5o/ar F.~rgy Vol. 21. pp. 351-352 0038-092d78/1001-0351/$02.00/0 © Penltmon Press Ltd., 1978. Printed in Great Britain

TECHNICAL NOTE

Tota l s o l a r i r r a d i a n c e a t T a b l e M t n , C a l i f o r n i a 1926--777

R. C. WILLSON-'[: and C. P. BUTLER§ Jet Propulsion Laboratory of the California Institute of Technology, Pasadena, California, U.S.A.

(Received 24 April 1978: in revised form 22 May 1978: received/or publication 30 May 1978)

The Smithsonian Astrophysical Observatory measured the total solar irradiance on a regular basis at Table Mtn, California from 1926 to 1952. The Table Mtn site, 100 kin. NE of Los Angeles at an elevation of 2460 m, was chosen for its exceptional observing conditions. It has a high incidence of clear sky and the water vapor content of the overlying atmosphere usually remains constant while observing. Located at the eastern edge of the Angeles National Forest adjacent to the Mojave desert, solar viewing at the site is protected from incursions of smog-laden air from the Los Angeles basin by the combination of high interven- ing mountains and the normally lower altitude of the pollution- trapping inversion layer.

The Smithsonian observations were made on a year-round basis on every clear day. Measurements of total and spectral irradianee were made at solar zenith angles corresponding to sec Zo = 2.50, 2.20, 2.00, 1.80 and 1.50. The principal purpose of the Smithsonian observations was the derivation of the time dependence of the solar total and spectral irradiance outside the atmosphere, and its correlation with changing patterns of the earth's climate. The project was terminated in 1952 when the SAO¶ became convinced that un- certainties introduced by unobservable UV and IR regions of the solar spectrum together with atmospheric variability exceeded the scale of solar variations of climatological significance[l].

The relationship between the Smithsonian 1913 scale, to which their observations were referenced, and the International System of Units (S.I.) was well defined by 1973124]. The 1913 scale is 2.5 per cent higher than the SAO water flow pyrheliometer realization of the S.L published in 1932. The 1913 scale was retained after 1932 for continuity of data, but with the under- standing that it exceeded their best S.I. realization[l, Vol. 5]. Using new cavity pyrheliometers developed in the last decade to define the radiation scale in S.I. units, it has been determined that the SAO 1932 water flow scale realization is within -+0.5 per cent of the S.I. [4, 5].

The S.I. traceability of the SAO data facilitates its use in studying potential variations in the solar "constant"--earth at- mosphere transmittance product over a long time base. P. Butler, one of the Field Directors at Table Mm. for the SAO, recognized this possibility and suggested the effort to the Jet Propulsion Laboratory, present operators of the observatory. .

Researchers at JPL have used the Table Mtn site frequently since the mid 1960's for radiometer comparison tests, as part of a basic research effort in the development of a new generation of cavity pyrheliometers[6, 7]. No systematic effort has been made since 1952, however, to continue the specific observation pro- gram employed by the SAO.

tThis paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS7-100, sponsored by the National Aeronautics and Space Administration.

~Member of Technical Staff. §Research Affiliate. ISmithsonian Astrophysical Observatory. The traditional

Smithsonian acronym is APO. ICommission on Instruments and Methods of Observation of the

World Meteorological Organization.

In July 1977 pyrhelinmetric measurements were resumed at Table Mtn at the same set of solar zenith angles used by the SAO for solar constant observations. The purpose was to determine whether a significant irradiance change had occured at the site since the SAO observation period. Measurements of solar irradiance were made by instruments calibrated at Table Mta by comparison with Willson's Active Cavity Radiometer (ACR) No. 310, whose uncertainty in S.I. units has been shown to be less than -+0.3 per cent[8]. ACR 310 is one of the five instruments recognized by the CIMO of the WMO J that define the World Radiometric Reference Scale (1976)[5, 9]. The SAO 1932 scale is known to lie within -+ 0.5 per cent of the S.I. as defined by ACR 31014].

The sequence of observations made at Table Mtn during July 1977 was identical to that by Smithsonian observers for a "short method" day. Only those days were chosen when the sky was free of cirrus clouds and layered haze around the sun. Obser- vations were made each day at four elevations of the sun above the horizon, corresponding to sec Zo---2.50, 2.20, 2.00 and 1.80. The SAO made more solar irradiance observations at a solar

zenith angle (Zo) of 60 ~ than at any other over the years. Here we compare their results and ours for Zo = 60 ° as the most represen- tative subset of all data. The mean direct solar irradiance at Table Mtn, California for all SAO data taken in July at 7_,o = 60 ° from 1926 through 1952 is 985 W/M 2 with a standard error of + 1.0 per cent (S.I. units). The maximum and minimum values of .the SAO July means are 1015 and 933 W/M 2, respectively. The mean number of SAO observations in the month of July over the 27 year period is eleven.

The mean direct irradiance in July 1977 was 994 W/M 2 with a standard error of -+0.9 per cent (S.I.). The total number of observations in July 1977 at Zo = 60 ° was 15.

CONCLU~ONS The mean direct solar irradiance at Table Mta for July over

the 1926-77 period has not changed by more than the standard errors associated with the means of the SAO and JPL obser- vations. There is, therefore, no evidence of change in the at- mospheric transmission-solar "constant" product exceeding about -+ 1 per cent. The limitations of the single month of 1977 data are recognized and a continuation and expansion of the measurement program is planned. In it we will use the oppor- tunity afforded by upcoming JPL space flight experiments, in which ACR's will determine the total solar irradiance outside the earth's atmosphere with -+0.1 per cent S.I. uncertainty, to determine the atmospheric transmittance for total solar flux [10].

REFERENCES

I. Annals of the Astrophysical Observatory of the Smithsonian Institution, Vols. I-7, Smithsonian Institution, Government Printing Office, Washington, 1900. 1908, 1913, 1922, 1932, 1942, 1954.

2. R. C. Willson, Experimental comparisons of the International Pyrheliometric Scale with the absolute radiation scale. Nature 239, 208 (1972).

3. J. R. Latimer, On the Angstrom and Smithsonian absolute pyrheliometric scales and the International Pyrheliometric Scale 1956. Tellus 25, 586 (1973).

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352 Technical Note

4. C. Fr6hlich. The relation between the IPS now in use and Smithsonian Scale 1913, Angstrom Scale and Absolute Scale. Proc. Syrup. Solar Rad., 13-15, Nov. 1973. Rockville, Maryland (1975).

5. C. Fr6hlich. The New International Pyrheliometric Reference Scale. Report of Working Group on Radiation Measurement Systems. Commission for Instruments and Methods of Observation. World Meteorological Organization (1976).

6. R. C. Willson, Radiometer Comparison Tests. JPL Pub- lication 900-446, Jet Propulsion Lab, Pasadena, California (19711.

7. R. C. Willson, Results of the 1972 Table Mtn. Radiometer and raation scale comparisons. In[ormal JPL Rep. Jet Pro- pulsion Lab, Pasadena, California (19721.

8. R. C. Willson, Active cavity radiometer. Appl. Opt. 12, 810 (1973).

9. C. Fr6hlich. Private Communication (1977). 10. R. C. Willson, Current pyrheliometry for total solar it-

radiance observations. Proc. Electro-Opt./Laser 77 ConL and Expos., Anaheim, California, Oct. 1977. p. 430 (1978).