An analysis of global radiation measurements and fractional cloud cover observations made in the Israel Meteorological Service’s network of climate stations demonstrated a significant decrease in the transmittance of solar radiation through the atmosphere during the last 60 years. The major cause was the reduced transparency of clouds. Under completely overcast skies with complete cloud cover transmission in the industrialized central coastal region decreased from 0.41 in the mid-20 th century to 0.21 in the first decade of the 21 st century. Under cloudless skies the reduction in the transmission of global radiation was less, from 0.79 to 0.71, and not statistically significant. Similar but somewhat smaller changes were observed in the less industrialized central hill region. Multi-linear analysis showed that since 1970, 61% of the measured decline in global radiation was attributable to changes in fractional cloud cover but only 2% to the marked increase in local fuel combustion; there was no statistically significant interaction between the two parameters.
The first reports of widespread and significant changes in the solar irradiance at the Earth’s surface [
In the present study we examine the effect of the fractional sky cover and the transmissivity of clouds, a major factor influencing Eg↓, based on the changes measured in Israel during the last 60 years.
The importance of clouds was apparent in an analysis of Eg↓ measurements in Israel’s industrialized central coastal plain which showed that the trend in global dimming was smaller during cloudless days and seasons than during all sky conditions [
Over the Eastern Mediterranean a simulation study of radiation transfer during the 1983 to 2013 period showed that on an average annual basis the effect of clouds, aerosols and water vapor reduced Eg↓ by 63 W∙m−2, 18 W∙m−2 and 9 W∙m−2 respectively, accounting for 70%, 20% and 10% of their combined radiative effect; it should be noted that the simulation used a constant aerosol load [
A study of changes in net solar radiation over the entire Mediterranean basin, based on the GEOS-5 climate model processing of satellite data, indicated that between 1970 and 2012 spatial and temporal trends were primarily controlled by variations in cloud optical depth, although the analysis was unable to distinguish between the roles of the extent of cloud cover and cloud radiative properties [
On a global scale the onset of global brightening in the 1980’s [
A major difficulty in distinguishing between the role of aerosols and clouds as the cause of trends in global radiation is due to their complex interaction [
In this study we make this distinction using a simple statistical approach to separate aerosol effects on Eg↓ into the direct effect observable under cloud-free skies and the indirect effects which include the influence of aerosols on the formation, magnitude and duration of clouds as well as on their radiative properties, i.e. reflection and absorption of solar radiation.
Our analysis is based on measurements of global radiation and observations of total cloud cover made at climate stations in Israel during the 60-year period 1954-2014, supplemented with data of total fuel consumption since 1970 used as a proxy measure to quantify the effects of local emissions of anthropogenic aerosols.
Measurements of Eg↓ were made with regularly calibrated thermopile pyranometers at the 23 sites in Israel shown in
represented by the standard error, varied between 248 ± 6 W∙m−2 in the first decade of measurement to 225 ± 4 W∙m−2 in the last decade. Measurements of Eg↓ were subject to the quality control procedures recommended by the World Meteorological Organization and have been corrected to the current World Radiometric Reference scale [
In addition to mean annual values of Eg↓ three series of mean monthly values normalized to their extra-terrestrial values, i.e. as Clearness Indices CI [
Additional details of the Eg↓ measurements made before 1995 can be found in Stanhill and Ianetz [
Observations at 08, 14 and 20 Israel Standard Time, were analyzed as mean monthly values after conversion from oktas to fraction of sky covered C. Six stations in the Israel Meteorological Service’s climate station network were selected to represent the five major climate regions on the basis of completeness and quality of the observations and the data was subject to the quality control procedures recommended by the World Meteorological Organization. Locations of the stations are shown in
In the absence of land based or satellite observations covering the 60-year period under study, national statistics of monthly values of total fuel consumption in units of TOE, thousand tons of oil equivalent F, were used as a proxy for the anthropogenic aerosols emitted by local fossil fuel combustion. The data is available from the Central Bureau of Statistics at http://www.cbs.gov.il/energy/new.enr.nach.eng.new.huz.html.
After 2000 measurements of aerosol optical depth, AOD were available from the MODIS Terra satellite for a 1˚ pixel centered on Israel (http://giovanni.gsfc.nasa.gov/giovanni/#service=ArAvTs&starttime=2000-03-01100.00:00Z&endtime+2016-04-30T23:59Z&bbox+39.6948.31.5857,35.4858,32.4866&data=MOD008).
Annual mean values of global radiation averaged for all available sites are presented in
E g ↓ = − 0.49 ± 0.055 N + 1205.8 , R 2 = 0. 56 , P < 0.00 1 ( 1955 - 2 0 13 )
C = − 0.000803 ± 0.000255 N + 1.895 , R 2 = 0.23 , P < 0.08 ( 1955 - 2013 )
F = 165 ± 23 N − 32150 , R 2 = 0.91 , P < 0.0001 ( 1970 - 2013 )
There was no significant difference between trends in Eg↓ measured at Bet Dagan and in Jerusalem [
Analysis of variation of the multi-linear relationship between Eg↓, C and F over the 1970 to 2013 period for which data for all parameters was available indicated that the relationship of Eg↓ to C and F was highly significant while the interaction between C and F was not. After removing the interaction term the coefficient of determination was 0.25. Replacing F with log10F raised the value of R2 to 0.31; justifications for the use of a logarithmic scale for fossil fuel consumption are discussed in Section 4.1.3.
The final relationship was,
E g ↓ = ( − 116 .2 ± 81 .4 ) C − ( 31 .97 ± 15 .1 ) log 10 F + 381 .6 ± 74 .2 , R 2 = 0.31 , P < 0.001
After normalization to the mean annual extra-terrestrial value of solar radiation, 360 W∙m−2, the relationship for clearness index CI, illustrated in
C I = ( − 0.371 ± 0.26 ) C − ( 0.102 ± 0.048 ) log 10 F + 1.219 ± 0.24 , R 2 = 0.31 , P < 0.001
The multi-linear analyses of monthly values were based on Eg↓ measurements from Bet Dagan as this was the only site with an almost complete series of monthly values for the 1970-2013 period analyzed. The use of data from this site as a proxy for the national mean is justified by the highly significant correlation between the two series (P < 0.01) and the near unity of their slope, 1.004. Values of Eg↓ were converted to CI to remove the major effect of seasonal variation in Sun-Earth geometry. As was the case with the annual values, analysis of variance of the multi-linear relationship indicated that the interaction between cloud cover and fossil fuel consumption was not significant, R2 = 0.003, P > 0.23. After eliminating this term stepwise regression indicated that cloud and local fossil fuel consumption together accounted for almost two thirds of the inter-monthly variation in CI (R2 = 0.63) with clouds accounting for 0.61 of the variation and fossil fuel consumption only adding another 0.02 to the coefficient of variation. The multi-linear relationship for monthly values, illustrated in
C I = ( − 0.511 ± 0.034 ) C − ( 0.0606 ± 0.023 ) log 10 F + 0.920 ± 0.062 R 2 = 0.63 , P < 0.0001
A comparison of the clearness index CI with observations of fractional cloud cover C is shown in
C I = − 0.471 C + 0.756 , R 2 = 0.51
as by the quadratic equation
C I = − 0.087 C 2 − 0.405 C + 0.744 , R 2 = 0.51
both relationships are highly significant (P < 0.01) and by extrapolation yield similar values for cloudless skies (i.e. C = 0), CI = 0.76 and 0.74 respectively, as well as for overcast, completely cloud covered skies (i.e. C = 1), CI = 0.28 and 0.25 respectively.
Time trends during the entire 1956 to 2013 period of measurement were examined by repeating the linear analyses for each of 10 successive periods of six years, this period was selected to provide sufficient data to yield statistically highly significant relationships (P < 0.01). The parameters of the linear regressions together with extrapolated values of normalized Eg↓ for both cloudless and completely overcast skies, are presented in
Period | Slope | Intercept | R2 | Sky transmission | |
---|---|---|---|---|---|
Cloudless | Overcast | ||||
CENTRAL COASTAL PLAIN, BET DAGAN. 1956-2013 | |||||
1956-1961 | −0.377 | 0.79 | 0.63 | 0.79 | 0.41 |
1962-1967 | −0.439 | 0.759 | 0.51 | 0.76 | 0.32 |
1968-1973 | −0.482 | 0.779 | 0.77 | 0.78 | 0.3 |
1974-1979 | −0.541 | 0.789 | 0.74 | 0.79 | 0.25 |
1980-1985 | −0.424 | 0.708 | 0.58 | 0.71 | 0.28 |
1986-1991 | −0.544 | 0.775 | 0.71 | 0.78 | 0.23 |
1992-1997 | −0.536 | 0.761 | 0.66 | 0.76 | 0.23 |
1998-2003 | −0.518 | 0.731 | 0.61 | 0.73 | 0.21 |
2004-2009 | −0.548 | 0.78 | 0.64 | 0.78 | 0.23 |
2010-2013 | −0.564 | 0.773 | 0.58 | 0.77 | 0.21 |
1956-2013 | −0.471 | 0.756 | 0.51 | 0.76 | 0.28 |
CENTRAL HILL REGION, JERUSALEM. 1954-2014 | |||||
1954-1963 | −0.336 | 0.774 | 0.96 | 0.77 | 0.44 |
1968-1975 | −0.241 | 0.732 | 0.48 | 0.73 | 0.49 |
1986-1995 | −0.431 | 0.722 | 0.85 | 0.72 | 0.29 |
1996-2005 | −0.388 | 0.75 | 0.89 | 0.75 | 0.36 |
2006-2014 | −0.475 | 0.776 | 0.91 | 0.78 | 0.3 |
THREE SITES, 1953-1961 (for details see Stanhill, 1962) | |||||
1953-1961 | −0.262 | 0.771 | 0.56 | 0.77 | 0.51 |
THREE SITES, 1992-1994 (for details see Stanhill and Ianetz, 1997) | |||||
1992-1994 | −0.464 | 0.729 | 0.8 | 0.73 | 0.27 |
S = − 0.00275 Y + 4.967 , R 2 = 0.617 , P < 0.01
The intercepts of the relationships, that is, the transmission of completely clear, cloudless skies, also decreased with the mid-year of measurement but the decrease was small and not statistically significant.
The results of a comparison of monthly measurements of CI and observations of C made at three sites in Jerusalem between 1954 and 2014 are presented in
A trend of decreasing CI was found for completely overcast skies, S = −0.00298Y + 6.28, R2 = 0.546, P > 0.15 which, although non-significant, was similar to the trend found in the Central coastal plain, as was the much smaller change in the transmission of completely cloud free, clear skies.
A highly significant linear relationship between the 94 monthly values of CI and observations of C measured between 1953 and 1961 is presented in
A comparison of the relationship between 1953 and 1961 with that derived from a group of measurements made at matched sites between 1992 and 1994 [
Under overcast sky conditions CI was highly significantly inversely related to the fossil fuel combust ion (F in units of MTOE) both on logarithmic and linear scales as shown in
C I c l o u d = − 0.1574 log 10 F + 0.8349 , R 2 = 0.732
C I c l o u d = − 0.0114 F + 0.3135 , R 2 = 0.697
Under cloudless skies the relationships were not statistically significant:
C I c l e a r = − 0 .2811 log 10 F + 0.8608 , R 2 = 0.019
C I c l e a r = − 0.0017 MTOE + 0.7728 , R 2 = 0.019
Over the 1970-2013 period the relationship CIcloud = −0.0114F + 0.3135, on a unit area basis (total area 20,770 km2) indicates that the combustion of each unit TOE, by definition equivalent to 41.868 GJ, reduced Eg↓ by an average of 4.1 W∙m−2.
Under cloudless conditions both the log and linear relationships indicate much smaller effects of fuel combustion which were not statistically significant.
At the start of the period studied the accuracy of daily values was assessed at 5% [
The spatial representativeness of the mean of values of Eg↓ measured at individual sites, which was related to the national mean data of cloud cover and fossil fuel consumption to derive the relationships presented in Sections 3.2 and 3.3, was assessed as the standard error of the national means of Eg↓. Thus, the uncertainty in the areal mean varied between 2.4% for the first decade of measurements to 1.7% in the last decade of the previous century, values similar to the 2% absolute mean error found in a study comparing measurements at 778 sites with that of their surrounding 3˚ grid area [
In the absence of a local objective measurement series of known accuracy it is not possible to assess the accuracy of the standard subjective synoptic observations of cloud cover used in this study. However the combined effect of between observer variability [
The use of local data of primary fuel consumption as the proxy for aerosol load assumes that the aerosol load produced bears a constant relationship to the advected aerosols. Another limitation is the neglect of changes and trends in the type and composition of the fuels used and in the efficiencies of the combustion processes; changes that can be expected to have led to a reduction in the aerosol emissions per unit ton of oil equivalent. The use of log10F values in Equations (1) and (2) is justified by the logarithmic sensitivity of cloud properties to aerosol load [
The values of the cloudiness index for cloudless and cloud covered skies in Israel found in this study as listed in
Replacing the subjective synoptic observations of cloud cover at Bet Dagan with measurements of sunshine duration increased the coefficient of determination in the relationship with normalized global radiation to R2 = 0.81; similar high coefficients of determination were found at five other sites covering a wide range of climates and aerosol emissions. At Bet Dagan and the two other urban sites the transmission of cloud was found to decrease with time [
Trends in the direct and indirect aerosol effects derived from the intercepts and slopes of the relationships between CI and C shown in
By contrast the increase in the indirect aerosol effect over the 60 years was significant. The decrease in cloud transmittance is highlighted by a comparison of the values listed in
cloud covered skies transmitted half of the top of atmosphere irradiance; during the later period only a third was transmitted.
Similar results showing major changes in cloud transmission and minor changes in clear skies were reported in an analysis of high frequency direct and diffuse solar radiation measurements at seven USA sites between 1996 and 2011 [
The significant relationship between cloud transmission and fossil fuel consumption in Israel during the period under study shown in
Changes in cloud, both in the fraction of sky covered and in their radiative characteristics, played a major role in determining the global radiation measured in Israel during the last 60 years. Highly significant inverse linear relationships between normalized Eg↓ and cloud cover indicate that a reduction in cloud transmission occurred in both the central coastal plain and central mountain region with a much smaller change in the transmission of cloudless skies. Analysis by stepwise regression indicated that since 1970 changes in cloud cover accounted for 61% of the changes in Eg↓ while the major increase in local fossil fuel consumption, serving as a proxy for anthropogenic aerosol emissions, only accounted for an additional 2% of the changes. Although the interaction between cloud cover and fossil fuel consumption is not statistically significant the indirect aerosol effect demonstrated in this study suggests that an important microphysical interaction may exist.
We thank Amos Porat, Talia Horowitz and Vera Lybansky of the Israel Meteorological Service who provided the data on cloud cover and global radiation used in this study. The assistance of I. Lynsky of Tel Aviv University who supplied the MODIS measurements is gratefully acknowledged. We also thank Marcel Fuchs for useful discussion of the results presented in this paper.
The authors declare no conflicts of interest regarding the publication of this paper.
Paudel, I., Cohen, S. and Stanhill, G. (2019) The Role of Clouds in Global Radiation Changes Measured in Israel during the Last Sixty Years. American Journal of Climate Change, 8, 61-76. https://doi.org/10.4236/ajcc.2019.81004