ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-1805-2018Reconstruction and analysis of erythemal UV radiation time series from
Hradec Králové (Czech Republic) over the past 50 yearsEUV radiation in Hradec Králové, 1964–2013ČížkováKláracizkova.klara@hotmail.comLáskaKamilhttps://orcid.org/0000-0002-5199-9737MetelkaLadislavStaněkMartinDepartment of Geography, Faculty of Science, Masaryk University, Brno, 611 37, Czech RepublicSolar and Ozone Observatory, Czech Hydrometeorological Institute,
Hradec Králové, 500 08, Czech RepublicKlára Čížková (cizkova.klara@hotmail.com)7February20181831805181827July20174September201721December201721December2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/1805/2018/acp-18-1805-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/1805/2018/acp-18-1805-2018.pdf
This paper evaluates the variability of erythemal ultraviolet (EUV) radiation
from Hradec Králové (Czech Republic) in the period 1964–2013. The
EUV radiation time series was reconstructed using a radiative transfer model
and additional empirical relationships, with the final root mean square error
of 9.9 %. The reconstructed time series documented the increase in EUV
radiation doses in the 1980s and the 1990s (up to 15 % per decade), which
was linked to the steep decline in total ozone (10 % per decade). The
changes in cloud cover were the major factor affecting the EUV radiation
doses especially in the 1960s, 1970s, and at the beginning of the new
millennium. The mean annual EUV radiation doses in the decade 2004–2013
declined by 5 %. The factors affecting the EUV radiation doses differed
also according to the chosen integration period (daily, monthly, and
annually): solar zenith angle was the most important for daily doses, cloud
cover, and surface UV albedo for their monthly means, and the annual means of
EUV radiation doses were most influenced by total ozone column. The number of
days with very high EUV radiation doses increased by 22 % per decade, the
increase was statistically significant in all seasons except autumn. The
occurrence of the days with very high EUV doses was influenced mostly by low
total ozone column (82 % of days), clear-sky or partly cloudy conditions
(74 % of days) and by increased surface albedo (19 % of days). The
principal component analysis documented that the occurrence of days with very
high EUV radiation doses was much affected by the positive phase of North
Atlantic Oscillation with an Azores High promontory reaching over central
Europe. In the stratosphere, a strong Arctic circumpolar vortex and the
meridional inflow of ozone-poor air from the southwest were favorable for
the occurrence of days with very high EUV radiation doses. This is the first
analysis of the relationship between the high EUV radiation doses and
macroscale circulation patterns, and therefore more attention should be
given also to other dynamical variables that may affect the solar UV
radiation on the Earth surface.
Introduction
Solar ultraviolet (UV) radiation causes a wide variety of environmental and
health effects, including the deceleration of the rate of photosynthesis,
damage of DNA structures, and the increased risk of skin cancer in humans
(e.g., Diffey, 1991; Caldwell, 2007). The scientific interest in solar UV
radiation has significantly increased since the 1980s, when it was discovered
that the many adverse environmental and health effects of the solar
ultraviolet (UV) radiation had been reinforced by the thinning of the ozone
layer (e.g., Farman et al., 1985; Krzyscin and Borkowski, 2008). The effects
of UV radiation on human skin differ according to the wavelength, so the
erythema action spectrum (McKinlay and Diffey, 1987) was developed to model
the susceptibility of human skin to sunburn. Therefore, it is important to
assess the changes in the erythemal ultraviolet (EUV) radiation over a longer
period of time and relate them to processes and events that are involved in
the attenuation of solar radiation passing through the atmosphere (Lucas et
al., 2006).
The incident UV irradiance is affected by a broad range of atmospheric and
environmental factors, of which the total ozone column (TOC) is arguably the
most studied one. Ozone, together with molecular oxygen, absorbs all UV-C and
a part of UV-B radiation, which are the most harmful ones to organisms (Bais,
2015). The changes in UV radiation are further modulated by cloudiness and
cloud types, atmospheric aerosols, solar zenith angle, and the altitude and ground surface
characteristics of the location (Kerr, 2005). According to numerous studies,
atmospheric circulation and stratospheric temperature greatly affect the
intensity of global ozone losses (e.g., Orsolini and Limpasuvan, 2001;
Schnadt and Dameris, 2003; Hofmann et al., 2009). In the 1970s–1990s, TOC
trends in central Europe showed a decline by about 3–4 % per decade,
especially in spring, while the recovery of the ozone layer has been reported
since the end of the 1990s (e.g., Krzyścin et al., 1998; Krzyscin and
Borkowski, 2008; Vaníček et al., 2012; De Bock et al., 2014).
So far, the analyses of EUV radiation trends have been carried out in
different locations across the Northern Hemisphere. A major increase in EUV
radiation doses was observed in the 1980s and 1990s, and according to Ziemke
et al. (2000), the trends ranged from 4.5 % per decade in the
Mediterranean region to 8.6 % per decade in East Asia. The studies from
central Europe reported an increase in EUV daily doses ranging from 2 to
17 % per decade (Rieder et al., 2008), but most commonly between 5 and
6 % per decade (e.g., Den Outer et al., 2010; Krzyścin et al., 2011).
According to Rieder et al. (2008), two-thirds of the ascertained trends were
attributed to the changes in TOC, and the remaining one-third to the combined
decrease in cloudiness and aerosol optical depth. In spite of the ozone layer
recovery since the mid-1990s, this period
does not reveal any statistically significant changes in EUV irradiation
(Hadzimustafic et al., 2013). At the same time, an estimate of future trend
in UV radiation remains unclear because it is often overlaid by the influence
of cloudiness and aerosols, whose trends significantly vary at different
European sites (Bais et al., 2015).
The solar UV radiation reaching the Earth's surface can be measured either by
broadband or narrowband UV radiometers, or by spectrophotometers, like the
Brewer spectrophotometer. Unlike the broadband radiometers, the Brewer
spectrophotometer provides the most accurate UV radiation observations and it
is therefore considered a reference instrument (e.g., Gardiner et al., 1993;
Anav et al., 1996; Hülsen et al., 2008). Nevertheless, the UV radiation
measurements taken by the Brewer spectrophotometers are sparse and often
cover only a short period of time, which complicates their incorporation into
the evaluation of long-term trends and effects on human health (Rieder et
al., 2008). Therefore, in order to assess the UV radiation variability in the
past, time series reconstructions are essential. In central Europe, several
UV radiation reconstructions have recently been performed (e.g., Krzyscin et
al., 2004; Rieder et al., 2008; Den Outer et al., 2010); however, the
reconstructed time series analyses do not extend to the last decade.
The EUV radiation time series presented in this study covers five decades
(1964–2013) and uses radiative transfer modeling and the most precise EUV
radiation measurements available in
Hradec Králové (Czech Republic). The evaluation of the reconstructed
time series for the given location focused on (1) the long-term variability
and trends in EUV radiation, (2) the relationships between EUV radiation
doses, TOC, and cloud cover, and (3) the days with very high EUV radiation
daily doses and their relationship to large-scale atmospheric circulation
patterns.
Study site
The data used in this study have been obtained at the Solar and Ozone
Observatory of the Czech Hydrometeorological Institute, which has been
operating since 1951 and focuses mostly on ozone and solar radiation
measurements (Vaníček, 2001). The observatory is situated on a small
hill in the south of Hradec Králové (50.180∘ N,
15.833∘ E) at an altitude of 285 m a.s.l. The building stands away
from local pollution sources and the southern horizon is open. During the
studied period (1964–2013), the localization of the station was not altered
and, except from the change of the instruments, there are no other known
sources of data inhomogeneity. All the instruments installed at the Hradec
Králové observatory are described in Vaníček (2001) and
Vaníček et al. (2015) and have been calibrated regularly according
to the standards corresponding to the Guide to Meteorological Instruments and
Methods of Observations, World Meteorological Organization (WMO, 2014).
Erythemal UV radiation time series reconstructionInput data
The measurements of UV radiation at the Hradec Králové observatory
started in 1994 (Vaníček, 2001), so in order to analyze the
five-decade-long time series, the EUV radiation time series had to be
reconstructed. As input, total ozone column (TOC), atmospheric optical depth
(AOD), surface albedo, water vapor
column, and global radiation from the Hradec Králové observatory have
been used.
Total ozone column
TOC is one of the most important variables affecting the intensities of EUV
radiation. The mean daily TOC values for the period 1964–2013 were compiled
from the Brewer spectrophotometer MK-IV B098 and the Dobson spectrophotometer
D074 measurements. The missing values (approximately 16 % of all days)
were estimated from ERA-40 and ERA-Interim reanalysis datasets (Uppala et
al., 2005; Dee et al., 2011). Related instruments and the method used for the
mean daily TOC time series completion are described in Vaníček et
al. (2012).
Aerosol optical depth
Since there are no long-term atmospheric aerosol observations at the study
site, annual climatological cycles of AOD were constructed based on the
existing records. This method can approximate the climatological behavior of
aerosols, but it does not consider any yearly variations (Lindfors et al.,
2007). The mean daily AOD values for λ= 320 nm (AOD320) were
measured by the Brewer spectrophotometer MK-III B184 in the period
2005–2013. The mean daily AOD values for λ= 550 nm (AOD550)
for the period 2001–2013 were obtained from the Moderate Resolution Imaging
Spectroradiometer (MODIS) (Remer et al., 2005). The total amount of days with
assigned AOD values, from which the climatological cycle was calculated, was
61 % for AOD320, and 46 % for AOD550.
Surface albedo
Surface albedo is a very important variable determining the intensity of
solar UV radiation. In winter months, surface albedo changes may increase the
intensity of solar UV radiation by up to 20–30 % (Blumthaler and Ambach,
1988; Krzyścin et al., 2004). The mean daily surface albedo for
shortwave radiation in the period 1964–2013 was calculated using an
ensemble of multilayer perceptron neural networks with the observed state of
the surface as the single relevant predictor and the measured shortwave
albedo as the predictand. Only the data from noon hours were used, and the
possible annual course of the relationship was taken into account with the
help of circular neurons added to the input layer. Data from the period
2000–2014 (4269 cases) were used to develop the model, while being randomly
divided into training set (70 % of data entries), testing set (15 %),
and validation set (15 %). The trained network was then applied to
estimate the albedo in the period 1964–1999, when the measured shortwave
albedo was not available. The shortwave albedo was then transferred to the
albedo for UV radiation using Eq. (1). The coefficients were estimated based
on the shortwave albedo for different states of the surface, because for
most surfaces, UV albedo is very low but for snow-covered surfaces it
approaches the values of shortwave albedo (Feister and Grewe, 1995; De Paula
Corrêa and Ceballos, 2008).
ALBUV=0.05forALBSW<0.211.36ALBSW-0.24for0.21≤ALBSW≤0.65ALBSWforALBSW>0.65,
where ALBUV is the surface albedo for UV radiation and
ALBSW is the surface albedo for shortwave radiation.
Water vapor column
Atmospheric water vapor absorbs a significant part of the global solar
radiation (e.g., Solomon et al., 1998); therefore, the total water vapor
column was also taken into account when performing the radiative transfer
calculations. The daily means of water vapor column in the period 1964–2013
were obtained from the ERA-40 and ERA-Interim reanalysis datasets.
Solar radiation
Global solar radiation was used in the EUV radiation time series
reconstruction in order to estimate the cloud effects on EUV radiation (e.g.,
Schwander et al., 2002). In the period 1964–2013, the global radiation was
measured using the CM5 and CM11 pyranometers by Kipp & Zonen
(Vaníček et al., 2015).
At the Hradec Králové observatory, UV radiation has been measured by
the Brewer spectrophotometer MK-IV B098 since 1994. In 2004, the
double-monochromator Brewer spectrophotometer MK-III B184 was installed.
Using the spectroradiometric method, these instruments are able to detect
solar UV radiation in the wavelength interval 280–400 nm (UV-A and UV-B
radiation). Both instruments are calibrated regularly every 2 years against
the B017 world traveling standard, and the calibration uncertainty is up to
1 % (see, e.g., Vaníček, 2002; Vaníček et al., 2015). The
EUV radiation time series obtained by the Brewer spectrophotometer B184 in
the period 2005–2013 helped to determine the EUV radiation reconstruction
equations and to validate the model (see Sect. 3.2, “Methods and model
validation”).
Methods and model validation
The reconstruction of the 1964–2013 EUV radiation time series from the
Hradec Králové observatory was performed using a radiative transfer
model and empirical nonlinear equations. The time series was originally
reconstructed with hourly resolution, because better results are achieved
when a higher time resolution is applied (Rieder et al., 2008). Then, the
EUV radiation daily doses were calculated from the reconstructed data.
First, the clear-sky EUV and global radiation were modeled using the DISORT
solver of the libRadtran radiative transfer package (Mayer and Kylling,
2005). The input parameters contained the information on the location
(latitude, longitude and altitude), TOC, albedo, annual climatological
cycles of atmospheric aerosols (AOD320 and AOD550), and, in the
case of clear-sky global radiation simulation, total water vapor column.
The all-sky EUV radiation was calculated based on global radiation
measurements, which is a common method for reconstructing UV radiation doses
(e.g., Krzyscin et al., 2004; Lindfors et al., 2007; Rieder et al., 2008). The
reconstruction was based on the cloud modification factor
(CMFEUV), which is the ratio between cloudy-sky EUV radiation
(EUVcloud) and clear-sky EUV radiation (EUVclear), as shown
in Eq. (2). Correspondingly, the CMF for global radiation (CMFGLB) was
calculated as the ratio between cloudy-sky (GLBcloud) and clear-sky global radiation (GLBclear), as shown in Eq. (3) (Lindfors et
al., 2007).
CMFEUV=EUVcloudEUVclearCMFGLB=GLBcloudGLBclear
Therefore, the reconstructed all sky EUV radiation (EUVrec) can be
expressed as the modeled EUVclear multiplied by CMFEUV.
CMFEUV was obtained using the global radiation observations, since it
can be expressed as the function of CMFGLB and solar zenith angle (SZA)
(e.g., Lindfors et al., 2007; Bilbao et al., 2011). To obtain the best-fit
function and to minimize the RMSE, a multiple nonlinear regression
expression, shown in Eq. (4), was used. The coefficients used to calculate
CMFEUV are given in Table 1.
CMFEUV=a+bSZACMFGLBc+dSZA⋅k+lCMFGLB+mCMFGLB2
The validation of the model outputs was performed using the individual
measurements of EUV radiation from the period 2005–2013. The individual
records were coupled with the global radiation data, and the entire data set
was then randomly split into two independent data sets with roughly the same
number of entries. The first data set, which included only observations taken
with SZA smaller than 75∘, was used to develop the multiple nonlinear
regression model for the CMFEUV estimation. The second dataset,
which was used to test the model, included EUV irradiance values for all SZA
values. The irregular time interval between the individual Brewer
spectrophotometer EUV irradiation records did not allow for daily doses
integration; therefore, in order to evaluate the model performance, the daily
mean EUV irradiation was calculated and assessed. Table 2 shows the mean
error, the root mean square error and the determination coefficient r2
between the reconstructed and observed mean daily EUV radiation for the whole
period and for the seasons (spring: MAM; summer: JJA; autumn: SON; winter:
DJF). The errors are lowest in summer and highest in winter, while the
correlation between measured and reconstructed daily doses is highest in
autumn and lowest in winter, but it exceeds 94 % in all seasons. Figure 1
shows the absolute and relative differences between the observed and
reconstructed mean daily EUV radiation. Although the absolute differences are
small, the relative differences are largest in winter (the mean relative
difference makes up to 10 % in February). On the other hand, in spring,
summer and autumn, the mean relative differences are in the range of
0–3 %, and less than 20 % of values exceed the 10 % difference
between the observed and reconstructed daily means.
The coefficients used to determine CMFEUV based on CMFGLB
and SZA by the means of nonlinear regression.
The mean (a) absolute and (b) relative differences between the
observed and modeled EUV irradiance and their standard deviations (SD) at
the Hradec Králové observatory in the period 2005–2013.
Methods of the time series analysis
The analysis of the reconstructed 1964–2013 EUV radiation consists of three
parts: (1) the annual variations and trends, (2) the factors affecting EUV
radiation, and (3) the analysis of days with very high EUV radiation doses.
The data were processed using the software STATISTICA® 10, R
Project for Statistical Computing (R Core Team, 2014), and
ArcGIS® 10.
To perform the analyses, TOC and surface albedo data (Sect. 3.1) were used.
Hourly cloud cover data in octas were included in the analysis in order to
evaluate the cloud effects on EUV radiation. There are no cloud cover records
directly at the Hradec Králové observatory; therefore, the data from
the Hradec Králové, Pouchov synoptic station (50.246∘ N,
15.843∘ E; 243 m a.s.l.; 8 km north of the Hradec Králové
observatory) were used. The missing values in the time series were
supplemented with data from the Pardubice, airport synoptic station (located
in flat lowlands at 50.013∘ N, 150739∘ E; 225 m a.s.l.; 20 km
south of the Hradec Králové observatory). The linear regression
relationship between these two datasets was strong and statistically
significant (r2= 73 %). For every day, the weighted mean of
cloud cover was calculated, while the weight was set as the mean clear-sky
EUV radiation at the certain time of the day. The gaps in the aggregated
daily cloud cover time series (7 % of days) were then filled by a
nonlinear regression using the global radiation daily doses from the Hradec
Králové observatory (r2= 76 %).
The trends in EUV radiation, as well as TOC and cloudiness, were studied
using locally weighted scatterplot smoothing (Cleveland, 1979). The current
trends in TOC were evaluated using linear regression in the period
1995–2013, which was selected for this assessment because in 1995, total
ozone reached its minimum over central Europe (Krzyscin and Borkowski,
2008). The effect of SZA, TOC, cloud cover, and surface UV albedo on EUV
radiation daily doses and their monthly and yearly means throughout the
entire study period and the five decades was analyzed using partial
correlation coefficients (rpart). The level of statistical significance
α was set to 0.05 in all the tests performed in this study.
Mean error (ME) and its percentage value (ME%), root mean
square error (RMSE) and its percentage value (RMSE%), and the
determination coefficient (r2) for the reconstructed time series and
for the individual seasons at the Hradec Králové observatory for
1964–2013.
The annual means of (a) TOC, (b) cloud cover, and (c) daily EUV
radiation doses fitted with the locally weighted scatterplot smoothing curve
(red), and the mean annual variation of (d) TOC, (e) cloud cover, and (f) daily
EUV radiation doses at the Hradec Králové observatory in the
individual decades of the period 1964–2013.
The emphasis was placed on days with very high EUV radiation doses (days with
EUV90+). They were defined based on the 90th percentile of the EUV
radiation daily doses for each month. Therefore, 1827 days with EUV90+
were selected and further analyzed. The monthly threshold values of
EUV90+ for the period 1964–2013 are displayed in Table 3. The days with
EUV90+ were then assessed with respect to trends and explained with
regards to TOC, cloud cover and surface UV albedo. Low TOC was defined as the
50th percentile of each month and lower. According to Alados-Arboledas
et al. (2003), high intensities of EUV radiation can be observed under the
cloud cover of 4 octas or less, therefore, low cloud cover threshold was set
to this value. High albedo was set to 0.3, which is, according to De Paula
Corrêa and Ceballos (2008), the lowest albedo of consistent snow cover.
The pressure field in the days with EUV90+ was investigated using
principal component analysis (Storch and Zwiers, 1999). Two geopotential
heights were chosen: 1000 hPa, which is well representative of surface
pressure, and 70 hPa, which is the level near the ozone layer maximum where
the ozone changes were most evident (Kirchner and Peters, 2003). The field of
geopotential heights for pressure levels 1000 and 70 hPa in the period
1964–2013 was obtained from the NCEP/NCAR reanalysis (Kalnay et al., 1996).
The study area was defined as a rectangle between 40∘ W–40∘ E
and 20–70∘ N with a spatial resolution of
2.5∘. The daily values of the principal components were then
correlated with the Northern Atlantic Oscillation (NAO) and Arctic
Oscillation (AO) indices (Wallace and Gutzler, 1981; Thompson and Wallace,
1998). Although NAO and AO are not independent from each other (e.g., Vallis
et al., 2004), they are linked to slightly different processes in the
atmosphere. While AO is zonally symmetrical and describes mostly the polar
vortex, NAO relates mainly to the see-saw variability of the Icelandic Low
and Azores High pressure regions (Ambaum et al., 2001). Therefore, in this
study, AO and NAO were assessed separately.
Results and discussionAnnual variations and trends
During the period 1964–2013, the annual means of TOC (Fig. 2a) show a large
relative variability (coefficient of variation cv= 3.5 %) with
diverse trends in the individual decades (Table 4). In the 1960s and 1970s,
the annual mean TOC at the Hradec Králové observatory fluctuated
between 340 and 360 DU. The decline in TOC started at the beginning of the
1980s, reaching the minimum in 1993 following the eruption of Mount Pinatubo
(annual mean of 315 DU). Then the TOC fluctuated at lower values than at the
beginning of the studied period (320–350 DU). The most distinct decline in
TOC was recorded in spring; the weakest decline was observed in autumn (Fig. 2d).
From 1995 on, the linear trends in TOC were not statistically significant
in any particular month, but there was an increase in winter (January:
1.2 ± 0.7 DU per year; February: 1.3 ± 0.9 DU per year) and a
decrease in summer (August: -0.4 ± 0.4 DU per year). The trends in TOC,
which are linked mostly to the increase in ozone depleting substances in the
stratosphere, but also to the changes of atmospheric dynamics, show a very
similar development to other parts of central Europe (e.g., Trepte and
Winkler, 2004; Hood and Soukharev, 2005; Harris et al., 2008). The negative
trends in warm seasons after 1995, which can be linked to the changes of
circulation patterns affecting the concentration of ozone depleting
substances in the atmosphere, were also recorded at other parts of central
Europe (Krzyscin and Borkowski, 2008; Vaníček et al., 2012;
Krzyścin and Rajewska-Więch, 2015). The annual variation of TOC
recorded at the Hradec Králové observatory, with its minima in
autumn (October) and maxima in spring (March or April), is connected to the
natural variability of TOC in Europe, which is linked to the transport of
ozone from low latitudes (Zvyagintsev et al., 2015).
Annual cloud cover means (Fig. 2b) exhibit a larger relative variability
than TOC (cv= 5.3 %) and changeable trends (Table 4). Cloud cover
maxima were recorded in winter (November or December) and minima in summer
(August), while in some years there was a secondary maximum present in June
or July (attributed to prevailing convection). Over the period 1964–2013,
cloud cover did not show any compact trend in any of the months (Fig. 2e).
There was a period of high cloud cover during the end of the 1970s and the
beginning of the 1980s, which was most pronounced in July and October, with
the maximum yearly mean cloud cover in 1981 (6.0 octas). This period was
followed by a decrease in cloud cover in the 1990s, which was very distinct
in January, May, July, and October. Since the 1990s, the yearly mean cloud
cover increased to up to 5.9 octas in 2013. According to Wibig (2008), who
observed similar patterns in Poland, the most important factor determining
the changes in cloud cover is the variability of the occurrence of different
cloud types, for example the intensification of convection in summer months.
The changes in the 1990s seem to be driven especially by the decreasing
amount of sulfur dioxide emitted into the atmosphere (e.g., Krüger and
Graßl, 2002).
Linear trends and standard error of annual means of TOC, cloud cover
and EUV radiation doses for the individual decades of the period 1964–2013
at the Hradec Králové observatory; asterisks mark statistically
significant trends (α= 0.05).
Of the three studied variables, the annual mean daily doses of EUV radiation
(Fig. 2c) show the largest relative variability (cv= 7.4 %).
The trends in the annual mean daily doses of EUV radiation were also
changing over the study period (Table 4). The EUV radiation doses declined
until the end of the 1970s, which might indicate a connection to the
increase in cloudiness in this period. In the 1980s and 1990s, the EUV doses
increased steeply due to the decrease in TOC, with the maximal mean daily
dose of EUV radiation being recorded in 2003. The slight decline in EUV
radiation doses since about 2005 was mostly attributed to the changes in
cloud cover. The relative increase in EUV radiation doses was most
pronounced in spring and summer, less in autumn and winter. The overall
changes in EUV radiation doses were not statistically significant in
January, September, October, or December. These results are in accord with
most other publications focusing on the long-term variability of EUV
radiation in central Europe (e.g., Rieder et al., 2008; Den Outer et al.,
2010; Krzyścin et al., 2011). The very high EUV radiation doses in 2003,
which were caused by the anomalously low cloud cover due to high pressure
episodes in summer, were also recorded at other European stations (Den Outer
et al., 2010; Rieder et al., 2010). The annual variations of EUV radiation
doses, shown in Fig. 2f, are clearly following the changes in SZA with
maxima in summer (June, July) and minima in winter (December). The shift of
the annual maxima from June to July can be attributed to the annual
variations of TOC (Seckmeyer et al., 2008).
Partial correlation coefficients (rpart) of the effects
of (a) SZA, (b) TOC, (c) cloud cover, and
(d) surface UV albedo on the daily doses in individual months and
decades; the decades are labeled only by their initial years.
The number of days with EUV90+ in the individual studied
decades, and their explanation by TOC, cloud cover, albedo, and their
combinations, for (a) the entire year, (b) spring,
(c) summer, (d) autumn, and (e) winter.
Factors affecting the erythemal UV radiation doses
The importance of the factors affecting the EUV radiation doses varied based
on the chosen time period, i.e., daily doses and their monthly and annual
means. Over the period 1964–2013, the dominant factor affecting the annual
means of EUV radiation daily doses was TOC (partial correlation coefficient
rpart=-0.75), yet the effects of cloud cover and surface UV albedo
were also statistically significant (rpart=-0.31, resp. 0.23). The
monthly means of EUV radiation doses were affected mostly by SZA, followed
by TOC, surface UV albedo, and cloud cover (Table 5). The effect of observed
variables changed during the individual months. In winter (January,
February, December), the effect of surface UV albedo was greater than the
effect of cloud cover and TOC, whereas in June and August, the monthly means of
EUV radiation doses were most affected by TOC. In all other months, cloud
cover was the most important factor. As shown in Table 5, the daily doses of
EUV radiation were most affected by SZA, cloud cover, and then by TOC and
surface UV albedo (rpart=-0.88, -0.59, -0.35, and 0.22,
respectively). During the individual months, cloud cover was the most
important factor affecting the EUV radiation daily doses; TOC had the most
pronounced effect in April. Furthermore, the effect of SZA, TOC, cloud cover
and surface UV albedo varied throughout the study period. The effect of SZA
did not change much over the decades; it ranged from rpart=-0.84 to
rpart=-0.87. It was the strongest in the months when SZA is the
most variable (spring and autumn months), but it had insignificant effect in
the months near the solstices (Fig. 3a). The effect of TOC was most
pronounced in the decade 1984–1993 (rpart=-0.34), when the
thinning of the ozone layer was most significant. Figure 3b shows TOC had
the strongest effect on the daily EUV radiation doses in the spring months,
but also in late summer and early autumn, especially in the decades
1984–1993 and 1994–2003. The effect of cloud cover was the strongest in
the decade 1994–2003 (rpart=-0.63), while the weakest relationship
was found in the decade 2004–2013 (rpart=-0.57). As shown in Fig. 3c,
the effect of cloud cover also differed in individual seasons, with the
strongest influence in the summer months of the decades 1964–1973,
1984–1993 and 2004–2013, and in the spring and autumn months of the decade
1974–1983. Surface UV albedo (Fig. 3d) was only affecting the EUV radiation
doses in winter months; therefore, its effect throughout the decades was the
weakest of all the studied variables (rpart ranging from 0.16 in the
decade 1984–1993 to 0.25 in the decade 2004–2013).
Partial correlation coefficients of the effect of SZA, TOC, cloud
cover, and UV albedo on EUV radiation daily doses and their monthly means
throughout the period 1964–2013; asterisks mark statistically significant
correlations (α= 0.05).
The results concur with similar studies performed at other European
locations, showing that TOC is the main driver of long-term EUV radiation
changes, while SZA and cloud cover are the dominant factors affecting EUV
radiation in the short term (Den Outer et al., 2010; Herman, 2010; De Bock
et al., 2014). The changes in EUV radiation in the 1970s have been
attributed to cloud cover variability, which is in accord with the high
rpart shown in Fig. 3c. The latter increase in EUV radiation doses was
caused mainly by the strong total ozone loss. For example, in the late 1980s
to early 2000s, the changes in TOC were responsible for the majority of
changes in EUV radiation in Austria, especially in spring (Rieder et al.,
2008). At the beginning of the new millennium, the effect of cloud cover
grew stronger in contrast to the previous decades (Krzyścin et al.,
2004).
Very high erythemal UV radiation daily doses
The EUV radiation doses reconstructed for the Hradec Králové
observatory within the chosen study period have been examined with regards
to very high values (EUV90+). Based on the 90th percentile of each
month (see Sect. 4 and Table 3), a total of 1827 days with EUV90+ were
selected for further analysis. In the period 1964–2013, the number of days
with EUV90+ increased by 8.1 ± 1.1 days (i.e., 22 ± 3 %) per
decade (r2= 55 %), but the increase was variable in
time. In the 1960s and the beginning of the 1970s, the number of days with
EUV90+ decreased, followed by a steep increase, which stopped at the
end of the 1990s. The number of days with EUV90+ increased again at the
beginning of the 21st century. The general increase in days with
EUV90+ was statistically significant in all seasons except autumn,
while it was most evident in spring (an increase by 3.2 ± 0.5 days in
10 years, r2= 47 %). The factors affecting the occurrence of days
with EUV90+ in all the studied decades are shown in Fig. 4, while the
thresholds of these factors are described in Sect. 4. There was a
statistically significant negative correlation of the yearly number of days
with EUV90+ and mean yearly TOC (r2= 61 %) and with mean
yearly cloud cover (r2= 23 %). The effect of TOC was strongest in
spring (r2= 53 %) and weakest, but still statistically
significant, in winter (r2= 8 %). Cloud cover showed the most
significant effect in summer (r2= 17 %) and its effect was
insignificant in autumn. 82 % of days with EUV90+ were recorded when
the TOC was low (less than the 50th percentile of each month), but in
spring and summer this value exceeded 94 %. Moreover, 74 % of days
with EUV90+ occurred when the daily mean cloud cover was 4 octas or
lower; in summer this value increased to 87 %. High albedo was observed
in 19 % of the days with EUV90+, in winter in 74 % of days with
EUV90+. Most of the unexplained days with EUV90+ occurred in
autumn, when the studied factors were also less likely to combine.
Therefore, the yearly number of days with EUV90+ is both in total and
in all seasons affected more by TOC than by cloud cover. The increase in
days with high EUV radiation doses was also observed in Austria and
Switzerland, especially in the 1990s. Low TOC and its combination with
partly cloudy or cloudless skies were also the most frequent causes of days
with high EUV radiation doses (Rieder et al., 2010).
The values of the first and second PCA modes of the 1000 and
70 hPa geopotential heights for the days with EUV90+ in spring, summer,
autumn, and winter (represented by the color legend on the right).
Variability explained by each of the modes is indicated by the relative
value in the bottom-left corner. The red dot marks the location of the
Hradec Králové observatory.
The correlation coefficients between the 1000 hPa first PCA
component and the NAO and AO indices; asterisks mark statistically significant
correlations (α= 0.05).
The relationship between the very high EUV radiation doses and the
atmospheric circulation was studied using principal component analysis
(PCA) and the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO)
indices. The first and second PCA modes (Fig. 5) show that the occurrence of
days with EUV90+ is linked to NAO, AO, and the shape and strength of
the Arctic circumpolar vortex. The 1000 hPa first component clearly
indicates the effect of the Azores High promontory in central Europe. The
intensities of the Azores High and Icelandic Low are statistically
interconnected (Wallace and Gutzler, 1981); therefore, both these pressure
systems are linked to the occurrence of days with EUV90+. This can be
supported by the statistically significant negative correlation coefficients
between the 1000 hPa first PCA component and the AO and NAO indices (Table 6).
Although the relationship between air pressure systems and shortwave
radiation is expressed mostly through the redistribution of clouds
(Chiacchio and Wild, 2010), the results presented in this study indicate
that low TOC is the dominant factor affecting the incidence of days with
EUV90+. Low TOC occurs mostly during the NAO positive phase, which
leads to stationary planetary waves, stronger zonal winds, and therefore a
colder Arctic circumpolar vortex (Orsolini and Limpasuvan, 2001; Schnadt and
Dameris, 2003). A cold polar vortex allows the development of polar
stratospheric clouds, which leads to greater ozone depletion over the
Northern Hemisphere (Harris et al., 2010). Moreover, during the positive NAO
phase, a ridge of high air pressure develops over central Europe (Hurrell,
1995), which usually leads to little cloudiness, enabling a more frequent
occurrence of days with EUV90+. The second 1000 hPa PCA mode can be
associated with the spatial pattern of high- and low-air-pressure areas
south of Iceland and over Scandinavia. It can therefore be understood as the
meridional airflow over the North Sea, either from or to the area of the
Arctic circumpolar vortex.
The zonal character of the 70 hPa first PCA mode in spring, autumn, and
winter (Fig. 5) is linked to the strength of the Arctic circumpolar vortex.
It seems to be the dominant stratospheric factor affecting the amount of
ozone and therefore also the occurrence of days with EUV90+ in these
seasons. The Arctic circumpolar vortex decays in summer (Harvey et al.,
2002), so the polar front cyclones can easily penetrate it, upset its
symmetry, and affect the transport of ozone from the lower latitudes.
Subsequently, this situation also favors the occurrence of days with
EUV90+, which is especially affected by the influx of air from the
southwest. The first PCA mode in summer and the second PCA mode in other
seasons document this meridional component of atmospheric circulation, which
allows ozone-poor air from the tropics to flow into central Europe as well
as high-latitude regions. Moreover, these conditions are often accompanied
by high air pressure leading to clear-sky or partly cloudy situations (Stick
et al., 2006), thus having major influence on the incidence of high EUV
radiation doses.
Concluding remarks
In this paper, the reconstruction and analysis of the EUV radiation daily
dose time series from the Solar and Ozone Observatory in Hradec
Králové (Czech Republic) have been performed for the period
1964–2013. The time series was reconstructed using a radiative transfer
model and an empirical relationship of the observed EUV and global solar
radiation. The model was verified based on EUV radiation observations of the
Brewer spectrophotometer B184 in the period 2005–2013. The selected methods
applied for the time series reconstruction gave the best estimates of EUV
radiation using the input and validation data (solar radiation, TOC, albedo,
AOD, water vapor column) available for the given location. This approach
significantly improved the quality and accuracy of the reconstructed EUV
radiation time series, which was extended to the entire 1964–2013 period.
The study focused on the evaluation of general variations and long-term
trends, as well as the factors affecting EUV radiation doses. The results showed
that the EUV radiation daily doses were characterized by large variability
and changeable trends in the individual decades. Due to increasing cloud
cover and TOC fluctuations, the EUV radiation doses slightly decreased in
the 1960s and 1970s. The rapid increase in EUV radiation daily doses,
observed in the 1980s and 1990s, was linked to the TOC decline, supporting
the findings of previous studies on TOC variation and past EUV radiation
doses. At the beginning of the new millennium, EUV radiation fluctuations
were mostly attributed to the changes in cloud cover. Moreover, the results
confirmed that the annual means of EUV radiation daily doses are most
affected by the changes in TOC, while the monthly means are most influenced
by cloud cover or, in winter months, by surface UV albedo. The daily EUV
radiation doses were mainly affected by SZA, but the effect of SZA was
insignificant in the summer and winter solstice months.
The number of days with very high EUV radiation doses (EUV90+)
increased significantly throughout the study period, especially in the
spring months. The occurrence of days with EUV90+ was mostly affected
by low TOC, but clear or partly cloudy skies also had a significant role,
especially in summer. In winter months, the increased surface albedo also
significantly affected the occurrence of days with EUV90+. Therefore,
it was clearly indicated that the main factors affecting the long-term
changes of the occurrence of days with EUV90+ are seasonally dependent.
For the first time, the relationship between the high EUV radiation doses
and synoptic weather situations was evaluated using principal component
analysis and large-scale atmospheric circulation patterns at the 1000
and 70 hPa geopotential heights. The results suggest that the days with
EUV90+ occur most likely during the positive NAO phase, when the Azores
High promontory reaches over the area of central Europe. These conditions
lead to a cold and stable Arctic circumpolar vortex, which contributes to
accelerated ozone depletion over the Arctic region. Moreover, a high air
pressure ridge over central Europe frequently causes cloudless skies and
subsequently higher EUV radiation doses. In summer, the influx of ozone-poor
air from the southwest in the upper levels of the atmosphere can also
contribute to the occurrence of days with EUV90+.
Compared to the existing long-term datasets, the reconstructed EUV radiation
time series is one of the longest in central Europe, and therefore it can be
used for the investigation of the long-term photobiological effects on
organisms, or for skin cancer research. Furthermore, the study brought
valuable knowledge of regional EUV radiation variability and trends in the
past decades. It especially provided a closer look at the days with very
high EUV radiation doses and their relationships with other variables,
including the atmospheric circulation patterns. Nevertheless, the EUV
radiation doses and the trends described in this study might be
significantly affected by ozone layer recovery and by the changes in
atmospheric circulation patterns predicted by the atmospheric chemistry and
climate models (e.g., Inglesias-Suarez et al., 2016). It must, however, be
anticipated that the scope of the study has not allowed to take into account
other possibly important factors, such as the El Niño–Southern
Oscillation or Quasi-Biennial Oscillation, which will be considered in
further research.
The data used for this study that were obtained at the
Solar and Ozone Observatory, Hradec Králové (SOO-HK), and other Czech
Hydrometeorological Institute (CHMI) stations are property of CHMI, and
therefore can not be made publicly accessible. Data will be provided upon
request by Ladislav Metelka, the head of SOO-HK
(metelka@chmi.cz). Data
on the total ozone content and water vapor column have been taken from the
ERA-40 and ERA-Interim reanalysis issued by the European Centre for
Medium-Range Weather Forecasts; the aerosol optical depth data have been
provided by the NASA Goddard Earth Sciences Data and Information Services
Center (at http://giovanni.gsfc.nasa.gov); and the geopotential height
fields were acquired from the NCEP/NCAR reanalysis of the Earth System
Research Laboratory, National Oceanic and Atmospheric Administration
(https://www.esrl.noaa.gov/psd/data/reanalysis/reanalysis.shtml).
KL, LM, and KČ drew up the
research idea; KČ and LM prepared the data;
MS wrote the libRadtran script; and KČ
carried out the time series reconstruction and the analyses. KČ and
KL prepared the manuscript with contributions from LM.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Quadrennial Ozone
Symposium 2016 – Status and trends of atmospheric ozone (ACP/AMT
inter-journal SI)”. It is a result of the Quadrennial Ozone Symposium 2016,
Edinburgh, United Kingdom, 4–9 September 2016.
Acknowledgements
The research was supported by the project of the Czech Hydrometeorological
Institute no. 03461022 “Monitoring of the ozone layer and UV radiation in
Antarctica”, which is funded by the State Environmental Fund of the Czech
Republic, and by the project of Masaryk University MUNI/A/1419/2016. The
article outputs contribute to the CzechPolar2 and ECOPOLARIS working group
(CZ.02.1.01/0.0/0.0/16_013/0001708). Edited
by: Sophie Godin-Beekmann Reviewed by: two anonymous referees
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