Solar radiation at Rome, Italy, and its environment

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Again, latitude, sunshine duration and cloud cover are midway among the others in Italy, so that this zone can be considered as representative for many.
Arch. Met. Geoph. BioId., Ser. B, 27,205-212 (1979)

ARCHlV FUR METEOROLOGIE GEOPHYSIK UND BIOKLIMATOLOGIE © by Springer-Verlag 1979

551.521.1(45) Laboratorio di Chirnica e Tecnologia dei Radioelementi, C.N .R., Pad ova, Italy, and Centra Studi e Applicazioni Risorse Energetiche (C.S.A.R.E.), Venice, Italy

Solar Radiation at Rome, Italy, and Its Environment D. Camuffo With 3 Figures Received November 17, 1978

Summary Routinely collected data of solar radiation over a 12 year period have been studied for different zones near Rome. The region under consideration extends between Tyrrhenian Sea and Apennines Mountains and is characterized by three stations situated in urban, rural and mountain sites. Again, latitude, sunshine duration and cloud cover are midway among the others in Italy, so that this zone can be considered as representative for many regions of this country. Variations in time and space on the falling radiation, due to different climatological effects are evidenced. Averaged and extreme values of sunshine duration and global radiation are discussed, especially with reference to the local cloudiness. As the formation of orographic clouds over the Apennines cause a depletion on solar energy greater than the effect of a layer of about 2 km of turbid air over the plain, the latter seems more convenient in order to use solar energy for practical applications. Zusammenfassung Sonnenstrahlung in Rom und in seiner Umgebung In einer zwolfjiihrigen Beobachtungsperiode gewonnene Daten der Sonnenstrahlung werden flir verschiedene Gebiete nahe von Rom untersucht. Die in Betracht gezogene Region erstreckt sich zwischen Tyrrhenischem Meer und den Apenninen und ist durch drei Stationen charakterisiert, die in der Stadt, im Freiland und im Gebirge gelegen sind. Da die geographische Breite, die Sonnenscheindauer und die Bewolkungsverhiiltnisse dem Mittel von Italien ungefahr entsprechen, kann dieses Gebiet auch als repriisentativ flir viele andere Regionen des Landes betrachtet werden. Zeitliche und riiumliche Anderungen der einfallenden Strahlung zufolge verschiedener klimatologischer Auswirkungen werden dargelegt. Mittel- und Extremwerte der Sonpenscheindauer und def Globalstrahlung werden besonders mit Bezug auf die lokalen BewolkungsverhaItnisse besprachen. Da die Bildung orographischer Bewolkung liber den Apenninen eine Verminderung der Sonnenstrahlung bewirkt, die groBer ist als die Wirkung einer ungefiihr 2 km miichtigen Schicht trliber Luft liber der Ebene, scheint letztere fLir die Verwertung def Sonnenenergie flif praktische Anwendungen glinstiger zu sein.

0066-6424/79/0027/02051$ 01.60

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D. Camuffo

1. Introduction As the Sun is the principal source of energy on the Earth, differences in the heat balance between ground and atmosphere cause variations on meteorological effects and climate. These in turn are responsible for the amount of falling radiation. Thus, together with the quantity of solar energy available for practical utilization, all the related meteorological phenomena are to be examined. Again, with the knowledge of the local climatology, the more convenient fields and ways of utilization can be recognized. With this purpose we will discuss here the averaged and extreme values of solar radiation over a 12 year period at Rome, Italy, together with their time and space variations. The data are routinely taken by the Meteorological Service of Italian Air Force and yearly published by ISTAT [6]. The data relative to Rome and its hinterland have been especially chosen for the following reasons: (i) the latitude and in addition sunshine duration and cloud cover are midway among the others in Italy, as evidenced by Cicala [2], Guerrini Lavagnini Vivona [4, 5] and Camuffo [1]; (ii) the region extends between the sea and Apennines like almost all the regions in Italy; (iii) moreover the urban site, the rural area and the mountain site near Rome are characterized by a sufficient series of data; (iv) the distribution of global radiation, which is the most important parameter for practical applications, was never discussed for this region. Thus a study of this zone can be usefully considered as representative for many regions in Italy by people interested in problems of solar energy utilization.

2. Data Analysis The altitude of the Sun, i. e. the angular elevation above the horizon, is determined by astronomical factors and it is of primary importance in order to compute the solar energy falling on a tilted surface, and in particular on a solar panel. It is to be noted that the solar elevation (X at noon ranges at Rome between 0 24 30' and 71 °30', so that the introduction of non linear effects due to the optical thickness of the atmosphere causes a strong seasonal dependence on the falling energy. As the sunshine duration is not important as the global radiation when the aim is the use of solar energy, we will limit here the discussion to the outstanding conclusions. The largest scatter of data regarding the sunshine duration is observed during the transition seasons, characterized by weather conditions which are less stable than during summer or winter. However, during summer the situation appears to be rather different from the corresponding one during winter for two reasons. The first is that during winter

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the sunshine duration is only a small fraction of the daylight which obviously represents the maximum duration theoretically possible. The second one, as a consequence, is related to the distribution of the extreme monthly totals. In fact, during winter, the mean is not midway between the extreme values, but generally more close to the lowest ones, thus making the highest ones an exception, as it generally does for the northern countries. On the contrary, during summer the lowest limits more than the highest ones, seem to be characterized by exceptions, as for the southern climates. Again, the sunshine duration is outphased with reference to the corresponding astronomical quantity, thus indicating a delay of the solar-induced meteorological effects. In addition to the regular, astronomically induced changes on the duration of daylight, the atmosphere too is effective in obscuring the sun, so that the ratio of sunshine to daylight duration is a very expressive and useful parameter. This is represented in Table I in a percentage unit. The sunshine duration is shown ranging between less than half the possible maximum during winter and 3/4 during summer. The cited delay and the quick variation during autumn are evident. Summer appears as the more convenient period to receive and accumulate solar energy. However, besides the seasonal trend, the variation with topography of sunshine duration is to be also considered. Clearly the sunshine depends on local climatology, e. g. the increase in atmospheric turbidity due to the human activity in urban areas, the development of cumulus clouds in proximity of the coast when the sea breeze is developing, on the formation of slope or Lee clouds when the humid air overflows the mountains. In Fig. I the data relative to an urban site (Rome), rural area (Vigna di Valle) and mountain (Terminillo, 1875 m over m.s.l.) are reported. The sources of reference are the already cited ones. The durations of sunshine at Rome and Vigna di Valle are very similar. The small variations can be ascribed to the heat island effect due to the presence of the city: during summer in enhancing the formation of evaporative cumulus clouds and during the mid-season in reducing mist. However, over Terminillo the situation is quite different: the orographic clouds noticeably reduce the sunshine. A comparison between the sunshine durations at the different places gives us some information about the solar radiation falling on the region under examination. However, if the three sites under consideration can be characterized by some differences in their own climate, it follows Table 1. Monthly Variation of the Ratio of Sunshine to Daylight Duration at Rome. Percentage Unit Jan.

Feb. March April

50

48

60

56

May

June

July

Aug.

Sept.

Dct.

Nov.

Dec.

62

65

76

75

68

62

45

40

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208

that the amount of energy reaching the ground depends also on the type of associated meteorological phenomena. It is evident that the amount of solar energy reaching the ground is dependent on the cloud cover, as well as the sunshine duration, which is a parameter more easy to measure even if less useful. However, a linear relationship between sunshine S and solar global energy G reaching the earth can, but only poorly, characterize a site. As an example Cicala [2) found for Vigna di Valle G = 18(5 + S) during winter

10

8

4

2

f m a m j

a son d

Fig. 1. Sunshine duration at Rome (dot), on a rural site (Vigna di Valle, white star) and on the mountain (Terminillo, 1875 m over m.s.l., black star)

and G = 36 (7 + S) during summer. One notes that the same coefficients for the summertime for Vigna di Valle (43 0 lat North) were used for Venice (45 0 lat North) and Trapani (38 0 lat North). Since the falling energy is strongly dependent upon both solar altitude and atmospheric optical thickness, the meteorological effects result in being more effective in causing variations in terms of energy rather than in sunshine duration. This is shown in Table 2 and Fig. 2, where the intensities of solar radiation on an horizontal surface (ly day-I) at Rome and over its hinterland respectively, are reported. The largest variations happen during summer, due to the development of cumulus clouds in the early afternoon. Even if short-lived, the cloud activity being caused by strong solar radiation can determine in turn considerable effects on falling solar energy. This is

209

Solar Radiation at Rome, Italy, and Its Environment

the reason of the wide range of variability both in time (Table 2) and in space (Fig. 2) during summer. Thus the atmospheric turbidity due to antropogenic activity appears to be responsible for the differences in falling energy over Rome and Vigna di Valle, whereas the sunshine durations are quite similar. The slightly favourable situation at Rome during March and November can be correlated to the slightly greater sunshine duration, as discussed with reference to Fig. 1.

600 >.

III

"C

400

>.

200

fmamj

jasond

Fig. 2. Daily global solar radiation on a horizontal surface on the urban, rural and mountain sites. Symbols as in Fig. 1

In absence of clouds energy is depleted from the direct solar beam through absorption and scattering by air molecules, water vapour, dust and smoke. Hence a great advantage is expected in receiving radiations at elevations above the mixed layer, which concentrates the largest amount of atmospheric turbidity. Practically this may be accomplished with mountain stations. However, the up-lifting of humid air over slopes and orographic disturbances can determine the formation of clouds. These in turn cause the largest depletions on the falling solar energy. This is the case of the Table 2. Mean and Extreme Monthly Totals ofDaily Global Solar Radiation (in Langley per Day) on a Horizontal Surface at Rome, During a 12 Year Period (1 Langley = 1 cal/cm 2 =41868 Joule/m2) Jan. max 168 mean 143 124 min

Feb. March April

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

224 198 163

575 477 385

577 517 437

580 532 475

500 465 393

384 355 292

275 252 209

164 149 133

143 114 99

349 286 226

436 387 345

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Terminillo, which is representative of the Apennines in the Roman hinterland. It is to be noted however that during winter, when synoptic disturbances prevail on local effects and the lower atmosphere reaches its maximum humidity, the balance between a more frequent cloud cover over Terminillo and the absorption on 1875 m of low atmosphere is but only little in advantage for the mountain site. The above data characterize the regularities of the monthly variation of radiant flux on a horizontal surface and are determined by solar height, duration of sunshine, hourly amount of cloudiness and atmospheric transparency in the site under consideration. However, these data represent only the relative seasonal distribution of radiation monthly totals expressed in units per day and cannot be considered as the radiation totals of each day during each month. Again, it is of primary importance to know how many days for each month we have in order to utilize direct solar energy. Table 3 presents the monthly distribution of cloud cover over Rome, by considering only clear and cloudy days. The used data are published by the Meteorological Office [1). During summer cloudy days are unusual and the general weather conditions indicate dominance of clear days and afterwards shortlived perturbations due to the development of the sea breeze. During winter and the mid-seasons the cloudy days are generally more frequent than the clear ones and the days with the variable cloudiness characterize about half of the period. It is to be noted that, even if in the yearly total the number of clear days appears to be quite large, i. e. 117 clear days and 74 cloudy days, during winter the situation is reversed. It has been mentioned above that cloud cover cause the largest variations on the falling solar radiation. Now transmission, reflection, diffusion and absorption of solar energy upon clouds depend on cloud type and thickness. As discussed by Fritz [3], the low clouds are the more effective ones in reducing solar radiation. Thus, in Fig. 3 the seasonal percentage frequency of the different heights of the base of low clouds during overcast conditions at Rome is shown. The observations relative to morning (0800 LST), mid day (1400 LST) and evening (1900 LST) are separately reported in order to outline the daily trend. During the whole year and notably in summertime the base of clouds is generally higher than 1500 m. In wintertime the base of low clouds appears to be subsiding during day, whereas in the other seasons and particularly during spring, the opposite tendency is noted. This

Table 3. Monthly Distribution of Clear and Overcast Days at Rome Jan. clear 6 overcast 7

Feb. March April 7 10

5 11

5 7

May

June

July

Aug.

Sept.

Gct.

6 7

16 3

20

18 0.8

11 3

10

0.8

8

Nov.

De(

8 7

5 9

Solar Radiation at Rome, Italy, and Its Environment

211

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900

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10

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20

30%

10

20

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b

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10

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20

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