Possibilities to detect trends in spectral UV irradiance

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  • Theor. Appl. Climatol. 81, 3344 (2005)DOI 10.1007/s00704-004-0109-9

    1 Institute of Meteorology and Climatology, University of Hannover, Germany2 Finnish Meteorological Institute, Kuopio, Finland3 Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Greece

    Possibilities to detect trends in spectral UV irradiance

    M. Glandorf1, A. Arola2, A. Bais3, and G. Seckmeyer1

    With 8 Figures

    Received February 11, 2004; accepted October 6, 2004Published online December 22, 2004 # Springer-Verlag 2004

    Summary

    It is investigated how long-term UV trends can be assessedby analysing the longest time series of measured spectralUV irradiance in Europe, which have been started in theearly 1990s in Thessaloniki, Greece and Sodankylaa,Finland. It can be concluded that both time series do notyet show an unambiguous yearly trend in UV irradiance.The regression lines show no uniform behaviour and varyirregularly in strength and from one solar zenith angle to thenext if all sky conditions are analysed. It is emphasised thatthese findings do not disagree with previous studies, thatsignificant changes in UV irradiance have been observedover Europe especially in spring.

    Our study introduces a new method to estimate therequired time series length for trend detection using themeasured time series in combination with model calcula-tions. At Sodankylaa, a reduction of the total ozone columnof 5.7% per decade has been observed from 1979 to 1998.A positive UV trend due to such conditions may be detectedafter 12 years at the earliest. For Thessalonki, a decrease intotal ozone of 4.5% per decade has been observed. Acorresponding increase of UV irradiance should be detect-able after 15 years. It should be noted that a constant ozonetrend over the whole period had to be assumed for thisanalysis.

    Since 1990 there has been a considerable variability oftotal ozone, but no steady decrease could be observed. Con-sequently, no general UV increase could be expected due toozone changes. Even if there was a constant ozone trend overthat period it is shown that even the longest European timeseries of UV irradiance are still too short to show distincttrends. However, this does not imply that no changes haveoccurred, it only shows that the large natural variability of

    UV irradiance has so far hindered the identification of unam-biguous trends. The only way to find significant and consis-tent UV trends is the continuation of high-quality long-termmeasurements of spectral UV irradiance.

    1. Introduction

    The main objectives for taking long-term system-atic measurements of surface ultraviolet (UV) ra-diation are to establish a global climatology of UV,both average and extreme values, and to quantifyany long-term changes that may have occurred as aresult of changes in stratospheric ozone or othervariables (Kerr and Seckmeyer et al., 2003). SolarUV radiation is known to have adverse effects onthe biosphere including terrestrial and aquatic eco-systems as well as public health. Especially forhuman beings, exposure to UV radiation from theSun is associated with skin cancer, acceleratedageing of the skin, cataract, or other eye diseases(Seckmeyer et al., 2001). During the past decades adepletion of stratospheric ozone has been detectedwhich brought up concern about increased surfaceUV radiation levels. Increases in UV radiationassociated with ozone decline have been observedby spectral measurements at a number of siteslocated in Europe (e.g. Zerefos, 2002). In recentyears the possible recovery of the stratospheric

  • ozone layer has been discussed as well (Reinselet al., 2002). If these scenarios, that are complicatedby complex interactions with climate change, arecorrect, a significant decrease in UV irradiance canbe expected.

    A number of publications focus on the searchfor the evidence of long-term changes in UV ir-radiance. Analyses of changes in UV doses havebeen performed, for example, by Herman et al.(1996), Seckmeyer et al. (1997), Weatherheadet al. (1997), Udelhofen et al. (1999), Lindforset al. (2003), Trepte and Winkler (2004), andEngelsen et al. (2004). Spectrally resolved rou-tine measurements of solar UV irradiance inEurope only started in the 1990s. Recent studieson possible long-term changes in spectral UVirradiance hence are hampered by the limitednumber of years of available data. Zerefos (2002),Zerefos (1997), Arola et al. (2003), and Lakkalaet al. (2003) for example analysed time seriesof spectral UV irradiance measurements. Zerefos(2002) detected a UV increase of 2% per year at305 nm for cloudless skies, but he concludes thatfew years of the time series are not enough forreliable conclusions on long-term changes. Thesame conclusion is drawn by Arola et al. (2003)who analysed the time series of Thessaloniki andSodankylaa focusing on the factors influencingshort- and long-term changes in spectral UV irra-diance. Lakkala et al. (2003) evaluated changesin Sodankylaa spectral UV data: significant changesin summer monthly mean irradiances could notbe detected. Previous studies using spectral datahave been limited to particular wavelengths andsolar zenith angles (SZAs). In this study we pres-ent an analysis of a wide range of SZAs andwavelengths.

    In addition to the trend analysis of the mea-sured UV time series we investigate the requiredlength of a time series of UV irradiance. Prior tothis study the possibilities of the detection oftrends have been investigated by Weatherheadet al. (1998) who provide a general statisticalstudy on the detection of trends. The resultingtables give an estimation of the number of yearsto detect a trend (of any geophysical parameter)in dependency on the autocorrelation of the noiseof the time series and the standard deviation ofthe noise. Lubin and Jensen (1995) calculated theelapsed time after the onset of ozone depletionbefore the dose rate for skin cancer has increased

    by a certain factor. This deduced time period de-pends on the standard deviation of the cloud-induced interannual variability, and the resultsare provided as global maps. Our study intro-duces a new method for an estimation on therequired time series length for trend detectionusing the measured time series in combinationwith model calculations.

    2. Data and methods

    Spectral UVirradiance from Thessaloniki, Greece,(40.5 N, 22.9 E) and Sodankylaa, Finland,(67.4 N, 26.6 E) has been analysed with theaim to search for trends in UV irradiance. Thesestations were chosen because they comprise thelongest time series of spectral irradiance withinEurope, about 1214 years each, and they repre-sent different climate zones within Europe. Bothtime series were recorded by Brewer single mono-chromator spectroradiometers (Bais et al., 1996;Lakkala, 2001).

    2.1 Instrumentation

    For the global UV measurements both instru-ments contain a Teflon diffusor with a diameterof about 35 mm, protected by a quartz dome. TheUV scans are recorded in a wavelength range of290330 nm in Thessaloniki and 290325 nm inSodankylaa, both in steps of 0.5 nm.

    Single monochromators have the disadvantageof stray light (Seckmeyer et al., 2001) especially atthe short wavelength end where the UV changesdue to ozone changes are most pronounced. How-ever, for this study priority was given to the lengthof the time series. Stray light becomes importantin the wavelength region below 300 nm (Bais et al.,1996), thus it was decided to focus on the wave-length range between 300 and 315 nm. The upperlimit was chosen according to the decreasing in-fluence of ozone on the UV radiation towards theUVA range.

    At both stations the instrument is calibratedmonthly while the stability of the instrumentsspectral responsitivity is controlled once per week(Arola et al., 2003). Moreover, for the purpose ofquality assurance both Brewer instruments havetaken part in several intercomparisons for UVspectroradiometers (Kjeldstad, 1997; Bais et al.,1996).

    34 M. Glandorf et al.

  • 2.2 Data extraction

    The spectral data have been submitted to theEuropean UV Database (EUVDB; FMI, 2003;Seckmeyer, 2000). This relational database al-lowed us to extract all data with SZAs between30 and 65 (Thessaloniki) and between 44 and65 (Sodankylaa) and to separate it into subsetswith constant SZA as a function of the wave-length. A limited band of only 1 width wasselected. A SZA of e.g. 62 is defined tocomprise all SZAs with 61.5 < 62:5. Inaddition to the data set that comprises all sky con-ditions a cloudless sky data set has been ex-tracted from the EUVDB. For such cases theEUVDB provides the option to search for spectrawith specified atmospheric conditions. The selec-tion of cloudless sky data relies on comparisonsof measured UV spectra with modelled UV spec-tra for the various atmospheric scenarios.

    The dependence of the SZA on UV irradianceis very strong. Therefore the influence of the SZAmust be removed for the analysis of the data. Evenwith a relatively small variation of the SZA, erro-

    neous results may be obtained (Arola et al., 2003).In order to remove this effect all measurementsare normalised to specified SZA values by meansof model calculation. Each irradiance is multi-plied by a factor that represents the difference inUV irradiance caused by the difference betweenthe SZA during the measurement and the speci-fied SZA value. Additionally all measurements arealso normalised for a constant Sun-Earth-distance.

    When studying irradiances at single wave-lengths wavelength stability is crucial (Arolaet al., 2003). Thus, all spectra were correctedfor possible wavelength shifts using the SHIC-rivm program developed by Slaper (2002).The remaining uncertainty due to wavelengthalignment of up to 0.02 nm is negligible.

    2.3 Data analysis

    Regression lines were fitted to all time series ofspectral UV irradiance; an example for 306 nm isgiven in Fig. 1. With an analysis of the residualsthe assumption of a linear trend was justified.

    Fig. 1. Spectral irradiances at different SZAs at 306 nm from Thessaloniki. Each data point represents one measurement atthe given combination of wavelength and SZA. The solid line represents the regression line. Strength and sign of the changevary. Please note that the range of the y-axis varies due to the large dependence of UV irradiance on SZA

    Possibilities to detect trends in spectral UV irradiance 35

  • For the interpretation of detected gradients it isessential to have information about its statisticalsignificance. The test after Mann (1945) is anappropriate method to estimate the significancewithout requiring the data being normally distrib-uted. It assesses a relative increase or decrease inthe time series but gives no detailed informationabout the temporal behaviour of this change. Ithas to be noted that the resulting significancelevel ( error probability) refers to the existenceof a positive or negative gradient and does notconfirm the magnitude of the calculated increaseor decrease rate.

    For the estimation of the required number ofyears we utilise the original, 1214 year long timeseries and their variability. It can be suspectedthat this time series adequately reflects the natu-ral variability of UV irradiance. The initial timeseries is detrended and extended. Afterwards an

    artificial trend is superimposed on the data whichrepresents a UV increase due to a given ozonetrend scenario. The extended time series is thenanalysed by a linear regression. We considereda time series to be long enough if the calculatedUV upward gradient caused by the given ozonedecrease is significant at the 1% level.

    The following list gives an overview of thesingle steps of this method:

    1. A possible linear trend from the original timeseries is removed.

    2. An artificial trend, which corresponds to anincrease in UV irradiance due to a realisticdecline in total ozone column, is superim-posed on the data.

    3. A linear regression analysis is carried out.4. The significance of the calculated gradient is

    determined.

    Fig. 2. The process of extending the time series.Top: the original detrended time series (Thessaloniki,304 nm, 63 SZA). Middle: extension of the timeseries from 13 years to 52 years by appending theinitial data. Bottom: the extended data set after themultiplication with factors representing an increasein UV irradiance due to a 4.5% decrease in totalozone

    36 M. Glandorf et al.

  • 5. Criterion: A time series is considered longenough if the calculated upward gradient ofthe regression line has a significance level of1%:a) Significance level 1% ! time series is

    long enough.b) Significance level >1% ! time series has

    to be extended: the original sequence isattached and the process continues withstep 2 again.

    The superposition of the artificial trend is doneby multiplication of the spectral irradiances withfactors that represent the increase of UV irradi-ance due to a certain ozone decrease for clearskies. These factors have been determinedthrough simulations with the radiative transfermodel UVspec (Kylling and Mayer, 2002).

    The Thessaloniki ozone trend rate is an averagedecrease for the years 19901998 (Zerefos, 2002).Information on the ozone trend for Sodankylaa(Arctic region, 65 N) was taken from Bojkovet al. (1998). The corresponding period is fromJanuary 1979 to December 1997. To gain moreinformation about the performance, additionalmodel runs with an imaginary ozone decrease of1.0% per decade were carried out. The completeyear analyses use the complete data set whilethe seasonal analyses comprise only data of therelevant months. A possible winter scenario for

    Sodankylaa is not considered as the UV irradiancemeasurements in winter are too sparse.

    Figure 2 shows the process of time seriesextension from the original (uppermost paneldiagram) to the extended time series with theintroduced ozone trend (diagram at the bottom).

    3. Results and discussion

    Figure 1 provides an overview of spectral irra-diances measured at Thessaloniki for the arbi-trarily chosen wavelength 306 nm. It can be seenthat magnitude and sign of the regression linevary for different SZAs. While there is a slightdecrease in UV irradiance at SZAs of 30, 35,40, and 45, slight increases at 50, 55, 63,and 65 and a strong increase at 60 SZA canbe observed. With regard to the variation of theregression lines this selection is representativefor other combinations.

    Figure 3 shows calculated changes of irradianceper decade for Thessaloniki. Each single datapoint in this diagram represents the calculatedin- or decrease of UV irradiance of a particularcombination of SZA and wavelength. These cal-culated UV changes per decade range from20%per decade up to almost 80% per decade. For themaximum data point the data set is added in anextra display. Lines indicating SZAs between 30and 43 are coloured light grey, SZAs between

    Fig. 3. Change of irradiance perdecade in percent as a function ofwavelength for Thessaloniki. Dif-ferent symbols and linestyles indi-cate different SZAs (see legend).The top symbol represents SZA 54

    Possibilities to detect trends in spectral UV irradiance 37

  • 44 and 54 a...

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