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Keywords:

  • total solar radiation;
  • sunshine duration;
  • solar energy;
  • global warming;
  • climate change

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and conclusions
  8. References

The article presents characteristics of long-term variability in sunshine duration and total solar radiation in Krakow, reconstructed based on the relative intensity of total radiation (global transmission). The analysis is based on hourly sunshine duration data from the period 1884–2010 and total radiation intensity data from the period 2003–2007. Results show that the variability of incident solar radiation depends on global factors – circulation-type and anthropogenic-type factors, in particular by industralization between the 1950s and 1980s – which are affected by local conditions. Copyright © 2013 Royal Meteorological Society

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and conclusions
  8. References

Climate change and its causes are issues that receive a great deal of attention in the climatologic literature today. Solar radiation is the most important factor in shaping the climate. It stimulates physical, chemical, and biological processes occurring at the surface of the Earth and in the atmosphere. The energy balance of our planet depends on the amount of solar energy that the Earth-atmosphere system receives. For this reason, understanding of long-term variability in the amount of solar radiation reaching the Earth may help explain the causes of the current climate warming.

Research on long-term solar conditions is possible because it can be based on the analysis of long measurement series. The first regular continuous actinometric measurements in the world were only initiated in the middle of the 20th century. Recording of sunshine duration was initiated about 100 years earlier. In 1853, Campbell performed the first measurements using a ‘self-registering solar sundial clock’ designed by himself (Stanhill, 2003). The Royal Meteorological Society adopted an improved Campbell–Stokes heliograph for common use in 1880. The heliograph was used in Great Britain and British colonial territories as well as in some European countries including Poland.

This type of heliograph was installed on the roof of the Astronomical Observatory in Krakow in June of 1883. The Krakow heliograph has been in operation without interruption at the same location ever since.

The measurement instrument was replaced only once, in 1942. First heliograph, produced in England was replaced by heliograph produced in Germany and the instruments worked together for 353 days (from 12 May 1941 to 30 April 1942). Again, in order to link two data series, both heliographs registered sunshine duration from 1 September 1957 to 31 August 1958. The data obtained were used to compare the readings of the new and the old heliograph and homogenize the measurement series. The calculations also took into account the annual course of the optical mass of the atmosphere, the durations of the Sun's location higher than 5 degrees above the horizon and the coefficient of transparency and vapor pressure. The new values of the corrections acquired a regular annual course. After they have been applied to the readings of the old heliograph, tests showed the homogeneity of the entire series (Lewik et al., 2010).

Although sunshine duration is determined by the length of time of solar radiation flow only, it also supplies certain data on the magnitude of total solar energy that is reaching the surface of the Earth during the day. The statistical association between sunshine duration and solar radiation intensity has been used by many researchers in the climatologic literature (Martinez et al., 1984; Linacre, 1992). To compute sums of total radiation, Black's formula (1956) has been used most frequently.

The association between these elements may be described in a more precise manner than if it is based on monthly totals of sunshine duration and total radiation. Hourly values are used while using a function involving a transmission coefficient (relative average hourly radiation intensity), depending on sunshine duration inline image, where R is a measured average hourly value of total radiation (Wm−2), and Rs is a short-wave radiation intensity at the upper limit of the atmosphere for a given day d and hour t (Duffie and Beckman, 1994).

The application of relative total radiation intensity (global transmission) allows exclusion of the variability of total radiation intensity that is due to changes in the Sun's altitude over the horizon, seasonal changes, and solar day (24-h) change patterns.

The purpose of this article is to show long-term variability in incident solar radiation flow in Krakow based on hourly totals of total solar radiation computed using values of sunshine duration and total radiation intensity.

2. Materials and methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and conclusions
  8. References

Hourly values of sunshine duration from the years 1884–2010 read directly off Campbell–Stokes recorder heliograms as well as measurement results of total radiation intensity generated using an automatic Kipp & Zonen CM5 sensor during the years 2003–2007 were used in this article.

Both the sensor and the heliograph had been installed on the roof of the old Astronomical Observatory building of the Jagiellonian University. This location allows for unobstructed recording of solar radiation, which was confirmed by a measurement of the obstruction level. The hourly sunshine duration values recorded daily and the total solar radiation intensity measurement results recorded daily in 10-min intervals by the sensor were used for further computation. On the basis of these data, the total amount of total radiation received per 1 m2 of surface area per 1 h [MJm−2] was computed for every day from 1884 to 2010.

Relative radiation intensity (R/Rs) (U) and its association with sunshine duration U, at any given hour, is expressed by the following formula (Matuszko, 2009):

  • display math
  • display math(1)

where inline image (more precisely inline image) is given by the formula: inline image,

pv – the p-value of the insignificance of the square regression hypothesis test, R2 – coefficient of determination, s - residual deviation (error of the model), inline image– short-wave radiation intensity at the upper limit of the atmosphere for a given day d and hour t (Duffie and Beckman, 1994).

Therefore, the energy ΣR, in MJm−2, received by 1 m2 of surface area per hour t on a day d of the year was calculated by using the following formula (Matuszko, 2009):

  • display math(2)

After calculations had been performed, tabulated reports were prepared that were similar to monthly heliographic reports containing sums of total radiation values for every hour. Reconstructed hourly data were used to calculate daily, monthly, and yearly sums of total radiation for the years 1884–2010.

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and conclusions
  8. References

The average yearly amount of total radiation calculated based on data for the years 1884–2010 for Krakow was 3733.2 MJm−2. The long-term pattern of yearly amounts of total radiation is sinusoidal in shape and demonstrates slightly decreasing radiation values for the entire heliographic series (Figure 1). On the basis of data for the multi-year research period, a decrease in radiation totals is present in every month, reaching its maximum value in September and minimum values in April, October, November, and December.

image

Figure 1. Multi-annual course of annual sums of total solar radiation [MJm−2] in Krakow between 1884 and 2010.

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The decreasing trend in changes in total solar radiation observed for Krakow is also observed in many other metropolitan areas in Europe (Bogdańska and Podogrocki, 2000) and the USA (Stanhill and Cohen, 2005). The solar radiation totals were highest in the 1940s, exceeding 4000 MJm−2. Since 1951, a clear decreasing trend in radiation values can be observed with a minimum (3302.6 MJm−2) in 1980.

The 1990s were characterized by an increase in average yearly radiation values. This was due primarily to an increase in radiation during the summer and clearly weaker radiation in the winter (Figure 2).

image

Figure 2. Multi-annual course of monthly sums of total solar radiation [MJm−2] in Krakow between 1884 and 2010.

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Very high radiation totals were calculated for 1992 and 1994 (3904 and 3813 MJm−2, respectively). The same was true of the final years of the research period, especially in 2006 and 2009 (3888 and 3880 MJm−2, respectively). July and December of 2006 as well as April of 2009 were marked by the highest radiation totals for these months in the entire research period.

Analyzing the solar radiation pattern for particular months (Figure 2), it is clear that total monthly radiation values depend primarily on astronomical factors, while their variability from year to year depends on factors related to circulation. The lowest values and relatively low variability in values over the years are typical for December, January, and November (Figure 2). More hours of daylight and a greater angle of incidence of the Sun's rays in February yield radiation totals that are greater than in December by about 100 MJm−2.

In October, monthly totals often exceed 200 MJm−2 and in some years even reach 250 MJm−2. This is caused by astronomic and meteorological factors as well as cloudiness that is smaller in comparison to the winter months. September values are typically highly variable from year to year (Figure 2). September shows tendency for the most steeply decreasing trend in radiation amounts (−0.25 MJm−2 year) of all the months analyzed over the multi-year period (Figure 2). This is related to a maximum decrease in sunshine duration (over 40 h per 100 years) and to an increase in cloudiness (Table 1). As a comparison, the decrease in sunshine duration for the remaining months ranges from several hours to more than a dozen hours per 100 years (for example: October – 7.9 h/100 years, June – 15 h/100 years).

Table 1. Equations of the trend of average monthly values of cloudiness in Krakow in the years 1884–2010
MonthsEquation
  1. Statistically significant coefficients are in bold.

Januaryy = −0.024x + 122.67
Februaryy = −0.0026x + 79.757
Marchy = −0.0147x + 97.771
Aprily = −0.0244x + 113.29
Mayy = 0.0161x + 31.153
Juney = 0.0172x + 29.786
Julyy = −0.0067x + 73.882
Augusty = 0.0019x + 54.399
Septembery = 0.0284x + 4.3746
Octobery = −0.0445x + 153.51
Novembery = −0.0149x + 106.5
Decembery = −0.0244x + 126.99
Yeary = −0.0077x + 82.84

4. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and conclusions
  8. References

Examples of analysis of multi-annual changes in solar radiation based on measurements of sunshine duration may be found in the international climatologic literature (Stanhill and Cohen, 2005). However, they usually pertain to countries with higher sunshine duration, located at latitudes lower than Poland (e.g. the southern part of the USA, Israel, Turkey, and Spain). Most European sites have much shorter measurement series than those available for Krakow, Poland, which only allow for comparing sequences of heliographic data recorded since the middle of the 20th century.

The measurement of solar radiation duration is influenced by natural factors such as astronomical, geographical, and meteorological. Moreover, results may also be affected by factors related to the instrumentation and methodology used in the measurement of sunshine duration and in the interpretation of heliograms, as well as by local factors.

The recorded value of sunshine duration may be influenced by the kind of paper used for heliographic cards (colour of sunshine cards, clarity of the scale printed on a card, paper quality), the properties of the glass (transparency, colour, scratching) that heliographic spheres are made of, and the professionalism of the observers who clean the glass sphere, change strips, and interpret heliograms. Stanhill and Cohen (2005) emphasize the influence of methodological factors, which may lead to an incorrect interpretation of a decrease in sunshine duration resulting from environmental changes around a heliograph such as growing trees and industrialization.

Air pollution, chimney smoke present in an urban area, and clouds of anthropogenic origin may all decrease the influx of solar radiation. This is especially true in the winter. According to Brázdil (1991), under urban environmental conditions with a considerable air pollution, a heliograph starts to burn a trace when the Sun's angular altitude over the horizon is greater than 8°.

The decrease in recorded intervals of sunshine duration are also influenced by solar radiation attenuation and fog. This happens especially when the Sun's angular altitude over the horizon is low. It is difficult to accurately estimate the influence of the above factors on measurement results. In spite of certain inconveniences, the Campbell–Stokes heliograph remains the only sunshine duration measuring device providing access to multi-annual series of measured data.

The total values of solar radiation for Krakow, calculated based on the association of the total radiation intensity with sunshine duration, were compared with results of measurements performed at other research stations in Poland (Bogdańska and Podogrocki, 2000).

The reconstructed values of total radiation are slightly higher than empirical data. This is due to the fact that the measurement results of radiation intensity used for an algorithm come from the years 2003–2007 when sunshine duration totals were higher than the multi-annual average. It should be noted, however, that during the first part of the period of interest, 1884–1944, influx of solar radiation was greater than in later years and exceeded the multi-annual average (Figure 1).

A decrease in the influx of solar radiation amounts from 1951 to 1980 and then a slow increase during the most recent 20-year time period were observed in the multi-year span of sunshine duration values for Poland and Central Europe (Matuszko, 2009). Such similar trends within such a large area attest to a global background of the causes of variation in solar radiation amounts. Such global causes may be of a natural or an anthropogenic origin.

Climatologists do not completely agree when it comes to explaining the causes of these trends. With modern measurement technology at their disposal (telescopes, space probes), scientists know more about the Sun than ever before. Nevertheless, contemporary models do not provide answers to many basic questions regarding solar activity. A number of studies have been conducted, often demonstrating contradictory opinions on the close association between solar activity and short-term climate changes (Crowe, 1971; Budyko, 1975; Svensmark and Friis-Christensen, 1997, among others).

Studies based on satellite-generated data, indicating that the variability of the solar constant is in accordance with solar activity only in 0.2% (Foukal, 1992) shed new light on the solution of this problem. Thus, the amount of energy between the ‘solar maximum and minimum’ is rather small. It is likely that the atmosphere, with its self-regulating and buffering function, does not cause a rapid and full response of the climate system to the changing amount of energy emitted by the Sun. However, changes in atmospheric circulation, affecting the cloud cover and the degree of cloudiness and cloud genera present, modify the amount of solar radiation reaching the surface of the Earth.

Many scientists (Liepert, 2002; Warren et al., 2006; Norris and Wild, 2007; Ruckstuhl and Norris, 2009), call the years 1960–1985 the period of ‘global dimming’, because the results of measurements conducted during that time at many locations around the world have shown that solar radiation intensity decreased by about 4%. The decrease in the amount of solar radiation received at the surface of the Earth might have been related to the accumulation of soot and aerosols in the atmosphere, both types of substances being products of human activity, since their highest concentrations were found over urban areas.

In the late 1980s, the economy in the former Eastern European communist bloc collapsed. As a result, industrial production began to decline. The level of emission of pollutants also decreased. Effective measures were put in place in order to improve air quality.

This was most likely the reason why the transparency of the atmosphere increased, epecially in Eastern and Central Europe. A noticeable ‘brightening effect’ occurred, with an increased flow of solar radiation to the surface of the Earth, observable also in Krakow.

As stated earlier, the existence of similar trends in sunshine duration over the long term for many research stations in Europe may be due to atmospheric circulation that is reflected in changes in cloud cover. This hypothesis is confirmed by the multi-year values of average yearly cloudiness and yearly sums of total solar radiation calculated for Krakow. The two elements are mutually dependent in an obvious way: in any given year, a decrease in cloudiness is associated with an increase in the sum of total solar radiation. The reverse is also true: an increase in cloudiness is reflected by a decreased total radiation value for a given year (Matuszko, 2009).

Despite a fairly high yearly variability, similar cloudiness and solar radiation trends can be observed over the long term. From the beginning of this study period until the 1920s, there was a small decrease in both cloudiness and solar radiation. An increase in cloudiness and solar radiation was noted between 1921 and 1956, then a decrease until 1975, and then again an increase from 1976 on. The main cause of the variability in cloudiness is global and local atmospheric circulation (Lewik et al., 2010).

During the first part of the research period was mostly atmospheric circulation over Poland latitudinal, initially moving from north and then, since 1920, from south. In the years 1957–1975, strengthening of the zonal circulation from east was observed. That may have been the cause for increased cloudiness with the majority of layered clouds decreasing the radiation amount received. The zonal overturning circulation, flowing from east had strengthened since the second half of the 1970s until the end of the research period. This type of circulation is associated with a cloud cover where the majority of the clouds are convective clouds as well as with more fair and cloudless days (Matuszko, 2009).

Atmospheric circulation is most likely also the cause of the largest decrease in solar radiation in September. On the basis of data from the years 1884–2010, September is the month in which anticyclonic situations occur most frequently. Nevertheless, the largest decrease in atmospheric pressure in the 20th century is observed in the month of September (Matuszko, 2007), which might have contributed to increases in cloud cover. This is shown by a positive coefficient of the trend in cloudiness pattern, which is highest in September (Table 1).

A slightly decreasing trend in cloudiness occurs for the most part of the year, with the exception of September, May, June, and August. It is also important to note that in Krakow the smallest increase in temperature (0.8 °C) within the multi-year research period occurred in September (Matuszko, 2007).

In the 1950s, the influence of atmospheric circulation over Krakow might have been augmented by anthropogenic factors. A large steel mill started production in 1954. Industrial development began to dominate the city, which also began to expand physically. An increase in the emission of industrial dust and gases contributed to the creation of layered clouds and to a decrease in atmospheric transparency.

The emission of pollutants began to decrease in the 1980s due to a decline in industrial production. The decrease in radiation in the years 1951–1980 occurred in spite of a decrease in cloudiness and in the number of partly cloudy and cloudy days (Matuszko, 2007). That would confirm the hypothesis of ‘global dimming’.

An analysis of long-term changes in the influx of solar radiation should focus primarily on the role of the variability of cloud genera, which may be of key significance in explanation of this issue. On the basis of the research studies in numerous locations in Europe and in the USA (Wibig, 2008), it may be stated that trends in the variability of cloudiness over time are similar to those generated for Krakow (Matuszko, 2003) in terms of the cloud genera in cloud cover since the 1950s.

In the last 40 years in particular, an increase in the frequency of appearance of vertical clouds has been observed, as well as a decrease in the frequency of appearance of layered clouds, the increase in summer cloud cover by low level clouds, and a decrease in cloud cover of the winter sky. High level clouds were also observed more frequently.

The described change in the characteristics of cloud cover causes an increase in the sums of solar energy reaching the surface of the Earth in spite of the increase in cloudiness noted in some regions of the world. The clouds Cumulus, Cumulonimbus, Cirrus, Cirrostratus, and Cirrocumulus block only a small fraction of solar radiation due to their structure and properties (Matuszko, 2009).

The causes of changes in characteristics of cloud cover are in part natural and anthropogenic (Matuszko, 2007). Atmospheric circulation, which may be affected by local environmental conditions and radiation factors, especially in the case of low level clouds, is the deciding factor affecting the appearance of particular cloud genera.

Krakow's location in the so-called temperature-inverse Vistula River Valley, with poor air circulation, intensified further by the aerosol effect, favors the formation of Stratus clouds. Moreover, the role of local factors in shaping clouds (so-called nephologic systems) is expressed more by convection than circulation during the warm part of the year. It is also not unlikely that an increase in the frequency of Cumulus and Cumulonimbus clouds in the second part of the 20th century is related to the emission of heat into the atmosphere by man-made sources and to convection over hot concrete surfaces created in the city by increasing urbanization.

A decrease in the frequency of appearance of layered clouds, especially Stratus, is caused by dry air over the city. A decrease in atmospheric turbidity is due to the concentration of aerosols in an urban area. Limiting the emission of pollutants in recent years contributes to the decrease in the share of layered clouds in the cloud cover. It also contributes to changes in the atmospheric balance that intensify the creation of convective clouds (Wibig, 2008). An increase in the frequency of high level clouds may be related to the development of air travel (Henderson-Sellers, 1986), or to unveiling of high level clouds by lower level clouds due to a decrease in the amount of layered clouds. However, it could also be caused by atmospheric circulation factors.

The amount of solar energy that reaches the surface of the Earth probably also depends on volcanic eruptions. This hypothesis was confirmed in 1980 when the yearly sum of sunshine duration in Krakow and in other Polish cities was the lowest in the multi-year research period (Matuszko, 2009). Similar trends in sunshine duration over a wider area point to global causes of the decrease in incident solar radiation. However, in this particular case, besides the role of atmospheric circulation, the eruption of Saint Helen's Volcano in May of 1980 cannot be excluded as a possible factor.

Volcanic aerosols floating in the air may have directly or indirectly contributed to the limitation of solar radiation getting through by increasing the amount of cloud cover. According to Budyko (1975), volcanic dust may decrease the amount of solar radiation up to 20%. It was estimated (Stanhill and Cohen, 2005) that the eruption of the El Chichon Volcano in April of 1982 caused a decrease in sunshine duration in the USA by 137 h year−1 and a reduction of solar radiation intensity by 6.4 Wm−2 over the entire continent.

According to Schönwiese (1997), volcanic dust ejected into the stratosphere, consisting of minute particles of volcanic glass and ash, does not initially affect the climate in a substantial manner. However, several months after eruption when a large quantity of ejected gases had undergone physical and chemical transformation and had changed into aerosols, volcanic output does become significant for the atmosphere. Sulphate particles produce the strongest climate effects. The initial clouds of volcanic products propagate quickly eastward or westward, depending on wind direction in the stratosphere. Secondary volcanic clouds, containing among others, sulphate particles, have more significant impact on climate, as they propagate also north and south within a few months.

5. Summary and conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and conclusions
  8. References

The rise of air temperature in Krakow during the multi-year research period was not associated with an increase in solar radiation totals but rather with a small decrease. Therefore, it may be concluded that the cause of climate warming is not an increase in short-wave radiation received at the surface but it is the retention of the heat radiated by the Earth in air layers close to the surface or emission of man-made heat by anthropogenic sources.

Solar radiation amounts reaching the surface of the Earth during the multi-year research period varied from year to year. As well, steady increasing or decreasing trends over longer periods of time lasting several decades were observed.

Recent research indicates that in spite of significant fluctuations in the number of sunspots, the energy stream coming from the Sun into the upper limit of the atmosphere is constant (Lewik et al., 2010). Therefore, the causes of changes in solar radiation amounts measured at the surface of the Earth over the years are related to the Earth-atmosphere system.

Analysis of heliographic data from Krakow shows that the variability in solar radiation depends on global factors – both natural and anthropogenic – affected by local conditions.

It is worth noting that maximum yearly sums of sunshine duration did not occur in recent years but happened in the 19th century and in the first half of the 20th century. This is in contrast to what is observed in the case of other elements of climate such as temperature and precipitation. There were 2 years in the 21st century that were exceptions to the rule, because some of the highest sunshine duration values that occurred during the multi-year period occurred in these 2 years – the largest number of sunshine hours per month in July and December of 2006 and in April of 2009.

The fact that the maximum values of sunshine duration over the multi-year period did not occur in recent years confirms the assumption that global warming is related more to decreased radiation of surface heat out into space than to the solar radiation amount that is reaching the surface of the Earth. This difference may point to the smaller role of clouds in the transmission of the Sun's short-wave radiation than the long-wave radiation from the Earth. Thus, this suggests that cloud cover plays an even more important role in the retention of heat energy.

Attention should be paid to the methodology being applied in sunshine duration measurements. The homogeneity of heliographic data series needs to be verified before interpreting multi-annual changes in solar radiation reception. Factors such as the obstruction of the horizon, change in measurement location, or in measuring device, disqualify the given series. Today, the cause of serious errors in the analysis of changes in sunshine duration over multi-year periods may be a different threshold value for measurements performed using a Campbell–Stokes heliograph and electronic measurement recorders (Matuszko, 2009).

Electronic measurement devices are more sensitive (threshold value = 120 Wm−2) and record more hours of sunshine per day than does the Campbell–Stokes heliograph (threshold value = 279.2 Wm−2). On cloudy days, a traditional heliograph may not record any sunshine, yet an electronic sensor sends an impulse even at brief moments of sunshine. Hence, an analysis of the causes of multi-annual variability in sunshine duration should be done with caution when the measurement results come from both traditional and electronic measurement devices, as the increase in sunshine duration of recent years may be caused by the use of different measuring devices rather than by any other factors.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Summary and conclusions
  8. References
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