Changes in surface shortwave solar irradiance from 1993 to 2011 at Thessaloniki (Greece)

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 33: (213) Published online 7 December 212 in Wiley Online Library (wileyonlinelibrary.com) DOI: 1.12/joc.3636 Changes in surface shortwave solar irradiance from 1993 to 211 at Thessaloniki (Greece) A. F. Bais,* Th. Drosoglou, C. Meleti, K. Tourpali and N. Kouremeti Physics Department, Aristotle University of Thessaloniki, Thessaloniki, Greece ABSTRACT: The variability of shortwave downward solar irradiance (SDR) received at Earth s surface over Thessaloniki, Greece for the period is investigated, focusing on the effects from the aerosols variability on the irradiance trends derived for different solar zenith angles (SZA). Linear trends have been calculated for the entire dataset, for cloudfree cases, and for different SZAs, separately for each season. The global upward trend in SDR after 199 (.33% year 1 ) is reconfirmed and is found to depend strongly on SZA, ranging from.1 to +.6% year 1. The long term changes in aerosols in conjunction with the local aerosol patterns result in differences of up to.1% year 1 in the derived trends in SDR between morning and afternoon hours. Finally, based on the analysis of the cumulative sums of the differences in monthly averages of SDR from the long term mean we report signs of a slowdown in the upward trend in SDR during the beginning of the 2s. KEY WORDS solar irradiance trend; solar irradiation; dimming brightening; aerosol effects Received 3 May 212; Accepted 8 November Introduction In recent decades, the ount of solar radiation incident at the Earth s surface has undergone significant variations, with impacts on climate. These variations have been manifested from the analysis of ground-based measurements of shortwave downward solar irradiance (SDR) from various networks worldwide, as well as from satellite-derived estimates. Studies over the past two decades reported a worldwide decrease in SDR for the period , known as global dimming, followed by a decrease since the late 198s, known as global brightening (Wild, 29 and references therein). Similar variations in surface irradiance were also found under cloud-free conditions, which were attributed to opposite changes in the ount of aerosols, a major factor affecting surface solar radiation. Over Europe, longterm records of atmospheric transmission for cloud-free conditions show a consistent downward trend from the 195s to the 198s ranging between 4.9 and 6.7 W m 2 per decade and a partial reversal thereafter, caused by changes in aerosols (Liepert and Tegen, 22; Pinker, et al., 25; Stanhill and Cohen, 25; Romanou et al., 27; Ruckstuhl et al., 28; Ohvril et al., 29; Wild et al., 29; Wild, 212). Negative trends in aerosol optical depth over the Mediterranean were found from the analysis of MODIS data for the period 2 26 (Papadimas et al., 29; Koukouli et al., 21). Analysis * Correspondence to: A. F. Bais, Physics Department, Aristotle University of Thessaloniki, Cpus Box 149, Thessaloniki, Greece. abais@auth.gr of measurements by two Brewer spectroradiometers operating at Thessaloniki, Greece, also revealed negative trends in aerosol optical depth: Kazadzis et al. (27) and Zerefos et al. (29) reported trends of 3.5% year 1 at 355 nm for the period , and 2% year 1 at 32 nm for the period , respectively. In this study, we investigate the changes in SDR from 1993 to 211 using measurements conducted at Thessaloniki, Greece, focusing, in particular, on the influence of long-term changes in aerosols on the irradiance trends at different solar zenith angles (SZA). The dataset used here is shorter compared to the one used in Zerefos et al. (29), because the pyranometer measurements were recorded in sufficiently high time resolution to allow grouping of the data in SZA bins only since In addition, this dataset is of superior quality as it starts with a new instrument, whose absolute calibration has been reconfirmed twice during this period. 2. Data and methodology The SDR data used in this study cover the period from January 1993 to March 211, and were measured with a Kipp & Zonen CM-21 pyranometer operating continuously at the Laboratory of Atmospheric Physics of the Aristotle University of Thessaloniki (4 38 N, E, 8 m above sea level). The monitoring site is located near the city centre. The horizon is clear of obstructions except for the azimuth angles between 35 and 12 where obstacles of up to 1 elevation block the direct irradiance in the morning for SZA larger than 8. These data points were excluded from the analysis, as 212 Royal Meteorological Society

2 2872 A. F. BAIS et al. they contain only the diffuse component of solar radiation. The measurements are recorded every 1 min together with the standard deviation of the sples during each minute. The stability of the CM-21 pyranometer quoted by the manufacturer is better than ±.5% year 1.However, two recalibrations that were done at the Deutscher Wetterdienst Meteorologisches Observatorium Lindenberg in 25 and 211 revealed no change in its sensitivity, as its stability is better than.1% during the 19 years of operation. As the main focus of this study is the estimation of relative long-term changes in SDR, all data were adjusted to the mean Sun Earth distance and then they were grouped into SZA bins of 1. The lower troposphere over the city of Thessaloniki contains significant ounts of aerosols and air pollutants, either produced by local sources or transported from distant regions in neighbouring countries (Kazadzis et al., 27; Zerefos et al., 29). In order to separate the effects of clouds from the effects of aerosols, measurements recorded under cloud-free conditions were analyzed separately. Distinguishing cloud-free periods is always a challenge and different methodologies have been proposed (Vasaras et al., 21; Zhao and Li, 27; Kaskaoutis et al., 28; Taylor et al., 28) using mainly ancillary information from other instruments (e.g. direct, diffuse irradiance) or statistics of the variability of the irradiance measurements with the aid of radiative transfer models. In this study, the radiative transfer model, libradtran, (Mayer and Kylling, 25), was used to estimate the upper and lower limits of the expected SDR under cloud-free conditions for high and low aerosol optical depth, as it was estimated in Kazadzis et al. (27). These calculations were repeated for all SZAs. Then, for each SZA we assumed that all SDR measurements falling between the two limits refer to cloud-free cases. Values higher than the model estimates for low aerosol correspond to enhancement of radiation by clouds, whereas lower values to attenuation of radiation by clouds. In addition, we have tested the variability of SDR within a period of 5 min before and after each measurement, as in Vasaras et al. (21), to remove any cloud affected measurements that escaped the first screening. Linear trends were calculated only for the subsets of data containing more than 5 data points, and their significance was tested by Student s t-test analysis. It should be noted that there are no significant gaps in the dataset which would affect the calculation of the trends. 3. Results and discussion As a first step, we calculated the daily irradiation (daily sum of SDR) and the climatological mean for each day of the year. Then these daily mean values were used to remove the measurements natural seasonal variability, which is mainly caused by the seasonal variation of the SZA and the length of the day, and partly by the seasonal variability in cloudiness and aerosols. The result, as time series of anomalies from the climatological mean Departures from long-term mean (%) Year Figure 1. Long term anomalies (in %) of the daily shortwave solar irradiation at Thessaloniki, Greece, from 1993 to 211. The straight line represents the overall linear trend of.33% year 1, which is statistically significant at the 99% confidence level. is presented in Figure 1. The observed annual pattern with higher variability in winter and smaller in summer reflects mainly the day-to-day variability of clouds and to a lesser extend of aerosols. The derived linear trend is.33% year 1, statistically significant at the 99% confidence level. This positive trend is in accordance with earlier studies (Philipona et al., 29; Wild, 29) that reported the start of a brightening (increasing of atmospheric transparency) since the late 198s, attributed to changes in cloudiness and aerosols, but it is at least half of the trends found at other European sites. There is a clear evidence that the aerosol optical depth in the area decreases during the 199s and early 2s (Kazadzis et al., 27), but it is still high compared to other areas in Europe (Papadimas et al., 28). Zerefos et al. (29) have shown that for several years after the eruption of Mt. Pinatubo the optical depth of aerosols remained elevated, therefore the first few years of the data set used in this study may have been influenced by these additional volcanic aerosols Trends for different SZAs As a next step, we calculated the linear trends of SDR for data subsets corresponding to constant SZA, after adjusting them to the mean Sun Earth distance. Morning and afternoon data were analyzed separately in order to identify effects of the diurnal variation of the aerosols on the trends, as the tropospheric aerosol load in urban areas usually is more pronounced in the afternoon hours (Smirnov et al., 22; Zerefos et al., 212). This analysis is presented in Figure 2, where for the period a general increasing trend in SDR is seen, reaching.4 and.6% year 1, for cloud-free and all-sky conditions, respectively. This trend is statistically significant at the 99% level for all SZAs except for the ones close to 9. For all-sky conditions the trends depict a noisier pattern compared to cloud-free conditions and have greater uncertainty due to the high variability in cloudiness. Otherwise, the trends for cloud-free and allsky conditions are similar, at least for SZAs smaller than about Royal Meteorological Society Int. J. Climatol. 33: (213)

3 SHORTWAVE DOWNWARD SOLAR IRRADIANCE TREND 2873 (a) (b) CLOUD-FREE ALL SKIES Solar zenith angle (deg) Solar zenith angle (deg) Figure 2. Linear trends in surface shortwave solar irradiance under cloud-free (a) and all-sky (b) conditions as a function of SZA. The trends were calculated separately for morning (red dionds) and afternoon (blue triangles) data. Filled symbols refer to trends with statistical significance higher than 99%. The trend in SDR increases gradually with SZA, with a higher rate for SZAs between 5 and 8, particularly in the case of all-sky conditions. The causes for the SZA dependence of the trends are probably connected with the variability of clouds and aerosols, the major controlling factors of SDR, and particularly with the physical mechanisms governing the attenuation of radiation by clouds and aerosols. It should be noted that surface reflectivity can be safely assumed constant at this urban location. For cloud-free skies, the SZA dependent trend in SDR can be attributed to the long-term changes in the ount and/or the optical properties of the aerosols at Thessaloniki. Reduction in the optical depth of the aerosols results in increased direct irradiance and reduced diffuse irradiance. These processes depend strongly on the atmospheric path-length of radiation; hence on SZA. Generally, the effect of a negative trend in the aerosol optical depth on SDR should be more pronounced at large SZAs. To verify this, the monthly mean SDR at Thessaloniki under cloud-free conditions for the period of the study was simulated with the radiative transfer model LibRadtran (Mayer and Kylling, 25) using monthly mean aerosol optical depth measurements from a collocated Brewer spectroradiometer (update of Kazadzis et al., 27). The derived trend of the simulated SDR was.13 and.18% year 1, for 2 and 7 SZA, respectively, confirming qualitatively the results from the analysis of the observations. However, the difference of.5% year 1 in the model-derived trends between the two SZAs is small compared to the difference in the trends of Figure 2 ( 2% year 1 ). This discrepancy could be caused by the use of monthly mean aerosol optical depth in the simulated SDR, while the trends derived from the measurements are influenced by the actual day-to-day variability of the aerosols. In addition to the aerosol ount, the variability of solar irradiance at the surface may also be influenced by the absorption efficiency of the aerosols, which can be described by the single scattering albedo (SSA). Long-term changes in SSA would modify the model-derived changes in irradiance discussed above, either to lower values, if the SSA increases during the period of the study, or to higher values if the SSA decreases. However, measurements of the SSA at Thessaloniki are not available for the entire period, as the local AERONET station started operating only in late 24. For all skies, the effects of clouds on SDR are dominant over the effects of the aerosols, as clouds have much larger optical depths. The derived linear trends are mostly positive implying that either cloudcover has been decreasing over the years, or that clouds bece optically thinner. For SZAs smaller than 5, the trends are noisy, ranging between.1 and +.3% year 1, comparable on average with those for cloud-free conditions. Thereafter, the trends progressively increase with SZA up to.6% year 1, faster than for cloud-free skies. Given that fewer or thinner clouds would result in less reflected radiation to space, and that scattered solar radiation to space by clouds is larger for larger SZA, decreasing cloudiness would result in increased surface irradiance with SZA; hence in larger positive trends. For cloud-free skies, the trends are larger in the morning than in the afternoon; up to twice as much for SZAs between 3 and 7. Presently there is no clear explanation for the causes of this behavior. One could argue that the morning measurements of SDR are less affected by the local sources of aerosols due to the thinner mixing layer; hence the increasing with time atmospheric transparency is mainly a result of a gradual reduction in the background aerosols over the area. On the other hand, as the mixing layer thickens over the course of the day, the afternoon measurements of SDR are affected mostly by the locally produced aerosols. Although the optical depth of the local aerosols decreases with time, there is no information on the long-term variation of their absorption characteristics Consistency of trends in SDR In addition to the calculation of the overall linear trends we have applied on our dataset the cumulative sums (CUSUM) analysis which is useful in identifying regime shifts and small, sustained changes or slow fluctuations in the mean values (Shapiro et al., 21). Here, we aim at detecting change points in the time series of monthly 212 Royal Meteorological Society Int. J. Climatol. 33: (213)

4 2874 A. F. BAIS et al. (a) CUSUM TSI (%) (b) CUSUM TSI (%) CLOUD-FREE ALL SKIES 2 24 Year daily Year Figure 3. Cumulative sums of the monthly differences in surface shortwave solar irradiance from the long-term mean for the period (a) CUSUM of SDR at 63 SZA in the morning (red thin line) and afternoon (blue thick line) under cloud-free conditions. (b) CUSUM of SDR at 63 SZA in the morning (red thin line) and afternoon (blue thick line) under all skies. The line with open circles (green) corresponds to the CUMUM of the daily irradiation. mean SDR. The residuals of the monthly SDR values from the climatological average for each month were used to construct the CUSUM charts for the daily irradiation, and for the irradiance data recorded at 63 SZA. These charts are shown in Figure 3. A segment on a CUSUM chart with a downward slope indicates a period where the monthly mean SDR values tend to be below the climatological average, and a change in the sign of the slope indicates a change in the regime. For the cloud-free data set, the CUSUM follows a rather smooth pattern with a systematic negative slope and reaches a minimum in 22 for the afternoon data, and in 23 for the morning data. The upward slope after the minimum suggests that during this period most of the departures are positive and the monthly SDR values tend to be higher than the long-term average. The statistical significance of this change was tested with a bootstrapping procedure: The CUSUM was calculated for 1 sples, constructed by randomly reordering the original series, and the range of CUSUM values was compared with the range derived from the original series. It appears that for both series (a.m. and p.m.) of clear-sky SDR the confidence level for the observed change is close to 1%. For all skies, the cumulative sums have a more complex structure, induced by the cloud variability, and several change points can be identified. It is worth noting that the CUMUM pattern is very similar for the morning and afternoon irradiance data, as well as for the daily irradiation, suggesting that the long-term cloud effects are independent of the time of the day. While the minimum CUSUM occurs in , a clear upward tendency is evident only after 24. For all skies, the confidence level for the change points is very large (close to 95%) for the daily irradiation and the afternoon SDR at 63 SZA, and somewhat smaller (about 84%) for the morning SDR data. The above analysis leads to the conclusion that the positive linear trends in SDR, which were discussed above, are not representative for the entire period. At least since the early 2s the rate of increase becomes smaller and diminishes towards the end of the decade. This behaviour is particularly evident for cloud-free skies, suggesting that the lower troposphere over Thessaloniki, where most of the aerosols reside, has reached a point where aerosols no longer decrease and are levelling off (Kazadzis et al., 29). As the activities of the city are growing, and taking into account that a good fraction of the aerosols are transported from other regions (Amiridis et al., 25), it appears that the measures taken since the early 199s cannot lead to further improvement of the air quality. Therefore in the coming years the solar irradiance at the surface will likely level off or start decreasing Seasonal trends The trends presented in Figure 2 are biased from data obtained at different seasons, since at the latitude of Thessaloniki only SZAs larger than 63 are present throughout the year. The trends for the smaller SZAs do not include data from the cold period, thus are strongly influenced by the seasonal variation of the aerosols. For exple, the dataset for 3 spans from mid-april to end of August, hence it is influenced by higher aerosol ounts and reduced cloudiness in this season. Figure 4 shows the seasonal patterns of the derived trends as a function of SZA for cloud-free conditions to isolate the aerosol from the cloud effects. The general pattern found in the yearly trends also remains in the different seasons showing clearly that trends are increasing with increasing SZA. In spring and summer, the afternoon trends are slightly smaller, possibly associated with the built-up of the aerosols in the warm period that occurs mainly in the afternoon hours. The trends at large SZA are by up to.2% year 1 larger compared to the annual trends (the maximum seasonal trend occurring at 8 SZA is about.6% year 1 compared to.4% year 1 for the annual trend). As discussed above, the aerosol optical depth in the spring and summer is larger than in other seasons, therefore the reduction in the optical depth that has occurred during the 19s and 2s results in larger trends in these seasons, particularly at large SZAs where the effect is more pronounced. The autumn trends follow the general tendency, but are slightly smaller with unimportant morning to afternoon differences. Finally, there 212 Royal Meteorological Society Int. J. Climatol. 33: (213)

5 SHORTWAVE DOWNWARD SOLAR IRRADIANCE TREND 2875 (a) (b) (c) WINTER SPRING SUMMER Greece are used to investigate the long-term changes and their association to changes in the aerosols and clouds over the area. Daily solar irradiation increases with an average rate of.24 W m 2 year 1 (or.33% year 1 ). The trends for small SZAs are close to.1% year 1, but at large SZAs they increase up to.4 and.6% year 1, respectively, for cloud-free and all-sky conditions. The increasing trend in SDR for cloud-free conditions is attributed to the cleaning of the atmosphere from aerosols. Moreover, it is suggested that changes in aerosols and clouds are responsible for the SZA dependence of the SDR trends derived from the observations. For the se SZA the trend in cloud-free SDR is larger by up to.1% year 1 in the morning than in the afternoon, particularly for the warm period of the year. The analysis of the cumulative sums of monthly departures from the long term mean reveals a change in the rate of increase of SDR which likely occurs in the first half of the 2s. These findings are in accordance with the results of Philipona et al. (29), and suggest that the well-established brightening of the last few decades has slowed down or is no longer progressing Acknowledgements This study has been partially accomplished in the frework of the project Hellenic Network of Solar Energy, contract: 9 YN , co-funded by the Hellenic Ministry for Develoent, Competitiveness and Shipping and the European Commission under the National Strategic Reference Frework (d) AUTUMN Solar zenith angle (deg) Figure 4. Seasonal trends in surface shortwave solar irradiance as function of SZA for cloud-free conditions, separately for morning (red dionds) and afternoon (blue triangles) data. Filled symbols refer to trends with statistical significance higher than 99%. is no clear SZA dependence in the winter trends, due to the large noise in this subset of data. 4. Conclusions Shortwave solar irradiance measurements at Earth s surface conducted from 1993 to 211 at Thessaloniki, References Amiridis V, Balis DS, Kazadzis S, Bais A, Giannakaki E, Papayannis A, Zerefos C. 25. Four-year aerosol observations with a Ran lidar at Thessaloniki, Greece, in the frework of European Aerosol Research Lidar Network (EARLINET). Journal of Geophysical Research 11. DOI: 1.129/25JD619. Kaskaoutis DG, Kbezidis HD, Kumar Kharol S, Badarinath KVS. 28. The diffuse-to-global spectral irradiance ratio as a cloud-screening technique for radiometric data. Journal of Atmospheric and Solar-Terrestrial Physics 7: DOI: 1.116/j.jastp Kazadzis S, Bais A, Amiridis V, Balis D, Meleti C, Kouremeti N, Zerefos CS, Rapsomanikis S, Petrakakis M, Kelesis A, Tzoumaka P, Kelektsoglou K. 27. Nine years of UV aerosol optical depth measurements at Thessaloniki, Greece. Atmospheric Chemistry and Physics 7: DOI: /acp Kazadzis S, Bais A, Balis D, Kouremeti N, Zempila M, Arola A, Giannakaki E, Amiridis V, Kazantzidis A. 29. Spatial and temporal UV irradiance and aerosol variability within the area of an OMI satellite pixel. Atmospheric Chemistry and Physics 9: Koukouli ME, Kazadzis S, Amiridis V, Ichoku C, Balis DS, Bais AF. 21. Signs of a negative trend in the MODIS aerosol optical depth over the Southern Balkans. Atmospheric Environment 44: DOI: 1.116/j.atmosenv Liepert B, Tegen I. 22. Multidecadal solar radiation trends in the United States and Germany and direct tropospheric aerosol forcing. Journal of Geophysical Research-Atmospheres 17. DOI: 1.129/21JD76. Mayer B, Kylling A. 25. Technical note: the libradtran software package for radiative transfer calculations - description and exples of use. Atmospheric Chemistry and Physics 5: DOI: /acpd Royal Meteorological Society Int. J. Climatol. 33: (213)

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