estmate spectral dffuse and glbal radatn n hrzntal surface. Ths methd s valdated by cmparng wth measured drect and glbal spectral radatn n fur bands. The results shw a gd perfrmance n cludless cndtns. The analyss f the rat f surface values t extraterrestral nes revealed an verall depletn n the summer mnths. Als there was n evdence fr any tendency fr cnversn f radatnal cmpnents thrugh dfferent bands. The mdel presents excellent agreement wth the measured values fr (UV/G) rat. MIRAMARE TRIESTE May 1994 Permanent address: epartment f Astrnmy and Meterlgy, Faculty f Scence, Car Unversty, Gza, Car, Egypt.

2 1 Intrductn Many slar energy applcatns requre a detaled knwledge f spectra] rradance t evaluate the prpertes f absrbers and reflectrs. Als the UV radatn has receved great attentn n the last few years due t ts blgcal and eclgcal effects. On the ther hand, the use f meterlgcal data n the plant dsease had als llustrated the need fr the level f radatn at specfc bands. In the spectral dstrbutn f dffuse radatn, the shrter wavelengths play a mre mprtant rle than n that f drect radatn. Guzz et at. (1983) cmpared ther mdel aganst drect spectral rradance abve.68//m and aganst brad band ntegrated rradance. Ther results shwed sgnfcant underestmatn by the mdel, bth fr the brad band and fr the spectral drect rradances specally n the.68 l.ofm spectral range. Justus and Pars (1985) made mprvements t the smple spectral grup mdels f Leckner (1978), Iqbal (1983) and Brd (1984). The cmparsn f ther mdel wth measured data shwed suffcent accuracy fr many applcatns. Brd (1986), refned the Justus and Pars mdel, by cmparng wth result f rgrus radatve transfer cdes and wth measured spectra. In the present study, the mdel f Brd (1986) was develped t be used n estmates f spectral radatnal cmpnents f nterval f.1/^m n the urban clmate f Car. It was ntegrated fr spectral brad bands.3.53,.53.63,.63.69, and.692.8^m; and then cmpared wth the bserved data. An average spectral dstrbutn ver Car s als develped thrugh ths mdel fr (UV/G). 2 Methd f calculatn The drect nrmal rradance f the sun at grund level fr any wavelength A s gven by: I\ = h\ ~ T \ T a \ T W \ T OX T U \ where; r rx exp {M/(A 4 ( /A 2 ))}, r ax = exp(/?am r a ) T WX = exp {.2385 a wx U' w M r /( a wx U' w M r ) A5 } VI = [{exp( T a )}/1] (P/P f 75 (TJT) ' 5 T OX = exp(a X OZ M ) M r = 35/(1224cs 2 Z + l) 5 as Rgers (1967) M = M T {P/P) smlarly the dffuse rradance s gven by: x = {r x + a x + g x ) x where; r x ~ / A cs(z) T OX T HX T WX T aa (1. r ra )/2. a X = I O \COS(Z) T X T ux T w x T aax T r,\(l T as x)f] gx = (I x cs(z) + TX + x ) R sx R gx j{\ R sx R g x)

3 r aa \ = exp{(l UJ\) r ax M r ) r asx = exp(u; A N r T ax ) F s = 1.5exp{(AF s + BF 3 cs{z))cs(z)} where AF 3 = [ A G( ALG.4129)} and BF S = ALG{$.m + ALG{M24~ALG.5874)} R.x = <A < A ri. A [.5(1 r rx ) + (1 Then the glbal spectral radatn can be frmulated as A cmplete lst f nmenclature s gven at the end f the text. The mdel was appled wth nterval.1 fm and the rradance at fur man bands were ntegrated. These bands M.2.53)/«w; 6 2 (.53.63)/m, & s (.63.69)^m and & 4 ( )/m, whch are representng the bands f Eppley precsn spectral pyranmeters peratng n a regular way n the perd ( ). The sensrs f these pyranmeters were checked at shrt regular perds. 3 Results and dscussn The mdel was tested fr the perd ( ) and the spectral drect and glbal radatn were calculated frm the abve mdel. As the mdel allws t many nput parameter the annual varatn and atmspherc turbdty were swtched n ths mdel. The results f the mdel were cmpared t measured rradance n these bands n rder t verfy the accuracy f the mdel. Tw ndces were used fr these verfcatn rt mean square errr (RMSE) and mean bas errr. In Table 1 the RMSE and mean bas are gven fr glbal and drect fur spectral bands.

4 Table 1 Mdel perfrmance n seasnal bases upper values rt mean square errr lwer mean bas errr n % t the mean measured truncated t nearest nteger. Band Seasn Wnter Sprng Summer Fall Glbal A rect h h As Table 1 presents the sum f the errr analyss n a seasnal way, n the bands (b 2,b3 and 63) the RMSE values are lwer n summer than n wnter whch s nearly expected as the majr weather dsturbance exsts. An ncrease n RMSE wth zenth angle had been ntced an example n Fg.l, ths culd be attrbuted t path length ncrease and cnsequently the ncrease f bth absrptn and scatterng f slar radatn. Fr glbal spectrum there s an mprvement n the mdel perfrmance n bands (63, 6 4 ) but fr the rest there were nsgnfcant changes as n Fg.2. On the ther hand, fr drect beam rradance the nly changes that had been realzed were n the UVb&nd where at lw values f precptable water (pw) an errr f 32% had been recrded when the pw value was dubled (bcm) the errr reduced t 27%. The analyss f spectral bands ndcated that drect beam has very lttle cntrbutn n the glbal radatn n the shrtest wavelengths, t wll cntnue ncreasng t be equal t dffuse cmpnents at wavelengths.37,.53 and Q.72ftm fr ar masses 1,2 and 5 respectvely. It was als ntced that an ncrease f drect cmpnent va wavelength s mre rapd n wnter than ther seasns. In fact the man slar radatn ntensty les n the range /m.e. any dfference n the bundares f lne spectrum has relatvely neglgble errr at the ntegral spectrum. An example f the deplatn nsde the atmsphere s knwn fr June 1984 where the spectral cmpnent fr extraterrestral, glbal, dffuse and drect were presented n Fg.3. The energy f UV band n the dffuse cmpnent represents 8% f the glbal rradance, whch ndcates that scatterng n UV band (fej) s the majr factr rather than absrptn. The utput f the mdel n UV t the glbal was used n mnthly bases t check the general trend 3 * m ~

5 n UV band, ths was cmpared wth the measured fr the sake f mdel verfcatns. In Fg.4 the devatn f ths rat frm the mean value t avd the annual cycle was reprted fr bth measured and predcted va mnths. As t s clear frm the fgure the mdel can predct values clse t the measured nes, see fr example at mnth number 17 and 3. There s n evdence fr changng the rat (UV/G) n lng term and the shrt term fluctuatn des nt absrb any defned regularty. We d beleve that the rat (UV/G) reduces the errr n calculatn n a better way than each cmpnent separately. The spectrum frm (.3 3.9)/m s gven fr ar mass (m = 1 and m = 2) fr hrzntal drect beam and dffuse cmpnent s gven n Tables 2 and 3. It s clear hw the mdel s senstve t ar mass varatn where > change n UV wth seasn and ar mass up t dfferent wave lengths. 4 Cnclusn 1) The present mdel can be smulated t measure the average ntensty f dfferent bands t the glbal ne wth very hgh accuracy. 2) On mnthly bases there s n sgnfcant evdence fr ncreasng ntensty f slar spectral r cnversns frm band t anther especally n UV band. Acknwledgments One f the authrs (M.M.A.W.) wuld lke t thank Prfessr Abdus Salam, the Internatnal Atmc Energy Agency and UNESCO fr hsptalty at the Internatnal Centre fr Theretcal Physcs, Treste.

6 >... NOMENCLATURE a \, a w\, a u\ zne, water vapur and mxed gases absrptn ceffcents /?, a Angstrm turbdty parameters u>x r \, a \, g \ F sngle scatterng albed prtn f dffuse radatn due t Raylegh, aersl scatterng and multple reflectn rat f aersl frward t ttal scatterng T O \, Tw\, T U \, T Q A, T T \ s the ptcal depth fr zne, water vapur, mxed gasses, aersl and Raylegh scatter.

7 REFERENCES Brd, R.E. (1984): A smple slar spectral mdel fr drect nrmal and dffuse hrzntal rradance. Slar Energy, 32, Brd, R.E. and Carl Rdam (1986): Smple slar spectral mdel fr drect and dffuse rradance n hrzntal and tlted planes at the earth surface fr cludless atmsphere. J. Appled Meterlgy 96 Guzz, R., I.G. L Vecch, R. Rzz and G. Scalabrn (1983): Expermental valdatn f a spectral drect slar radatn mdel. Slar Energy, 31, Iqbal, M. (1983): An ntrductn n slar radatn. Academc Press, p.31. Justus, G.G. and M.V. Pars (1985): A mdel fr slar spectral rradance at the bttm and tp f cludless atmsphere. J. Appled Meterlgy, 95, Leckner, B. (1978): The spectral dstrbutn f slar radatn at the earth surface. Slar Energy 29, Rgers, C. (1967): The radatve heat budget f the trpsphere and lwer stratsphere, Rept. N.12, Planetary Crculatn Prject, epartment f Meterlgy, MIT.

8 Table 2 Spectral dstrbutn f drect (///) and dffuse () at mr = 1 n seasnal bases (KJM** 211 r 1) Seasn Wavelength.3u Wnter ' Sprng Summer Fall b.ll

9 Table 2 (cntnued) Seasn Wavelength Wnter Sprng Summer Fall

10 Table 3 Spectral dstrbutn f drect (///) and dffuse () at mr = 2 n seasnal bases (KJM"* 211 r 1) Seasn Wavelength Wnter Sprng Summer Fall C U.I A

11 Table 3 (cntnued) Seasn Wavelength Wnter Sprng Summer Fall

12 Fgure 1: ependence f RMSE upn zenth angle J 3 u / 'zenth. 'zenth.b4' 25 H A t zenth angle 1 6 7

13 Fgure 2: ependence f RMSE% upn zenth angle t 'zenth. bl' 'zenth. b2' K^H 25 NJ CO n zenth angle 5 6 7

14 n m ex u LO r rs; en CM CM CN CM 13

15 .25.2 Fgure 4: Mean devatn f (UV/G) va mnth (Jan.'83ec.'85) r 'measured' 'predcted' _L mnth