an evaluation of the effect of volcanic eruption on the solar radiation at six canadian stations

Pergamon 0038-092X( 95)00066-6

Solar Energy Vol. 55, No. 6, pp. 513-525, 1995 Copyright 0 1995 Elsevier Science Ltd

Printed in the U.S.A. All rights reserved 0038-092X/95 %9.50+0.00

AN EVALUATION OF THE EFFECT OF VOLCANIC ERUPTION ON THE SOLAR RADIATION AT SIX CANADIAN STATIONS

JOHN GARRISON Physics Department, College of Sciences, San Diego State University, San Diego, CA 92182-0325, U.S.A.

(Communicated by RICHARD PEREZ)

Abstract-Hourly global and diffuse radiation data from the six Canadian stations at Edmonton, Montreal, Port Hardy, Resolute, Toronto and Winnipeg have been analyzed for evidence of the effect of volcanic eruption on the amount of direct beam radiation and diffuse radiation. A strong reduction of clear hour direct beam radiation is seen during 1983 due to aerosols in the stratosphere following the eruption of El Chichon ia southern Mexico on 28 March and 3 and 4 April 1982. A corresponding increase in diffuse radiation and Angstrom turbidity coefficient is also seen during 1983. The effect of the aerosols from El Chichon is much reduced during 1984, but is still seen above levels from other sources. By combining all significant evidence found in these Canadian solar radiation data, the presence of an excess of volcanic aerosols associated with one or more of the series of volcanic eruptions occurring during the periods from about 1963 to 1968, near 1975, and just prior to the eruption of El Chichon is clearly seen. The amplitude of the effect of these series of eruptions on the data selected for examination here is approximately one-third the amplitude of the effect found for El Chichon. This study confirms that global radiation shows little change when volcanic aerosols in the stratosphere increase following an eruption.

1. INTRODUCTION

A few large volcanic eruptions occur each century which cause a corresponding large increase in the aerosol content of the stratosphere. This increase in stratospheric aerosol content in turn affects the radiation balance of the earth. The eruption must be highly explosive and send large quantities of sulfur dioxide gas into the stratosphere to give rise to a large, long lasting global effect on the stratospheric aerosol content. The Katmai eruption in Alaska in 1912, the Agung eruption in Indonesia in 1963, the El Chichon eruption in southern Mexico in 1982, and the Pinatubo eruption on southern Luzon in the Philippines in 1991 all have had a strong global effect on the incoming solar radiation during this century.

In the stratosphere, sulfur dioxide from volcanic eruptions is converted mainly to sulfuric acid-water aerosols of the order of micron to sub-micron size in a time of the order of a month. Many of the smaller of these aerosols remain in the stratosphere for several years. Less explosive eruptions generally do not carry material above the troposphere and much of this material is washed out of the troposphere in a matter of weeks. Eruptions with lower sulfur dioxide content may not make enough sulfuric acid-water aerosols to have a sizable affect on the incoming solar and long wave- length infrared emission radiation. This evi- dently was the case for the highly explosive

eruption of Mount St. Helens in the State of Washington, U.S.A. in 1980 (Kerr, 1982, 1983). For Mount St. Helens, much of the material was ejected more nearly horizontally, also re- ducing the aerosol input to the stratosphere.

Scattering of solar radiation by the sulfuric acid-water aerosols in the stratosphere is largely in the forward direction and absorption is small. Low absorption and predominantly forward scattering by these aerosols makes the reduction in global radiation following volcanic eruption small, while the scattering by these aerosols can change an appreciable amount of the direct beam radiation into diffuse radiation. This can have an important depressing effect on direct beam radiation.

A number of observers have reported the effect of volcanic eruption on solar radiation. Viebrock and Flowers (1968) report the significant reduction in direct beam radiation and a corresponding increase in diffuse radiation at the South Pole following the eruption of Agung in Indonesia in 1963. There is little change in global radiation in these data. The large effect of the eruption of El Chichon on the solar radiation data at Corvallis, Oregon in the U.S. is reported by Nagaraja Rao and Bradley (1983). They observe about a 14% decrease in clearness index k,, an increase of about 2.3 in the ratio of diffuse irradiation to global irradiation, and about a 28% decrease in the transmittance of the atmosphere for direct irradiation for clear days relative to normal clear day values for the

513

514 J. Garrison

period from late November through December of 1982. McDaniels and Vignola (1984) also report the strong effect of El Chichon on the solar radiation in the Pacific Northwest of the U.S. The variation of atmospheric turbidity for the 27 yr from 1957 to 1983 at Tucson, Arizona is reported by Szymber and Sellers (1985). These results show the effect of the significant volcanic eruptions occurring during this period. Stoffel and Nelson (1993) also have reported a large reduction in direct beam radiation and a corresponding increase in diffuse radiation with little change in global radiation following the eruption of Mount Pinatubo in the Philippines in 1991. The reduction in direct beam radiation and corresponding increase in diffuse radiation following the eruption of Mount Pinatubo in the Philippines is also reported by Michalsky et al. (1994). In addition, they report the large reduction in solar energy collection by the highly concentrating tracking collectors of the SEGS collector field in the Mohave Desert in Southern California. Solar energy collection by tracking collectors with high concentration ratios can be greatly depressed by volcanic aerosols. Solar energy collection by collectors with low concentration and wide acceptance angle or no concentration are affected much less by volcanic aerosols because the global radiation shows much less change.

The effect of volcanic eruption on solar radiation is studied here using the hourly global and diffuse radiation data for the six Canadian stations at Edmonton, Montreal, Port Hardy, Resolute, Toronto and Winnipeg. This study also attempts to use the effect of volcanic eruption on the solar radiation data to infer some properties of volcanic eruptions. The method of analyzing global and diffuse radiation used here is introduced by Garrison and Sahami (1995a, b) and is also discussed below. The application of this method to the study of the effect of volcanic aerosol on solar radiation data shows again here the usefulness of this method of analysis.

Some information concerning the six Canadian stations is listed in Table 1. These data are obtained from the Atmospheric Environment Service, Downsview, Ontario, Canada. Information on the collection of these data, properties and calibration of the instruments, and the processing of these data to yield hourly global and diffuse radiation is presented in Hay and Won (1978) and Latimer (1978).

The study by Garrison and Sahami (1995b)

Table 1. Station data

Station Lat Long

Edmonton 53.5 113.5 Port Hardy 51.0 127.1 Montreal 45.5 73.6 Resolute 74.5 95.0 Toronto 43.7 79.4 Winnipeg 49.9 97.2

Alt (m)

206

17

35 254

Years

8.1 7.6

20.2 27.5 17.4 8.1

provides evidence that the calibration of the instruments has been maintained over the years of the Canadian solar radiation data for five of these stations. Resolute, the sixth station, shows a slow monotonic decrease in clear hour global radiation from 1958 to 1984 which seems to indicate that calibration of the instruments was not maintained as well. The data for Resolute are used here because these data are of long duration, the decrease over time is small, and this decrease is approximately linear. The data taken at a seventh Canadian station at Goose Bay has not been used here. Although the effects of the significant volcanic eruptions are seen rather clearly in the data and the data are of long duration, the calibration problems for Goose Bay are much more severe than those of Resolute.

Two aerosol scales are created covering the years of the solar radiation data. These are two different estimates of the amount of volcanic aerosol in the stratosphere as a function of the year. The correlations of these scales with the time variation of radiation data are used to test for the effect of the volcanic aerosols on the radiation data. The formation and use of these scales is discussed below. The apparent slow decrease in measured solar radiation by the instrumentation at Resolute over 27 yr is expected to have a minimal effect on the numerical value of the correlation between the radiation data and the aerosol scales. This is because the time which volcanic aerosols spend in the stratosphere is of the order of one year, much shorter than the 27 yr of the radiation data.

Three measures of the properties of the solar radiation data are selected to search for evidence of the variation of the direct beam radiation and diffuse radiation with time. Two of these test the variation of diffuse radiation with time. These are kdpeak, the mean clear hour diffuse index, and (kd jrange, the mean diffuse index in the range 0.44-0.72 of the clearness index k,. The third tests the variation of the direct beam radiation with time arising from variation of the aerosol content of the atmosphere. This third

Volcanic eruption 515

measure is peak /?, the peak clear hour Angstrom Turbidity Coefficient. How these three measures are defined and how they are obtained from the data is discussed in the next section. A more complete account is found in Garrison and Sahami (1995a, b).

2. METHOD OF ANALYSIS

2.1. Processing of the solar radiation data Hours of global and diffuse irradiation data

are sorted into bivariate histograms according to their values of k,, the clearness index, and kd, the diffuse index. These bivariate histograms have 24 intervals of 0.04 in k, covering the range O-0.96, and 19 intervals of 0.04 in kd covering the range o-0.76. The hours of irradiation data are sorted into different histograms according to the depth of snow, S, the atmospheric precipitable water, W, and solar elevation, CI, occurring during each hour. A different histogram is established for each of two conditions of S, three ranges of W, five ranges of c(, for each station, and for each year.

Lower case symbols for S, W, and c( are used for indices to characterize each histogram and quantities obtained from them. Snow depth uses s = 1 for no snow and s= 2 for snow depths greater than 5 cm. Precipitable water uses w= 1, 2 and 3 to represent the three ranges of precipitable water in order of increasing precipi-

table water. Solar elevation uses a= 1, 2, 3, 4 and 5 to represent the five ranges of solar elevation in order of decreasing solar elevation (increasing zenith angle). The range of values of S, W, a, and the numbers for the corresponding indices s, w, a used with each of these ranges are presented in Table 2. The bivariate histogram of Table 3 shows the number of hours in each interval of k, and kd for the eight years from 1977 to 1984 combined for Montreal. This is forthecases=l(nosnow),w=2(1<W<2cm), and a=3 (30~~~43 deg).

For each interval of k, of each bivariate k,, kd histogram, (kd), the mean value of kd, is determined. For Montreal for 1977-1984 for s=l, w =2 and a= 3, this is the average of the kd values in each column in Table 3. Figure 1 shows (kd ) for the histogram of Table 3.

A plot of (kd) versus k, is the same for all histograms for the overcast condition corresponding to k, less than about 0.3, when k,= kd and kb is zero. Also, there are few hours with k, above the range of values for clear hours. Thus, tests for significant differences between histograms (histograms with different s, w and 1; different stations; or different year group) using values of (kd) are performed best in the partly cloudy interval of k, near and below the clear hour values of k, and above k,=0.3. The mean value of (kd) for the seven intervals of k, from 0.44 to 0.72, (kd)range, is obtained for each

Table 2. Range of values of variables

Variable Range Units

Snow depth s=o s<s cm Precipitable water O<W<l l< W<2 2<W<4 - cm Solar elevation 60<0( 43<a<60 30<a<43 18<G(<30 9<a<18 Dcg Index 1 2 3 4 5

Table 3. Bivariate distribution of K,, Kd values for Montreal S=Ocm, l<W<2cm, 3O<ctc43”

0.54 0.50 1 0.46 3 2 3 1 1 0.42 5 17 16 6 6 3 3 0.38 12 31 21 16 20 12 5 4 1 1 0.34 25 34 12 15 23 11 17 9 5 2 2 1 0.30 44 39 24 17 11 12 15 25 22 17 7 3

K, 0.26 1 62 24 10 5 9 8 12 16 13 30 31 24 5 3 0.22 3 76 12 5 3 4 6 4 9 8 18 28 31 35 9 4 2 0.18 106 6 1 2 2 4 1 6 9 32 71 46 9 1 0.14 1 94 9 1 3 4 8 91 104 19 1 0.10 2 125 7 1 41 226 34 0.06 2136 6 1 37 15 0.02 80 6

0.02 0.06 0.10 0.14 0.18 0.22 0.26 0.30 0.34 0.38 0.42 0.46 0.50 0.54 0.58 0.62 0.66 0.70 0.74 0.78 0.82 K,

516 J. Garrison

MONTREAL 1977 1984 s=l,w=2,a=3

0.4

0.3 1

0.1 I/ <Kd>a”ge

Ol/

0 0.2 0.4 0.6 0.8

Kt

Fig. 1. The average value of kd for each interval of k, for Montreal for the eight years 1977-1984 for hours with S, Wandain therangesS=O, l<W<2cm,and30<a<43”. These ranges have the identifying indices s= 1, w=2 and G( = 3 (the swa: case “123”). (kd)range is the average value of kd over the range values of k, from 0.44 to 0.72 shown by

the arrow.

histogram. These seven intervals are selected so as to cover the approximate region of k, where differences in (kd) for different years are expected to be most significant. The range from 0.44 to 0.72 is shown by an arrow in Fig. 1.

The total number of hours of data in each interval of k, is obtained for each bivariate histogram. For Montreal for 1977-1984 for s= 1, w = 2 and 2 = 3 this is the sum of the number of hours in each interval of kd in each column in Table 3. Figure 2 shows the number of hours of data in each interval of k, for the histogram of Table 3. Each bivariate k,, kd histogram generally has a clearly defined peak in the number of hours in each interval of k,, which occurs at higher values of k,. This peak is associated with hours for which the sky is clear or nearly clear in the direction of the sun. The position of this peak is estimated from the number of hours in each k, interval near the peak. The value of k,

MONTREAL 1977 1964 s=l,w=Z,a=3

500 1

0.02 0.22 0.42 0.62 0.82

Kt

Fig. 2. The number of data hours in each interval of k, for Montreal for the eight years from 1977 to 1984 combined. All hours have values of S, W and a in the ranges S=O, 1~ W < 2 cm, and 30 < G( < 43”. These ranges have the identifying indices s = 1, w = 2 and N = 3 (the SWG( case “123”). The peak in the number of hours occurring at high k, is associated with clear hours. The k, value at the peak is ktpeaL= 0.731 kO.003. The uncertainty of k,,,,, is determined from

the fluctuations of k,,,,, over the years of the data.

at the peak is called the peak or most probable value of k, and is given the symbol ktpeak. For Fig. 2 with 8 yr of data, ktpeak =0.731 k 0.003. The uncertainty in mean ktpeak is determined from its fluctuation from year to year. The uncertainty in ktpeak is larger for histograms for individual years. The width of the clear hour peak increases and the height of the peak decreases with decreasing solar elevation and increasing atmospheric precipitable water. Thus, the uncertainty in the determination of peak values is generally larger for higher values of precipitable water and lower solar elevations. Histograms with fewer hours of data, such as those with the highest solar elevation range for each station, also can provide a less precise determination of the peak values.

The method of Louche et al. (198:) is used to determine the value of a pseudo-Angstrom turbidity coefficient p’ for each value of the direct beam index, kb, for all the data hours. Values of j?’ in intervals of lower kb are larger and include the effect of cloud attenuation or attenuation by higher atmospheric turbidity. Values of/Y for high values of k,, for which there are no clouds in the direction of the sun for the entire hour provide an estimate of the Angstrom turbidity coefficient. The values of atmospheric ozone used for the calculation of p are obtained from Iqbal (1983, Table 53.2, p. 89).

Hours of global and diffuse irradiation data are also sorted into bivariate histograms according to their values of k,, and F. Each of these bivariate histograms has 24 intervals of 0.04 in kb covering the range o-0.96, and 80 intervals of 0.005 in j?’ covering the range o-0.40. As before, a different histogram is established for each of the two conditions of S, three ranges of W, five ranges of c(, for each station and for each year. Most bivariate histograms containing values of k, and j?’ for each hour show a clearly defined peak in the number of hours of data in each interval of kb which occurs at higher values of k,, where the sky is clear or nearly clear in the direction of the sun. The position of this peak is estimated from the number of hours in each kb interval near the peak. The kb value at the peak is called the peak or most probable clear hour value of k, with the symbol kbpeak.

An average of the values of /Y over the hours in the intervals of kb used to determine the location of kbpealr is an estimate of peak /$ the Angstrom turbidity coefficient at the peak. This value of peak p will be a good estimate of the true peak /I if there are few hours with a veil of


thin clouds in the direction of the sun or thicker clouds of short duration in the direction of the sun. Any hours with more cloud attenuation in the direction of the sun will tend to have a kb value well below the peak. +

The difference between ktpeak and kbpeak is given the symbol kdpeak. kdpcak is approximately equal to the mean diffuse irradiation associated with the clear hours in the peaks of k, and kb.

Histograms for each station identified by the values of s, w, and a are formed separately for each year of data so that the values of ktpeak, k dpeak 3 kbpeak, p and (kd)range obtained from them can be examined for changes from year to year.

2.2. Selection of irradiation data for exhibiting evidence of volcanic aerosols

For comparison of the irradiation data with the volcanic aerosol scales presented below, only the values of kdpeak, j? and (kd)range are used. k bpeak is not used, since the information contained in kbpeak is found in /?. ktpeak is not used since global irradiation is affected only slightly by volcanic aerosols. Other approaches are possible. Recently, Gueymard (1994b) has sug- gested using the ratio: kbpeak/kdpeak, because of the anticorrelation between kdpeak and kbpeak with changes in volcanic aerosol concentration in the stratosphere. An optimum strategy for the analysis has not yet been developed for this initial study. The quantities kdpeak, p and <kd )range and the s, w, IX cases which show most clearly the effect of volcanic aerosols on the Canadian irradiation data have been selected after a preliminary study of the data. This preliminary study included visual examination of the effect of El Chichon and the other eruptions occurring during the years of the data and also calculation of the correlation of these and ‘other measures of the properties of the irradiation data with the early versions of the aerosol scales discussed below.

In looking for changes in solar irradiation data over time arising from volcanic eruption, the selection of the s, w and a cases to be used are limited to those which are expected to show most clearly any changes. No histograms with w = 3 are used. These have higher tropospheric turbidity and less well defined clear hour peaks. No histograms are used with M= 1 (with fewer hours of data) or m= 5 (with less well defined clear hour peaks). Also, values of (kd)range obtained from histograms with s=2 are not used, since surface albedo varies considerably

with the condition of the snow and the presence of surface buildings, roads, trees and type of terrain. Resolute is the only station which uses VaheS Of kdpeak obtained from histograms with s = 2. Histograms with s = 2 and w # 1 are omit- ted from the analysis, since all stations have few hours of data with s = 2 and w different than one.

The effect of the eruption of El Chichon on the data is so clearly evident (as will be seen in Figs 3-7, 9, and 11 discussed later) that the emphasis of this work will be on detection of the lesser effect of eruptions occurring during the time preceding the eruption of El Chichon in 1982. Clear detection of this lesser effect is more of a challenge.

2.3. Irradiation data averaging The values of kdpeak, p, and (kd)range obtained

from histograms for individual s, w, a cases for each station show considerable fluctuation from year to year beyond that which is caused by changes in volcanic aerosol. These fluctuations are large because of the large number of histograms into which the hours of data are sorted and because only one year of data is used for each histogram. These fluctuations make the effect of changes from year to year in the amount of volcanic aerosol almost unobservable for individual s, w, a cases, except for the effect of the eruption of El Chichon. To reduce these fluctuations, all the s, w, 1 cases selected are averaged for each year for each station. In addition, kdpeak, p, and (kd)range, separately, are averaged for two or more of the six stations. These averages increase the ratio of volcanic aerosol signal to background considerably.

One further process of averaging can be performed. The values of kdpeak, /? and (kd)range all increase with increase in the amount of volcanic aerosol. They also have different averages and standard deviations for the years covered by these data. To average these quantities, they are modified by what will be called “normalization”. Normalization of the values of kdpeak, /3, and

(kd )range consists of subtracting from their values their averages over all years and then dividing this result by their respective standard deviations over all years. They then all have zero average and unit standard deviation and are called “normalized”. Equal weight is used in averaging normalized values of kdpeak, fl, and <kd )range. The numerical value of the correlation between two series of numbers does not change when the same constant is added to each member of either of the series, or when the same

518 J. Garrison

constant multiplies each member of either of the series. Thus, normalization does not change the numerical value of the correlation.

With the elimination of w = 3 cases from use in this study, the summer months are largely eliminated. This is the time when the quantities

k tpeak 2 kdpeak, kbpeak, peak P and (kd jrange are least sensitive to the effect of changes in the volcanic aerosol concentration. Other observers have found also that this is a time when the effect of volcanic aerosols on optical observa- tions in the atmosphere is at a minimum (see, for example, Michalsky et al. 1990; Gueymard, 1994a; Michalsky et al. 1994).

2.4. Formation of the volcanic aerosol scales Two volcanic aerosol scales are created for

this study: Aerosol Scale 1 and Aerosol Scale 2. They are two initial attempts to estimate the yearly variation of the amount of volcanic aerosol in the stratosphere with the year over the years covered by the Canadian irradiation data. The two series of numbers representing the two scales are uncertain. The information needed to obtain an improved scale seems to be largely unavailable. Each of these scales are compared to the variation with the year of kdpeak, (kd)range and peak p for different s, w, and a cases and their averages to test for correlation. The effect of the eruption of El Chichon on the Canadian solar radiation data is clearly evident. Thus, the volcanic aerosol scales created here serve pri- marily as tools to search for the effect of other eruptions on the solar radiation data.

The publications of Lamb (1970), Hammer (1977), Hammer et al. (1980), Simkin et al. (1981), Szymber and Sellers (1985), Legrand and Delmas (1987), and McClelland et al. (1989) are the principal sources of information which are used here in developing the volcanic aerosol scales. Aerosol scale 1 uses values of the volcano explosive index (VEI) for volcanoes found in Simkin et al. (1981) and McClelland et al. (1989). These two references list a rough estimate, to the nearest integer, of the VEI for known volcanic eruptions from early historical times up through 1985. The latter portion of these tabulations covers the years of Canadian solar irradiation measurements.

The concept of a volcano explosive index (VEI) for each eruption was introduced by Newhall and Self (1982). The VEI is a log scale index, like the Richter scale for earthquakes. The amount of material ejected by an eruption is proportional to lOVEI. Estimates of the VEI

for each eruption are much less well determined than the Richter magnitude for earthquakes. Further, the amount of sulfur dioxide injected into the stratosphere by each eruption is often poorly correlated with the VEI. Nevertheless, the VEI, as used here, provides a measure of the amount of aerosol introduced into the stratosphere by each eruption. This study has found no better measure.

To form Aerosol Scale 1, the tabulated values of VEI equal to and greater than three have been used for estimating a number proportional to the amount of volcanic aerosol in the stratosphere over Canada for the years covered by the solar radiation data. Eruptions with a VEI less than three do not penetrate into the stratosphere. For each eruption, the value of VEI minus three is used as the exponent of ten and this resulting value is multiplied by a transport loss factor (TLF) and a stratospheric penetration factor (SPF). The resulting number is taken to be the contribution of each eruption to aerosol scale 1 for the half-year in which it is considered to start its effect. The estimate of the aerosol added to the stratosphere each half year is obtained by summing the contributions from each eruption contributing during that half year:

Added aerosol =x{ 10VE’-3} *TLF*SPF.

(1)

The transport loss factor (TLF) represents the loss during transport from the latitude of the eruption to the latitude of the Canadian stations. The TLF is assigned a value for each of three latitude zones selected for this study. One zone covers the latitudes south of the equator. Another zone includes latitudes between the equator and 30”N. The last zone is for latitudes north of 30”N latitude. The TLF is not adjusted for each eruption separately.

To account for the possibility that the value of VEI is not proportional to the amount of aerosol reaching the stratosphere, the eruptions are multiplied by a stratospheric penetration factor (SPF). In this study, the value of SPF can be different for each integer value of VEI, but is not adjusted for each eruption individually.

The main effect of El Chichon shows up in the data of this study in 1983. The large eruptions occurred on 28 March and 4 April 1982. The first optical effects of aerosols from large volcanic eruptions are generally seen within a few weeks or months after the eruption for observers at latitudes differing from that of the


eruption. They are observed sooner at the same latitude as the eruption. The full effect of the eruption takes longer to develop in the stratosphere. For this study, eruptions occurring south of the equator are assumed to have their effect delayed by nine months before reaching Canadian latitudes; eruptions occurring between the equator and 30”N latitude are delayed by six months; and eruptions north of 30”N latitude are delayed by three months. These delays appear to be very roughly what is indicated by reports in the literature. The delay for a particular eruption will depend upon the conditions in the atmosphere at the time of the eruption, but different delays for individual eruptions in the same latitude zone are not considered here. The time of eruption and the specified delays are used to indicate which half year the contribution of a particular eruption to the aerosols in the stratosphere above Canada is assumed to start. This approach ignores the effect of changes in the seasons on the stratospheric aerosol concentration.

The “added aerosol” for each half year could be combined for successive pairs of half years to form a yearly volcanic aerosol scale to compare with yearly values of kdpeak, peak /3, and

(kd )range and their averages. However, a scale formed in this manner does not account for the time the aerosol from each eruption remains in the stratosphere. The added aerosol for each half year is assumed to have a half life for decay in the stratosphere. The amount remaining each year from earlier eruptions is added to the amount of added aerosol for each year and the logarithm of this quantity taken to form the value of the volcanic aerosol scale for that year.

Taking the logarithm has the effect of making the yearly numbers for aerosol scale 1 be distributed approximately normally. It also puts the scale in a form more similar to the data repre- sented by the values of kdpeak, peak p, and <kd )range. The yearly values of kdpeak, peak /I and <kd )range are distributed approximately normally. Having yearly values distributed approximately normally allows the use of the Student t-distribution for testing the significance of the correlation between the yearly volcanic aerosol scales and the values of kdpeak, peak p and <kd )range (see, for example, Cramer, 1951; Korn and Korn, 1961). It is found that taking the logarithm does not change the numbers for the correlation very much in magnitude or how they vary with delay year.

Subsequent to the initiation of calculations

using aerosol scale 1, it appeared desirable to form an alternate aerosol scale 2, because of the large uncertainties of aerosol scale 1. This helps to evaluate aerosol scale 1 and provides another test for the presence of the volcanic aerosols. This scale uses only the volcanoes listed in Szymber and Sellers (1985, Table 2). These volcanoes are all known to have contributed sig- nificantly to the aerosol concentration in the stratosphere. Aerosol scale 2 is formed in the same manner as aerosol scale 1. The volcanoes in Table 2 of Szymber and Sellers listed as “most significant” are arbitrarily assigned a VEI of four, while the volcanoes listed as “minor” are assigned a VEI of three in forming the scale. The SPF, TLF and half life are used with these VEI values in the same manner as for aerosol scale 1. It is found below that the two scales show approximately the same strong positive correlation between each scale and the irradiation data. This reinforces the conclusions reached by this study.

2.5. Adjustment of the volcanic aerosol scales The transport loss factor (TLF), the strato-

spheric penetration factor (SPF), and the half life for decay of volcanic aerosols in the stratosphere are treated as adjustable parameters in the two volcanic aerosol scales. These parameters are adjusted to approximately maximize the correlation between the volcanic aerosol scales and the solar irradiation data. This in turn can provide some information concerning the properties of the volcanic eruptions that have affected Canadian solar irradiation data. The maxima in the correlation obtained by variation of these parameters are rather broad. This probably arises mostly from the uncertainties in the information used to form the aerosol scales and the consequent uncertainty of the aerosol scales.

Correlation between the solar irradiation data and aerosol scale 1 has been calculated for a half life of l/2, 1 and 2 yr. The maximum positive correlation between the solar irradiation data and aerosol scale 1 for these three cases occurs for a half life of one year. The half life,of one year also represents roughly what is observed in the Canadian solar irradiation data for El Chichon and by others (see McCormick and Veiga, 1992; Michalsky et al. 1994). A strong positive correlation is obtained also for a half life of l/2 or for 2 yr.

There are two eruptions affecting the time covered by the Canadian solar radiation data

520 J. Garrison

which are classified with a VEI of five. The The volcanoes listed as “minor” in Szymber Bezymianny eruption in March of 1956 is one and Sellers (1985, Table 2) must have a signifi- (Simkin et al. 1981). This eruption barely shows cantly larger SPF of about seven in order to in the Greenland ice sheet conductivities maximize the correlation between aerosol (Hammer, 1977). It occurs 2 yr before the start scale 2 and the radiation data when the “most of the earliest Canadian solar radiation data significant” volcanoes have a SPF set equal to taken at Resolute. This information indicates one. This indicates that the “minor” volcanoes that the VEI should be reduced for use in contribute almost as much volcanic aerosol as aerosol scale 1. A VEI of four is used for the “most significant” volcanoes, within the this study. accuracy of this analysis.

Mount St. Helens is listed also with a VEI of five (McClelland et al. 1989). It is known to have had a stratospheric aerosol contribution much less than El Chichon (Kerr, 1982; 1983), Mount St. Helens is listed as “minor” in Szymber and Sellers (1985, Table 2). This information indicates that the VEI for Mount St. Helens should be reduced for use in aerosol scale 1. A VEI of four is used.

2.6. Comparison of the volcanic aerosol scales with the solar irradiation data

Each of the two major eruptions of El Chichon are assigned a VEI of four in the published tabulation (McClelland et al. 1989). The two eruptions of El Chichon, when combined have a VEI of five (McClelland et al. 1989). A VEI of five is used in aerosol scales 1 and 2.

The correlation between the normalized means of kdpeak, B and (kd)range and aerosol scales 1 or 2 is calculated. The years used for aerosol scales 1 and 2 are varied about the range of years covered by kdpeak, /3 and (kd)range in 1 or l/2 yr steps to test the delay in time of arrival of the aerosols after the eruption. The Student t-distribution applied to the values for the correlation yields approximately the probability that a positive correlation have this particular value or larger if the two sets of values compared are uncorrelated.

It has been found by varying the TLF that the correlation between both of the volcanic aerosol scales and the irradiation data is approximately maximized using the same TLF for all latitude zones. The maximum is broad and thus not sensitive to the values of TLF. The results of this approximate maximization seems to indicate that the volcanic aerosols from each eruption spread throughout all latitudes, north and south, approximately equally. This result appears to be in disagreement with differences seen when comparing the Northern Hemisphere results of Hammer (1977) for the conductivities of the different years of the Greenland ice cores with the Southern Hemisphere results of Legrand and Delmas (1987) for the Antarctic ice cores. The ice core results for particular eruptions indicate relative differences in H,SO, content of the ice between Northern and Southern Hemispheres which seem to indicate different transport losses.

3. PRESENTATION OF RESULTS

The effect of changes in the amount of volcanic aerosol in the stratosphere on the values

of ktpeak, kdpeak, kbpeak, peak P and (kd Lange will first be presented visually in several figures prior to discussing results of the correlation of the normalized means of kdpeak, peak /I, and

& )range and their averages with the two volcanic aerosol scales.

3.1. A visual presentation of the effects of volcanic aerosols on the solar irradiation data

The stratospheric penetration factors (SPF) are set the same for all values of VEI for aerosol scale 1. Using the same SPF for aerosol scale 1 for all values of VEI, approximately maximizes the correlation between the data and aerosol scale 1. This implies that the stratospheric aerosol contribution of each eruption is approximately proportional to the VEI of that eruption.

Figure 3 shows the average values of ktpealrr k dpeak, and hpeak for Montreal for each year from 1965 to 1984, inclusive. Averaging is over the four swa cases, 113, 114, 122 and 123, for each year. Here, for example, 123 means snow depth index s= 1 (S=O, no snow), precipitable water index w = 2 (1~ W < 2 cm), and solar elevation index a=3 (30” <a<43“). The dip

in kbpeak and rise in kdpeak in 1983 due to El Chichon is evident. The value of ktpeak appears largely unaffected by El Chichon. In Fig. 3, one can see possible evidence of an increased amount of stratospheric aerosol near and after 1965, near 1975 and in the 1980s prior to the El Chichon eruption, but this evidence are not visually very convincing.


MONTREAL

0.4

t

0

65 70 75 80 85

YEAR

Fig. 3. The average values of ktpcak, kdpeaL and kbgeak for Montreal as a function of the year from 1965 to 1984 inclusive. The values of ktpeak, kdFaL and kbpcak for each year are averages over the four SWG( cases: 113, 114, 122 and 123. The dip in k,,,, and the rise in kdpear in 1983 are the effect of the increase in aerosol content of the stratosphere following the eruption of El Chichon in 1982. An extended dip in k bFal: and rise in kdpeaL from 1965 to 1969 are very likely due to the series of eruptions over this period. The dip in k bpcaL and the rise in kdpeaL in 197551976 are probably mainly the effect of the eruption of Fuego in Guatemala in late

1974.

Figure 4 shows the average values of peak atmospheric turbidity /I for the years 1977-1984, inclusive. The averages are over the SW% cases: 113, 114, 122, 123, 132, 133, 213, and 214 for the five stations of Edmonton, Montreal, Port Hardy, Toronto and Winnipeg, combined. The averages for s = 1, w = 1, 2, 3 (labeled w = 1, w = 2, w = 3 in the figure) and s=2, w = 1 (labeled s = 2 in the figure) are all kept separate for each year. This figure shows clearly the effect of El

FIVE STATION

0.1 .

0.075

c s z 0.05

2 0.025 .

0’

76 78 80

YEAR

a2 a4

Fig. 4. The average value of the peak Angstrom turbidity coefficient p as a function of the year for the five stations of Edmonton, Montreal, Port Hardy, Toronto and Winnipeg. The four curves are averages over the values of peak p for all five stations. The curve labeled w = 1 is an average of the two swc( cases: 113 and 114. The curve labeled w = 2 is an average of the two swc( cases: 122 and 123. The curve labeled w = 3 is an average of two swa cases: 132 and 133. The curve labeled s=2 is an average of the two SWN cases: 213 and 214. The effect of the eruption of El Chichon on p in 1983 is clearly evident. As the value of w increases, the effect of volcanic aerosols from El Chichon on the data is reduced.

Chichon in 1983. It also indicates the reduction in the sensitivity of the data to the effect of volcanic aerosols from El Chichon with increasing precipitable water. This also has been observed by Michalsky et al. (1990), Gueymard (1994), Michalsky et al. (1994), and others. The average values in 1984 are still considerably above what might be considered approximately the background value of peak /I in the period from 1977 to 1979. The reduction from 1983 to 1984 gives some indication of the half life for decay of volcanic aerosols in the stratosphere. The presence of eruptions other than El Chichon during this period can affect the value of /I and reduce the accuracy of the determination of the half life for decay.

Figure 5 shows the average values of kdpeak for Resolute for the swz cases: 213 and 214; for Montreal and Toronto combined for the SWN cases: 113, 114, 122 and 123; and for Edmonton and Winnipeg combined for the SWN cases: 113, 114, 122 and 123. As in Fig. 3, the effect of El Chichon is clearly evident. Possible clues for an increased amount of stratospheric aerosol near and after 1965, near 1975, and in the 1980s prior to El Chichon eruption are present, but not visually very convincing.

Similarly, Fig. 6 shows the average of the

values of &Lange for the five stations of Edmonton, Montreal, Port Hardy, Toronto and Winnipeg for the years from 1965 to 1984 using the sw~r cases: 113, 114, 122 and 123. Only Montreal contributes to the averages indicated by the first three points of the graph, and only Montreal and Toronto contribute to the averages indicated by the points from 1968 to

0.3 -

0.25 -

0.05 - - MONTREAL. TORONTO

- EDMONTON. WINNIPEG

0

55 60 65 70 7s 80 85

YEAR

Fig. 5. The average kdpcak as a function of the year for Resolute, for Montreal and Toronto combined, and for Edmonton and Winnipeg combined. The two swo( cases: 213 and 214 are averaged for Resolute. The four swcl cases: 113, 114, 122 and 123 are averaged for Montreal and Toronto,

and Winnipeg and Edmonton, separately.

J. Garrison 522

0.32

0.3

i d z 0.28 h

: 0.26

0.24

FIVE STATIONS

65 70 75 80 85

YEAR

Fig. 6. The average of (kd)_“se as a function of year for the five stations of Edmonton, Montreal, Port Hardy, Toronto and Winnipeg, combined. The average involves the four swa

cases: 113, 114, 122 and 123, for each station.

1976, inclusive. The effect of El Chichon is clearly evident. Some evidence for an increased amount of stratospheric aerosol near and after 1965, near 1975, and in the 1980s prior to El Chichon is present, but is not visually very convincing.

3.2. Tests of correlation between the volcanic aerosol scales and the solar irradiation data

Figure 7 shows aerosol scale 1 plotted for the years 195551985 inclusive along with the nor-

6 AEROSOL SCALE Z

’ IRRADIATION DATA

-4

55 65 75 85

YEAR

Fig. 7. The top curve is aerosol scale 1. The eight curves shown beneath aerosol scale 1 are the irradiation data. The two irradiation data curves starting in 1958 are for Resolute. One is an average of normalized mean peak p. The other is an average of normalized mean peak kdpeak. Averages for both curves are for the two SWG( cases 213 and 214. The three normalized mean curves starting in 1965 are for Montreal. The three normalized mean curves starting in 1968 are for Toronto. The Montreal and Toronto curves are each for (kd)rangs, peak p and k the normalized means of <kd ,,..d~~~hd”k’,~~:“~~~ of the four SWG( cases: 113, 114, 122 and 123. The normalized means of peak /I are an average of the six SWG( cases: 113, 114, 122, 123, 213 and 214. The correlation of the eight irradiation curves with aerosol scale 1 is not visually

apparent.

malized mean of eight irradiation data sets. The eight irradiation data sets consist of: (1) the average of the two 213 and 214 cases of peak B for Resolute; (2) the average of the two 213 and 214 cases of kdpeak for Resolute; (3) the average of the values of peak /I for the six swu cases 112, 113, 122, 123, 213 and 214 for Montreal; (4) the average of the values of peak /I for the six swu cases 112, 113, 122, 123, 213 and 214 for Toronto; (5) the average of (kd)range for the six swa cases 112, 113, 114, 122, 123, and 124 for Montreal; (6) the average of (kd)range for the six SWG( cases 112, 113, 114, 122, 123, and 124 for Toronto; (7) the average for the values of k dpeak for the four swu cases 113, 114, 122, and 123 for Montreal; and (8) the average for the values of kdpeak for the four swa cases 113, 114, 122 and 123 for Toronto. It is seen by the figure that the correlation between the data and aerosol scale 1 is difficult to detect visually by observation of the plotted data, except for the effect of El Chichon.

In Fig. 8, the correlations of the eight data sets plotted in Fig. 7 with volcanic aerosol scale 1 are shown for different years of delay or advance between the years for the aerosol scale and the data set. Here, one year of delay corres- ponds to having each year of aerosol scale 1 compared to the following year of the irradiation data in calculating the correlation, for example. The delay is in addition to the delays of three, six and nine months already inserted for the three latitude zones in forming the aerosol scales. So that the effect of El Chichon

1 .oo

0.50

p

4 Z 0.00

8 -0.50

-1.00 (

-2 0 2 4

YEARS DELAY

Fig. 8. The correlation of each of the eight normalized mean irradiation data curves shown in Fig. 7 with aerosol scale 1 as a function of the years of delay between the aerosol scale and the irradiation data. The years of delay are in addition to that inserted in aerosol scale 1. All eight curves have positive correlation with aerosol scale 1 for years of delay

near zero.


is not included, and sufficient years of delay and advance can be used, the Canadian irradiation data for the correlations include only the years up to 1980, inclusive. It is seen that all the correlations are positive for the delay years at zero and are mostly positive for delay years near zero.

In Fig. 9 the normalized mean irradiation data for peak p, (kd)range, and kdpeak from Fig. 7 have been averaged with equal weights to yield one irradiation data curve to compare with aerosol scale 1. Figure 10 shows the corresponding correlation between these two curves as a

6.00

, AEROSOL SCALE 1 _

4.00

2.00

0.00

.2.00 1 Y

55 65 75 65

YEAR

Fig. 9. The irradiation data curve is the average of the normalized mean of (kd)rangcr k,,,,,, and peak j? for the three stations of Toronto, Montreal and Resolute. Aerosol scale 1 is also shown again for comparison. The correlation of the combined irradiation data curve with aerosol scale 1 is verv

1

0.5

0

-0.5

-1

apparent visually.

- CORRELATION

- PROBABILITY

DELAY (YEARS)

Fig. 10. The correlation of the irradiation data shown in Fig. 9 with aerosol scale 1 as a function of the years of delay between the aerosol scale and the irradiation data. The years of delay are in addition to the delay already inserted in aerosol scale 1. There is a strong positive correlation between aerosol scale 1 and the irradiation data for delay years near zero. Also shown is the probability that the irradiation data shown in Fig. 9 and aerosol scale 1 can be uncorrelated. The relatively large number of years involved in the correlation makes the strong positive correlation for delay years near zero very improbable for two curves which are

YEAR

Fig. 11. The upper curve is aerosol scale 2. The lower curve is the irradiation data shown in Fig. 9. Visually, the correlation between the two curves is very apparent. The arrows at the top of the figure indicate the time of eruption of the “most significant” volcanoes listed Szymber and Sellers (1985, Table 2). In chronological order these volcanoes are Bezymianny, Agung, Taal, Awu, Fernandia, Hekla, Fuego

uncorrelated. and El Chichon.

function of years of delay. Visually the positive correlation is seen clearly in Fig. 9. The calculated results in Fig. 10 indicate this positive correlation for delay years near zero. Also in Fig. 10 is the calculated probability that, for the correlation calculated and the number of irradiation data points used, the data and aerosol scale 1 be uncorrelated.

Figure 11 shows the irradiation data of Fig. 9 and aerosol scale 2. Figure 12 shows the corresponding correlation between the two curves of Fig. 11 as a function of years delay. Also shown in Fig. 12 is the probability that, for the correlation calculated and the number of irradiation data points used, the irradiation data and aerosol scale 2 be uncorrelated. Comparison of Figs 10 and 12 indicates that limiting the volcanoes used for aerosol scale 2 to those known to be significant contributors to the stratospheric aerosol concentration probably improves the correlation. The arrows at the top of Fig. 11 show the time of eruption of the “most significant” volcanoes listed in Szymber and Sellers (1985, Table 2).

4. DISCUSSION

The combined, normalized solar irradiation data in Figs 9 and 11 indicate that the increased volcanic activity during the periods near and after 1965, near 1975, and in the early 1980s prior to the eruption of El Chichon is clearly observable in the solar radiation data. The amplitude of the effect of these eruptions on

AEROSOL SCALE 2

IRRADIATION DATA

524 J. Garrison

1 .oo r

1::: -I -2 0 2 4

DELAY YEARS -0.50

I - CORREVITION COEFFICIENT

-1 .oo 1 - PROBABILITY

Fig. 12. The correlation of the irradiation data shown in Fig. 11 with aerosol scale 2 as a function of the years delay between the aerosol scale and the irradiation data. The years delay is in addition to that already inserted in aerosol scale 2. There is a strong positive correlation between aerosol scale 2 and the irradiation data curve for delay years near zero. Also shown, as a function of delay year, is the probability that the irradiation data shown in Fig. 11 and volcanic aerosol scale 2 be uncorrelated. The relatively large number of years involved in the correlation makes the strong positive correlation for delay years near zero very improbable for

two curves which are uncorrelated.

these data is about one-third that of El Chichon. The effect during the period near and after 1965 is greater than the effect near 197.5.

Figs 10 and 12 indicate that the average delay in time after eruption before the full effect of the eruption on the stratospheric aerosol content shows up in the data is possibly a few to several months longer than the delays of three, six or nine months for the three different latitude zones used in forming the aerosol scales.

The increase in volcanic aerosol near and after 1965 is seen also in the data of Hammer (1977). Both the increase in aerosol near and after 1965 and near 1975 are seen also in the data of Szymber and Sellers (1985), with the first increase also being the larger. The increase in stratospheric aerosol concentration near and after 1965 is due to several eruptions starting with Agung in Indonesia on 17 March 1963, while the increase near 1975 is appears to be due mainly to the eruption of Fuego in Guatamala on 10 October 1974.

5. CONCLUSIONS NOMENCLATURE

?? The volcanic aerosols from eruption of El Chichon have a pronounced effect on the Canadian solar irradiation data in 1983. The reduction in the mean peak direct beam index kb for hours with low precipitable water is of the order of 10%. The eruption of El Chichon increased the peak Angstrom turbidity

k, direct beam index kd diffuse index k, clearness index

k bpeak direct beam index at the clear hour peak k dpeaL diffuse index at the clear hour peak

<k, !::: clearness index at the clear hour peak average k, in the range from 0.44 to 0.72 of k,

S snow depth s snow depth index

coefficient by a factor of two or three for hours of low atmospheric precipitable water. The effect of increased volcanic activity during the periods from about 1963 to 1968 and near 1975 on the Canadian solar irradiation data each have an amplitude which is about one-third that of El Chichon. In the past, attention has concentrated on the few major eruptions, when considering the effect of volcanic eruption on the stratospheric aerosol concentration. The results here show that other eruptions can separately or in combination make significant contributions to the stratospheric aerosol concentration. This study again shows that volcanic aerosols have little effect on global radiation, but can cause a significant reduction in direct beam radiation and a corresponding increase in diffuse radiation. The half life for decay of volcanic aerosols in the stratosphere is a time of the order of 1 yr. The results shown by this study indicate that the Canadian solar radiation data are of good quality. The big advantage of these Canadian solar radiation data, besides their good quality, is in the many years of data and the number of different stations with useful data. Research data exist which are expected to be of higher quality. However, the number of years and the number of stations contained in these research data tend to be rather limited. Large data sets covering many continuous years are necessary for studies of the type discussed here.

AcknowledgemenrsPThe thoughtful suggestion in April 1993 by Chris Gueymard that the Canadian solar radiation data must contain information concerning volcanic eruptions led to the initiation of this work. We are indebted to the many individuals, unknown to us, who were involved in the careful collection and analysis of the Canadian solar radiation data and who put these data in the suitable form in which it came to us from the Canadian Climate Centre. David Wardle and Bruce McArthur of the Climate and Atmospheric Research Directorate of Canada kindly provided additional information on procedures and errors of measurement. Chris Gueymard, Bruce McArthur and Joe Michalsky reviewed the manuscript prior to submission for publication and provided many valuable comments and suggestions.


W atmospheric precipitable water w atmospheric precipitable water index z solar elevation angle a solar elevation index b Angstrom turbidity coefficient B’ nseudo-Angstrom turbiditv coefficient which

Latimer J. R. Radiation measurement, International Field Year for the Great Lakes. Techn. Man. Ser. 2, National Res. Council of Canada, The Secretariat, Canadian National Committee for the Intern. Hydrological Decade, Ottawa, Canada (1978).

Learand M. and Delmas R. A 220-year continuous record -of volcanic H,SO, in the Antarctic ice sheet. Nature 327, 671-676 (1987).

Louche A., Maurel M, Simonnot G., Peri G. and Iqbal M. Determination of Angstrom’s turbidity coefficient from direct total solar irradiance measurements. Solar Energy 38, 89-98 (1987).

m&ides cloud attenuation SPF stratospheric penetration factor TLF transport loss factor VEI volcano explosive index

REFERENCES

McClelland L., Simkin T., Summers M., Nielsen E. and Stein T. Global Vulcanism 1975-1985. Prentice-Hall, Englewood Cliffs, N.J. (1989).

McCormick M. and Veiaa R. Sage11 measurements of earlv Cramer H. Mathematical Methods of Statistics. Princeton

Univ. Press, N.J. (1951). Pinatubo aerosols. %eophysr Res. Lett. 19, 155-158 (1992).

Garrison J. D. and Sahami K. A study of the division of McDaniels D. K. and Vignola F. Impact of volcanic matter global irradiance into direct beam and diffuse irradiance from El Chichon on direct radiation in the Pacific North- at seven Canadian sites. Solar Energy 55, 493-504 west. Proc. Ann. Conz Am. Solar Energy Sot., Anaheim, (1995). (1995a). Calif. (1984).

Garrison J. D. and Sahami K. Analysis of clear sky solar radiation for seven Canadian stations. Solar Energy 55, 5055512 (1995). (1995b).

Gueymard C. Analysis of monthly average atmospheric precipitable water and turbidity in Canada and Northern United States. Solar Energy 53, 57-71 (1994a).

Gueymard C. Private communication (1994b). Hammer C. Past volcanism revealed by Greenland ice sheet

impurities. Nature 270, 4822486 (1977). Hammer C., Clausen H. and Dansgaard W. Greenland ice

sheet evidence of post-glacial volcanism and its climatic impact. Nature 288, 230-235 (1980).

Michalsky J., Pearson E. and LeBaron B. An assessment of the impact of volcanic eruptions on the northern hemisphere’s aerosol burden during the last decade. J. Geo- phys. Res. 95, 5677-5688 (1990).

Michalsky J., Perez R., Seals R. and Ineichen P. Degradation of solar concentrator performance in the aftermath of Mount Pinatubo. Solar Energy 52, 205-213 (1994).

Nagaraja Rao C. and Bradley W. Effects of the El Chichon volcanic dust cloud on insolation measurements at Cor- vallis, Oregon (U.S.A.). Proc. Ann. Co@ Am. Solar Energy Sot. (1983).

Newhall C. and Self S. The volcanic explosivity index (VEI): an estimate of explosive magnitude for historical volcanism. J. Geovhvs. Res. 87. 1231-1238 (1982).

Hay J E. and Won T. K. Proc. 1st Can. Solar Radiation Workshoo. Anril 17-19 (1978).

Iqbal M. An’&bduction to Solar ‘Radiation. Academic Press, Simkin T., Siebert’L., McClelland I., Bridge D.; Newhall C. New York (1983). and Latter J. Volcanoes of the World. Hutchinson Ross,

Kerr R. El Chichon forebodes climate change. Science 217, Pa (1981). 1023 (1982). Stoffel T. and Nelson D. Effects of the Mt. Pinatubo eruption

Kerr R. El Chichon climate effect estimated. Science 219, on solar radiation resources near Denver, Colorado- 157 (1983). some preliminary analyses. Proc. Ann. Conf. Am. Solar

Korn G. and Korn T. Mathematical Handbookfor Scientists Energy Sot., Washington, D.C. (1993). and Engineers Chap. 19, p. 622. (McGraw-Hill, New Szymber R. J. and Sellers W. D. Atmospheric turbidity at York (1961). Tuscan, Arizona, 195661983: variations and their causes.

Lamb H. Volcanic dust in the atmosphere; with a chronology J. Clint. Appl. Meteorol. 24, 725-734 (1985). and assessment of its meteorological significance. Phil. Viebrock H. and Flowers E. Comments on the recent Trans. R. Sot. Land.. A. Math. Phys. Sci. 266, 4255533 decrease of solar radiation at the South Pole. Tellus 20, (1970). 400-411 (1968).

an evaluation of the effect of volcanic eruption on the solar radiation at six canadian stations

Documents