UV-Indien Network: Ground-based measurements dedicated to the monitoring of UV radiation over the Western Indian Ocean.

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Introduction
Solar radiation ::: The ::: sun : produces a broad spectrum of radiation, in which wavelengths between 100 and 400 nm correspond to ultraviolet radiation (UVR).The UVR that reaches the Earth''s surface has a direct effect on human health, terrestrial and aquatic ecosystems, and on the degradation of material :::::::: materials (Bernhard et al., 2020).
UV has important effects on human health.These effects can be beneficial or harmful.For example, human exposure to UV-B ensures the synthesis of vitamin D, which is essential for bone mineralization.UVR is also known to be a powerful virus killer, which can explain some epidemiological pattern (Duan et al., 2003).On the other hand, UV-B can have a harmful effect by altering the DNA of cells.For humans, an increase in UV-B radiation increases the risk of skin cancer and the occurrence of cataracts and weakens the immune system.
The UV Index (UVI) was defined by the World Health Organization (WHO, 1995) to measure the amount of sunburnproducing UV radiation that reaches the surface.If the UVI is above 3, protection is necessary and, above 8, prolonged exposure can become dangerous.Indices above 11 are considered extreme.
Naturally, the geographical area corresponding to the tropics of the southern hemisphere (tropical and ocean-dominated environment) exposes populations to high levels of UV radiation.This is because the zenith angles of the sun are very small close to noon :: are :::: very ::::: small : all year round and the amounts of stratospheric ozone or atmospheric aerosols are lower than at higher latitudes in continental regions.This is the case in most of the southern Indian Ocean (La Réunion, Madagascar, Mauritius, Seychelles, Comoros).UV indices in these regions can exceed 10 almost all year round, which implies serious consequences for the health of the populations.For example, dermatologists in Reunion Island (21°S, 55.5 °E) have noted a rapid increase in sun-induced skin lesions (Observatoire Régional de la Santé de La Réunion, 2008).The number of these lesions triples : is :::::::: currently ::::::: tripling : every 10 years, with an accelerating progressionand affects : , :::: and more than 2% of the population :: is ::::::: affected.Radiation doses received by particularly exposed populations (children and outdoor workers) exceed dangerous thresholds in this region of the Indian Ocean almost all year round (Wright et al., 2013).Finally, it can be noted that all skin phototypes are concerned, including photo-types 5 and 6 (Sitek et al., 2016).An important part of this observed increase can be explained by lifestyle changes, and expansion in :::: with :: an ::::::::: expansion :: of seaside activities and a constant increase in exposure to the sun.The development of tourism, especially local tourism, is a determining factor.
When UV radiation penetrates the atmosphere, its intensity along the atmospheric path and on the ground depends on various parameters, including geometric or geographical factors such as the solar zenith angle (SZA), latitude and altitude.Cloud cover plays an important role, generally reducing the intensity of radiation, although under certain conditions cloud cover can also increase the intensity, as it is the case of fractional cloud cover (Sabburg and Wong, 2000).Aerosols, small particles of natural (marine aerosols, volcanoes ::::::: volcanic) or anthropogenic (biomass fires, fossil fuel combustion) origin, scatter and absorb UV radiation.If present in large quantities, they can strongly affect surface UV radiation or radiative forcing.The reflectivity of a surface, or albedo, is also involved in modulating the intensity of radiation and, finally, the concentrations of certain gases, such as ozone, in the atmosphere plays ::: play : an essential role.The amount of ozone (O 3 ) in the stratosphere largely drives UV levels on the ground because of ozone has a high absorption capacity in the UV wavelengths.O 3 is produced in the tropics under the influence of strong solar radiation and is transported towards the poles by the Brewer-Dobson circulation (BDC).
Ozone concentrations have been significantly depleted over the past 35 years due to emissions of halocarbons, which are both powerful greenhouse gases (GHGs) and ozone-depleting substances (ODS).The Montreal Protocol (1987) and its amendments are an undisputed success of environmental policy making and have slowly brought ODS amounts down almost to historical levels (Chipperfield et al., 2015).
However, the evolution of stratospheric ozone beyond the middle of the 21st century, when ODS emissions will be minimal and their atmospheric concentrations will continue to decrease, will be largely determined by the concentrations of certain greenhouse gases (GHGs) such as CO 2 , N 2 O and CH 4 (Eyring et al., 2013).On the other hand, the increase in CO 2 is expected to accelerate the BDC (Butchart, 2014;WMO, 2018) ::::::::::::::::::::::::: (Butchart, 2014;WMO, 2018).Ozone will then be transported more rapidly from the tropics to the poles and thus the total ozone column (TOC) will increase in mid-latitudes and decrease in the tropics.Since UV radiation at the surface is directly dependent on the total ozone column, UV can be expected to decrease in mid-latitudes and increase in the tropics (Butler et al., 2016;Lamy et al., 2019) :::::::::::::::::::::::::::::: (Butler et al., 2016;Lamy et al., 2019).Therefore, climate change, linked to the accumulation of greenhouse gases in the atmosphere, is also one of the phenomena identified to explain the increasing levels of exposure to UV radiation in the tropics.
The western Indian Ocean is mainly located in the tropical region of the southern hemisphere.This part of the world is therefore potentially impacted by the changes in radiation described above.However, this geographical area suffers from a very large measurement deficit, both for ozone and UV radiation on the ground.Studies associated with these topics are sparse in the literature.The originality of Reunion Island is that it is located in the heart of this region and and has benefited from high-quality measurements of ozone and aerosols for several decades thanks to OPAR (Reunion's Atmospheric Physics Observatory).Within the framework of various programmes funded by Europe, Reunion Council or CNES (French Space Agency), the University of Réunion Island has developed a specialized network for the measurement of UV radiation and cloud cover in the south-western part of the Indian Ocean, known as the UV-Indien network.The data-sets presented here are original and, as mentioned, concern an extremely poorly measured region of the globe, which highlights their importance.
They include the UV index at 9 stations, the cloud fraction measured at 4 stations.These measurements are ideally suited for assessing the effect of UV radiation on the health of people or ecosystems in this region.
The TROPOMI instrument is onboard the Sentinel-5 Precursor (S5P) polar-orbiting satellite launched on 13 October 2017 as part of the EU Copernicus programme.TROPOMI surface UV radiation products include irradiances with daily integrals at four different wavelengths, and dose rates with daily doses for erythema (CIE Standard, 1998) and vitamin D synthesis (Bouillon et al., 2006) action spectra.All parameters are calculated for overpass time, solar noon time, and for clear-sky conditions (no clouds, no aerosols).The TROPOMI UV algorithm (Lindfors et al., 2018) is based on two pre-computed look-up tables (LUTs): The first is used to retrieve the cloud optical depth from the measured 354 nm reflectance.This cloud optical depth, the total ozone column from the TROPOMI level 2 total ozone column product (Garane et al., 2019), the surface pressure, the surface albedo and the SZA are then used as input to the second LUT from which the UV irradiances and dose rates are retrieved.A post-correction for the effect of absorbing aerosols (Arola et al., 2009) is applied to the irradiances.The ground resolution for the UV products is 7.2 x3.5 : x ::: 3.5 : km 2 (5.6 x3.5 : x ::: 3.5 km 2 since 6 August 2019) at nadir.Only the estimates corresponding to the time of the overpass are chosen here.The UVI estimated in clear-sky conditions and in all-sky conditions will be used in this study and will both be called UVI-TROPOMI for ease of reading.UVI-TROPOMI computed for clear-sky conditions will be compared against UVI-RADIO or UVI-BENTHAM measured in clear-sky conditions and UVI-TROPOMI computed for all-sky conditions will be compared against UVI-RADIO or UVI-BENTHAM measured in clear-sky conditions.
OMUVBG is a product derived from the Ozone Monitoring Instrument (Levelt et al. (2006) in all-sky conditions (called UVI-OMUVBG hereafter).This estimate corresponds to the satellite overpass time.Absorbing aerosols are only corrected if the aerosol index is higher than 0.5 and if the measured reflectance at 360 nm is lower than 0.15 (Tanskanen et al., 2006).This could lead to overestimation for regions affected by absorbing aerosols (Tanskanen et al., 2007).
UVI-OMUVBG computed for clear-sky conditions will be compared against UVI-RADIO or UVI-BENTHAM measured in clear-sky conditions and UVI-OMUVBG computed for all-sky conditions will be compared against UVI-RADIO or UVI-BENTHAM measured in clear-sky conditions.
The offline surface UV is a product derived from the measurements of the GOME-2 instruments on-board the METOP-B and METOP-C satellites.The offline surface UV contains multiple variables related to UV radiation and human health: UVI, integrated UVB and UVA, and daily doses derived from different biological weighting functions (erythema, DNA damage, plant damage and vitamin-D synthesis).TOC and cloud measurements provided by the AC SAF Total Ozone product and the Advanced Very High Resolution Radiometer (AVHRR-3) reflectances are used with an RTM to compute the offline surface UV product (Kujanpää and Kalakoski, 2015).The product is :::: given on a regular 0.5 x 0.5°grid and UVI is computed at local solar noon (called UVI-GOME hereafter).
The horizontal resolution was approximately 80 km prior to 2016-06-21 and approximately 40 km afterwards.Following its continuous development, the CAMS model has been upgraded every year with components that significantly change the UVI forecasts.For instance, improved handling of cloudiness (implemented on 2017-01-24) and a new extraterrestial :::::::::::: extraterrestrial UV spectrum (implemented on 2017-09-26) have resulted in significant improvements (W.Wandji, 2018).As these changes only affect the data following the upgrade, the CAMS UV forecast is not a homogeneous time series , but should be considered as an evolving UV product with which measurements can be compared.More information on the IFS and CAMS-IFS models is available at https://atmosphere.copernicus.eu/node/326.UVI-CAMS computed for clear-sky conditions will be compared against UVI-RADIO or UVI-BENTHAM measured in clear-sky conditions and UVI-CAMS computed for all-sky conditions will be compared against UVI-RADIO or UVI-BENTHAM measured in clear-sky conditions.
The TEMIS UVI product is derived from the TOC at solar noon obtained by satellites (SCIAMACHY, GOME-2A or GOME-2B depending on the time period.Following the parameterization of Allaart et al. ( 2004), a first estimate of UVI is computed with the TOC.The final UVI under clear sky at local solar noon (called UVI-TEMIS hereafter) is obtained by correcting the first estimates for the effects of the Earth-Sun distances, surface albedo, elevation and aerosols (Zempila et al., 2017).The spatial resolution is 0.5 x 0.5°.

Calibration
The UV-Indien project is deploying broadband radiometers at various sites in the Indian Ocean.In order to recalibrate these radiometers with a frequency of about 2 years, we plan to regularly reposition the radiometers on the Moufia super measurement site (University of Reunion Island -20.902°S, 55.485°E), and thus co-locate with the BENTHAM spectrometer, for a period of 3 or 4 months.For the recent recalibration of the UVS-E-T 15-0124, located in St-Denis, the absolute differences (AD) of the UVI, the relative differences (RD) between the UVI measured by the radiometer and the UVI measured by BENTHAM were calculated.These differences are calculated per SZA band (+-5°) and for clear-sky conditions.We noted that the differences depend on the SZA.As the SZA increases, the absolute values of the UVI tend to decrease and, therefore the absolute difference also tends to decrease.Nonetheless, the RD increases as the SZA increases.This behaviour is observed for all radiometers currently compared to the Bentham (Table 3).While the UVS-E-T radiometer tends to overestimate UVI between 0°and 50°o f SZA, the SUV-E radiometer underestimates the UVI.The underestimation is greatest at high SZA.The SUV-E radiometers were installed colocally during the austral winter, when the SZA takes higher values.The differences with respect to the BENTHAM spectroradiometer were used as a reference to recalibrate the KZ Radiometers (UVI-RADIO).UVI-BENTHAM is cosine corrected, its temperature is stabilized and the instrument is regularly calibrated against standard 1000 W lamps, whose origin can be traced to the National Institute of Standards and Technology (Brogniez et al., 2016).We compared both measurements again for all-sky conditions.We noted that the ADs between each radiometer and the Bentham were less than 0.5.RDs were less than 5 to 10% except for the UVS-E-T 15-0124 around 25°of SZA, where the differences could reach about 11%.There was still a SZA dependency: as SZA increased, RD tended to increase, but the magnitude of this effect decreased after recalibration.Note that the radiometers installed in Mahé, Anse Quitor, and Antananarivo are still under the constructor's calibration and have not yet been compared to BENTHAM.These radiometers should be recalibrated during the coming year.
Table 3 presents the different radiometers and their current locations, along with the date of the next calibration.

Filtering
The radiometer data were filtered to define the UVI measurements made in clear sky.This filtering was done manually for 1h intervals.The filtering process is as follows: each daily UVI and cloud fraction profile is plotted together with an estimate of the clear sky profile using the Madronich analytical formula (Madronich, 2007).To calculate the Madronich UVI estimate, we use the total ozone column from either the OMTO3d satellite product or a co-located ground-based instrument (the SAOZ for the St-Denis station).An observer then selects the one-hour windows considered as clear sky according to the following criteria: -Difference between observed and estimated UVI (according to the Madronich Analytical formula) -Presence of clouds, a cloud fraction threshold of 0.25 is set.A UVI measurement performed -Shape and regularity of the UVI curve during the day.
A Gaussian (or semi-Gaussian) curve indicates a day (or half-day) not affected by the presence of clouds.As clouds generally have a high temporal variability, rapid development of clouds usually results in rapid variations of UVI over a few minutes.-Cloud cover can also be quasi-homogeneous and quasi-constant over the day, which does not lead to sudden variations in UVI, and this case is also excluded from the filtering.estimate : computed at this point, we looked for the closest UVI-BENTHAM or UVI-RADIO measurement.Finally, for each couple of measurements ::: pair :: of ::::::::::::: measurements, we checked the following requirements: the time difference between the estimate and the reference must ::: had :: to : be less than 5 minutes and the SZA difference must ::: had :: to : be less than 5 degrees.For these comparisons, the UVI-BENTHAM was also scaled up and linearly interpolated at a resolution of 5 min.For data from the OMUVBG and TROPOMI product, we selected UVI estimates located within 10 km away of the corresponding ground-based station :: we ::::::: selected.For CAMS, we took the closest grid point available for each station ::: was ::::: taken: 14.8, 9.6, 7.2 and 4.7 km from Antananarivo, Anse Quitor, Mahé and St-Denis respectively.For TEMIS, we took the closest grid point available for each station :::: was :::: used: 7.8, 13.0, 12.1 and 11.8 km from Antananarivo, Anse Quitor, Mahe and St-Denis respectively.Here, it was chosen to compare the satellite product or model forecast to ::: with :::::: results ::::: from ::: the four stations of the UV Indien Network ::: that :::: have ::: the :::::: longest :::: time ::::::: periods :::::: covered : (Antananarivo, Anse Quitor, Mahé and St-Denis)that have the longest time periods covered.Figure 2 shows the period covered by each data set at these four stations.BENTHAM has been collecting data since 2009 but, in this study, only the period between 2016 and now ::: the :::::: present : was chosen because the radiometers were only installed ::::::: installed :::: only : in 2016 or later (in 2017) depending on the stations.The Pearson correlation coefficient and absolute and relative differences were calculated for each ground-based instrument installed at each station.
The correlation between the BENTHAM at St-Denis and the other measurements is shown in Figure 3.All data sets are well correlated with the UVI-BENTHAM under clear sky conditions; Pearson correlation coefficients are greater than 0.9, except for OMUVBG (0.55) at Saint-Denis.Correlation results for the radiometers at St-Denis, Antananarivo, Mahé and Anse Quitor stations are available in the appendix (Figure A1, A3, A5 and A7 respectively).Correlation coefficients are greater than 0.8 at all stations except Anse Quitor (for TEMIS and OMUVBG), Mahé (for OMUVBG, GOME and TEMIS) and Saint-Denis (OMUVBG).On average, for all stations combined, OMUVBG is the product with the lowest correlation coefficients while CAMS shows the highest correlation coefficients.Mahé is the station that is least well represented station by most models, with correlation coefficients between 0.19 and 0.9.
The absolute and relative Differences ::::::::: differences between satellite product or forecast model and UVI-BENTHAM at St-Denis are shown in Figure 4 and Table 4.The clear-sky conditions are presented in blue on the boxplot (Figure 4) and the corresponding values are reported in Table 4.The all-sky conditions are presented in red on the boxplot (Figure 4) and the corresponding values are shown in brackets in Table 4. Looking at the median of the absolute differences (MED-AD :::::::::: Median-AD) under all-sky conditions, it can be seen that all data sets, with the exception of OMUVBG, overestimate UVI relative to the UVI-BENTHAM measurements; median overestimation ranges between 0.21 (TROPOMI) and 0.96 (TEMIS).In clear-sky conditions, the median absolute differences decrease but a UVI overestimation can still be observed for CAMS (+0.21) and TEMIS (+0.27).The smallest median difference is obtained for GOME (-0.01).UVI-RADIO is expected to be aligned with UVI-BENTHAM since it has been recalibrated with the UVI-BENTHAM measurements, so it will not be discussed further here.The mean of the absolute differences under all-sky conditions ranges from 0.34 +-: ± : 2.54 (OMUVBG) to +2.31 +-3.26 :: ± :::: 3.26 (TEMIS).Under clear-sky conditions, this absolute difference is smaller and ranges between -0.25 +-: ± : 0.73 (TROPOMI) and 0.47 +-: ± : 0.55 (CAMS).
UVI estimates by satellite or models are closer to instruments measurements in clear-sky conditions observed ::::::::: instrument ::::::::::: measurements ::::: made : on the ground : in :::::::: clear-sky ::::::::: conditions.This is mainly due to the spatial resolution and clouds representation ::::::::::: representation ::: of ::::: clouds.The satellite pixel or model grid point is representative of a 5.6 x 3.7 km region for the better ::: best defined satellite (TROPOMI) or a 0.5 deg x 0.5 deg region ( 55 ::: km x 55 km) (GOME, OMI and TEMIS).Thus the cloud cover considered is representative of this entire region but it is not necessarily that directly observed above the ground-based instruments.In addition, the pixels and grid points selected are not perfectly centred on the ground instruments.Finally, the four study sites are located in the tropics, present non-uniform topographic conditions : , and are very close to the sea (with the exception of Antananarivo).These conditions favour the rapid development of clouds and complicate the estimation of cloud cover over the site by the satellite (Lakkala et al., 2020).Thus ::::::: (Lakkala :: et ::: al., :::::: 2020).::::: Thus, : the clear-sky conditions observed on the ground may not be the same as those observed by satellites or models.This will induce : a : discrepancy between UVI derived from satellite and modelling and UVI observed at the ground.
Although TROPOMI has large relative differences, especially in Mahé, it is also the product with the most consistent differences.The :: Its relative and absolute standard deviation is the smallest at all stations.It should be noted that all satellite or model estimates give poor results at the Mahé station.An examination of ::: the percentage of clear days showed ::::: shows that, in Mahé, Anse Quitor and Antananarivo, there are only 16% clear days while, in St-Denis, there are 27% clear days.In addition, Anse Quitor and Antananarivo radiometers have a 1 min resolution, while those in Mahé and St-Denis have a 5 min resolution.The small number of clear-sky days combined with the larger temporal resolutionreduce , ::::::: reduces : the number of points compared in clear-sky conditions at Mahé.For OMUVBG, TROPOMI, GOME and TEMIS, which produce only one UVI estimate per day, the number of points compared ranges from 72 to 253.Mahé station is also closer to the equator than the other stations (at about -4 :: 4°:South) and is strongly influenced by the convection in this region.
In Antananarivo, the peaks of UVI mean and maxima are aligned for both clear-sky and all-sky conditions.In St-Denis the peak of mean UVI occurs later in the day for clear-sky conditions than for to all-sky conditions.In Anse Quitor, the peak of UVI is early in the day.This is due to the time zone used at Anse Quitor.Rodrigues Island, where Anse Quitor is located, shares the same time zone as St-Denis (Reunion island) or Mahé (Seychelles) but is farther east by about 8 degrees of longitude.In Mahé, the peaks of mean UVI, in all-sky and clear-sky conditions, are aligned.This is not the case for the maxima, which occur 1 to 2 hours earlier in all-sky conditions than in clear-sky conditions.The position of the UVI peaks can be explained by the variability of the cloud cover as will be shown in the next section.
The differences range between 1 during austral winter at St-Denis and 6 in austral summer at Mahé.The corresponding SZA differences ranges :::: range : from 0 to 6 °.
: °. : Looking at the few complete clear-sky days revealed a UVI SNOON and a UVI DMAX that were almost equivalent.However, as there were not enough clear sky measurements per month and per SZA bin, these results are not represented here.and November (SON, in orange) and June, July and August (JJA, in purple).The density of the corresponding data set for each month of the year is represented in Figure 7b. Figure 7c represents the difference of UVI maxima between clear-sky UVI and all-sky UVI for the same period.The UVI distribution is also available in Figure 6d.The same results are presented for St-Denis on :: in Figure 8.
Over Antananarivo (Figure 7), the annual mean diurnal cycle shows intermediate values of CF, of about 0.6 at the beginning of the the day.CF tends to decrease during the day and to increase in the late afternoon.Antananarivo is located in a mountainous region in the high lands :::::::: highlands of Madagascar at 1370m :::: 1370 :: m asl.The difference in CF in the four seasons is not statistically significant, especially for the JJA and SON seasons.Since the camera at Antananarivo was installed in April, 2019, there are not yet significant numbers of data points for certain months of the year.Nonetheless, since the radiometers at Antananarivo were ::::::::: radiometer :: at ::::::::::: Antananarivo :::: was : installed in 2016, there is a significant number of UVI data over the whole year (Figure 7d).The largest difference between maxima of UVI in clear-sky conditions and maxima of UVI in all-sky conditions can be observed for the DJF and JJA seasons (Figure 7c).CF diurnal profiles are quite different between the DJF season and the JJA season ::: and :::: JJA :::::: seasons, with DJF showing CF higher than the annual mean while JJA shows CF lower than the annual mean.

Data Availability
Data from the UV radiometers and the total sky imagers can be downloaded from Zenodo and are referenced under the following doi: https://doi.org/10.5281/zenodo.4811488(Lamy and Portafaix, 2021).
The data from the UV radiometers are also available on the WOUDC website at these addresses for the four stations studied here : -Saint-Denis, La Réunion : https://woudc.org/data/stations/?id=530 The TROPOMI surface UV radiation product used in this study is available at https://nsdc.fmi.fi/data/data_s5puv.php The AC SAF (GOME/METOP) data can be downloaded through the website at https://acsaf.org The TEMIS data can be downloaded through the website at http://www.temis.nl/uvradiation/UVindex.html The OMUVBG data can be downloaded through the website at https://disc.gsfc.nasa.gov/datasets/OMUVBG_003 The CAMS data can be downloaded through the website at https://apps.ecmwf.int
This two phenomenons ::::: These ::: two :::::::::: phenomena could explain both the observed high UVI and the discrepancy with satellite and model estimates.A drift in the radiometer calibration could also explain part of the difference.: In :::::::: addition, Mahé also presents a low :::::: presents :: a :::: small : number of clear-sky comparison points since the K&Z radiometer temporal resolution is 5 min and there are only 16% of clear-sky days.St-Denis and Anse Quitor are mountainous islands also under the influence of trade winds, where there is also frequent cloud formation in the late morning.A fractional cloud sky cover could induce UVI enhancements measured by the ground-based instruments while satellites and model could apply attenuation of the measured UVI instead.
, Tanskanen et al. (2007), Arola et al. (2009)).It is based on the Total Ozone Mapping Spectrometer (TOMS) algorithm to retrieve TOC.TOC, measured with OMI, along with climatological albedo, ozone and temperature profile, elevation, : and SZA are used as input to an RTM to compute a first estimation ::::::: estimate of the UVI under clear skies.The measured reflectance at 360nm ::: 360 ::: nm : made by the OMI is used to estimate a cloud modification factor (CMF).The CMF represents the attenuation of UV radiation by clouds and non-absorbing aerosols.The clear-sky UVI computed previously is multiplied by the CMF to obtain an estimation of UVI

Figure 2
Figure 2 represents the time period covered by all datasets at St-Denis.TEMIS shows the longest period covered as of 2002.TROPOMI is the most recent instrument and the UVI data starts at the end of 2017.For the ground-based instruments, UVI-BENTHAM has been acquiring data since 2009 and UVI-RADIO since the end of 2016.
Figure :: 5. :::: The : daily maxima of UVI are in red lines with filled circles, the corresponding sun zenith angles are in orange dashed lines with filled circles.The UVIs at local solar noon are marked with blue lines and crosses, ::: and : the corresponding sun zenith angles are shown as dashed orange lines with crosses.

Figure 4 .
Figure 4. Boxplot of differences between UVI-BENTHAM and Sat/Model Estimates at ST-DENIS.

Figure 5 .Figure 6 .
Figure 5.Diurnal Cycle of UVI at Anse Quitor, Antananarivo, Mahé and St-Denis.In all-sky conditions: Mean UVI in blue, Max UVI in red, First and Third Quartile delimit the blue shaded area.In clear-sky conditions:Mean UVI in green, Max UVI in dashed-red, First and Third Quartile delimit the green shaded area.

Figure 7 .
Figure 7. a) and b) Diurnal Cycle of CF at ANTANANARIVO.Annual Mean CF in black, first and third Quartiles delimit the blue shaded area.Seasonal Mean in green (DJF), red (MAM), orange (SON) and purple (JJA).c) and d) Diurnal Cycle of the difference between maxima of UVI in clear-sky conditions and in all-sky conditions at ANTANANARIVO.Annual Max UVI differences in black.Seasonal Max UVI differences in green (DJF), red (MAM), orange (SON) and purple (JJA)

Figure 8 .
Figure 8. a) and b) Diurnal Cycle of CF at ST-DENIS.Annual Mean CF in black, first and third quartiles delimit the blue shaded area.Seasonal Mean in green (DJF), red (MAM), orange (SON) and purple (JJA).c) and d) Diurnal Cycle of the difference between maxima of UVI in clear-sky conditions and in all-sky conditions at ST-DENIS.Annual Max UVI differences in black.Seasonal Max UVI differences in green (DJF), red (MAM), orange (SON) and purple (JJA)

Figure A2 .
Figure A2.Boxplot of differences between UVI-RADIO and Sat/Model Estimates at ST-DENIS.

Figure A3 .
Figure A3.Correlation between UVI-RADIO and Sat/Model Estimates at ANTANANARIVO.

Figure A4 .
Figure A4.Boxplot of differences between UVI-RADIO and Sat/Model Estimates at ANTANANARIVO.

Figure A5 .
Figure A5.Correlation between UVI-RADIO and Sat/Model Estimates at MAHE.

Figure A6 .
Figure A6.Boxplot of differences between UVI-RADIO and Sat/Model Estimates at MAHE.

Figure A8 .
Figure A8.Boxplot of differences between UVI-RADIO and Sat/Model Estimates at ANSE QUITOR.

Table 2 .
Satellites, Model and ground-based data.

Table 3 .
Radiometers of the UV-Indien Network.