We provide trend estimates for total, stratospheric, and tropospheric ozone columns over Reunion (21.1°S, 55.5°E) from 1998 to 2021, using only Système d’Analyze par Observation Zénithale and Southern Hemisphere Additional OZonesonde observations. Trends are derived using Trend‐Run, a multiple linear regression model, and a dynamic linear model (DLM) to identify potential turning points. Overall, total ozone exhibits a positive trend (3.0 1.5 DU/decade), with increases in both stratospheric (1.1 1.6 DU/decade) and tropospheric ozone (2.2 1.0 DU/decade). DLM identifies a turning point in stratospheric ozone in 2008, with a clear decrease in stratospheric ozone before this point and an increase afterward. We also determined changes in the lapse rate tropopause (LRT), the subtropical barrier position, and ERA5 wind and geopotential fields during the same period to investigate possible links between mid‐tropospheric ozone increase and transport‐related perturbations. Although trends in LRT height and temperature are barely significant, they suggest a recent deepening of the troposphere, indicative of climate change. Intensification of the anticyclonic gyre over Southern Africa and a weakening of the Mascarene anticyclone are found. This suggests that, independent of possible changes in ozone precursor emissions over Africa or South America, dynamics are driving increases of ozone and ozone precursors over Reunion from 1998 to 2021. Furthermore, the rate of Reunion's free tropospheric trends exceeds that observed at all other southern hemisphere ozonesonde stations, including those in tropical, subtropical and mid‐latitude regions.

Solar Ultraviolet Radiation (UV) corresponds to electromagnetic waves with wavelengths of 100-400 nm, constituting approximately 5% of the energy emitted by the sun. The risks and benefits of exposure to UV for life on Earth have been known for many years and include impacts on human health, materials, terrestrial and aquatic ecosystems, and biogeochemical cycles. Climate change, influenced by land use change and other factors, can increase or decrease the intensity of the incident UV depending on location, seasons, and changes in the atmospheric composition. UV intensity reaching the surface can be informed as the UV index. This dimensionless indicator often makes it easier for people to assess their UV levels and understand how to protect themselves from excessive sun exposure. In middle-income countries like Brazil and Argentina, networks, and instruments for monitoring UV are often sparse and poorly supported with both capacity and funding, and thus, obtaining reliable UV data is difficult. With only a few stations reporting long-term UV measurements, which significantly restricts its extrapolations to all populated areas, a way to continuous monitoring UV globally is through satellites. Similar to ground-based observations, satellite measurements are affected by instrument errors and are subject to uncertainties in the algorithms used to derive surface UV radiation. Therefore, evaluation of satellite-based estimates of surface UV against available ground measurements at many locations around the world is needed to characterize the errors toward further refinement of the surface UV estimates, especially in the Southern Hemisphere, where there has been relatively limited work to compare ground-based and satellite-derived UV. This study compares ground-based and satellite-derived UV Index levels from OMI (Ozone Monitoring Instrument) at overpass time during clear sky conditions, which are determined using LER (Lambertian Equivalent Reflectivity). A characterization of the diurnal and seasonal variability of the ground-based UV index levels will also be reported. The study period will be from 2005 to 2022, varying according to each data source, and comprises data from two Brazilian cities – Itajubá (22.41ºS, 45.44ºW, 885 m, Davis 6490 UV sensor), Santa Maria (29.4°S, 53.8°W, 476 m, Brewer Spectrophotometer MKIII #167), and from Buenos Aires in Argentina (34.58º S, 58.48°W, 25 m, Solar Light UV Biometer – Radiometer model 501). Comparing satellite-derived data with ground-based measurements helps validate the accuracy of satellite data, which can help identify any discrepancies and improve the satellite data retrieval algorithms, leading to more accurate satellite-derived UV products. Also, such a process of data verification is necessary should these data be used for long-term trend analysis or the monitoring of UV exposure risk and possible impacts on human health, as we intend to do in a future study, to understand better the dynamics of the space-temporal variability of the surface UV in South America.

On 15 January 2022, the Hunga volcano (20.5 degrees S, 175.4 degrees E) erupted, releasing significant amounts of water vapor (H2O) and a moderate quantity of sulfur dioxide into the stratosphere. The resulting volcanic plume traveled westward with the southern hemispheric stratospheric circulation, reaching the Indian Ocean and R & eacute;union (21.1 degrees S, 55.5 degrees E) within days. This study presents the first analysis of Infrared Atmospheric Sounding Interferometer (IASI) ozone data to investigate the impact of the Hunga eruption, and also incorporates Microwave Limb Sounder (MLS) and Ozone Mapping and Profiler Suite Limb Profiler (OMPS-LP) data, as well as ground-based measurements from R & eacute;union. IASI observations revealed a transient ozone depletion event in the first week following the eruption. OMPS-LP aerosol extinction profiles, sun-photometer measurements, and lidar observations characterized the plume's vertical and latitudinal extent, showing its presence over R & eacute;union at altitudes ranging from 26.8 to 29.7 km and its spread across more than 30 degrees longitude and 20 degrees latitude by 21 January. IASI ozone spatial distributions showed marked decreases in total and stratospheric ozone on that date, with the fifth percentile of the anomaly reaching -18.6 DU for total column ozone and -14.5 DU for stratospheric column ozone. A key finding, as shown by MLS profiles, is that the ozone reduction was confined to two separate layers (-0.7 +/-(1 sigma) 0.6 ppmv in the 14.68-12.12 hPa range, and -0.6 +/-(1 sigma) 0.5 ppmv in the 31.62-21.54 hPa range), each associated with a distinct aerosol cloud with excess H2O. This layered structure of ozone loss offers new insight into the chemical and radiative effects of the Hunga plume on stratospheric ozone.

To gain a deeper understanding of cloud variability over the Southwest Indian Ocean (SWIO) region, various measurement techniques can be used. Cloud data focused on two main sites (Antananarivo, Madagascar, and Reunion Island) were collected over nearly three years (September 2019 to June 2022) using ground-based all-sky camera and Meteosat Second Generation (MSG) satellite imagery. This study primarily examined cloud fraction, although additional analysis based on cloud types is also needed. Two image processing algorithms were used to estimate cloud fraction by analyzing the camera images: one provided by the manufacturer Reuniwatt, and the other, Elifan, initially developed by CNRS. Their performance was compared to assess their relative strengths. Additionally, comparisons with MSG satellite data were carried out to ensure consistency and evaluate the complementarity of ground-based and satellite observations. Despite differences in methodology, the various data sources showed strong agreement. The results indicate that in Antananarivo, during the dry season, a high morning cloud fraction (similar to 50 %) is typically observed, which gradually decreases throughout the day. In the wet season (December to April), cloud fraction varies between approximately 30 % and 60 %, with reduced cloudiness observed around midday in October and November. In contrast, in Saint-Denis, Reunion, skies are generally clear in the morning but become increasingly cloudy as the day progresses, reaching up to 80 % cloud fraction during the wet season and around 60 % during the dry season.

Understanding optical and radiative properties of aerosols and clouds is critical to reducing uncertainties in climate models. For over 10 years, the Observatory of Atmospheric Physics in Reunion (OPAR; 21.079° S, 55.383° E) has been operating three active lidar instruments, named lidar 1200 (Li1200), stratospheric ozone lidar (LiO3S), and tropospheric ozone lidar (LiO3T), providing time series of vertical profiles from 3 to 45 km of the aerosol extinction and backscatter coefficients at 355 and 532 nm as well as the linear depolarization ratio at 532 nm. This work provides a full technical description of the three systems, the details about the methods chosen for the signal preprocessing and processing, and an uncertainty analysis. About 1737 nighttime averaged profiles were manually screened to provide cloud-free and artifact-free profiles. Data processing consisted of Klett inversion to retrieve aerosol optical products from preprocessed files. The measurement frequency was lower during the wet season and the holiday periods. There is a good correlation between the Li1200 and LiO3S instruments in terms of stratospheric aerosol optical depth (AOD) at 355 nm (0.001–0.107; R=0.92±0.01) and with LiO3T in terms of Ångström exponent 355/532 (0.079–1.288; R=0.90±0.13). The lowest values of the averaged uncertainty in the aerosol backscatter coefficient for the three time series are 64.4 ± 31.6 % for LiO3S, 50.3 ± 29.0 % for Li1200, and 69.1 ± 42.7 % for LiO3T. These relative uncertainties are high for the three instruments because of the very low values of extinction and backscatter coefficients for background aerosols above Maïdo observatory. Uncertainty increases due to the signal-to-noise ratio (SNR) decrease above 25 km for LIO3S and Li1200 and above 20 km for LiO3T. The lidar ratio (LR) is responsible for an uncertainty increase below 18 km (10 km) for LiO3S and Li1200 (LiO3T). LiO3S is the most stable instrument at 355 nm due to fewer technical modifications and fewer misalignments. Li1200 is a valuable addition meant to fill in the gaps in the LiO3S time series at 355 nm or for specific case studies about the middle and low troposphere. Data described in this work are available at https://doi.org/10.26171/rwcm-q370 (Gantois et al., 2024).

This paper presents the observational, remote sensing, and model simulation used to analyze southern Brazil Antarctic ozone hole influence (SBAOHI) events that occurred between 2005 and 2014. To analyze it, we use total ozone column (TOC) data provided by a Brewer spectrophotometer (BS) and the OMI (Ozone Monitoring Instrument). In addition to the AURA/MLS (Microwave Limb Sounder) instrument, satellite ozone profiles were utilized with DYBAL (Dynamical Barrier Localization) code in the MIMOSA (Modélisation Isentrope du Transport Mésoéchelle de l’Ozone Stratosphérique par Advection) model Potential Vorticity (PV) fields. TOC has 7.0 ± 2.9 DU reductions average in 62 events. October has more events (30.7%). Polar tongue events are 19.3% in total, being more frequently observed in October (50% of cases), with medium intensity (58.2%), and in the stratosphere medium levels (55.0%). Already, polar filament events (80.7%) are more frequent in September (32.0%), with medium intensity (42.0%), and stratosphere medium levels (40.7%).

Since October 2016, 10 ground-based stations have been progressively equipped for continuous measurements of UV indices and atmospheric cloud fraction. All these instruments form the UV-Indien measurement network. The stations are homogeneously distributed in 5 countries of the Western Indian Ocean region (Comoros, France, Madagascar, Mauritius and Seychelles). The main scientific objectives of this network are to study the annual and inter-annual variability of the ultraviolet (UV) radiation in this area, to validate the output of numerical models and satellite estimates of ground-based UV measurements, and to monitor UV radiation in the context of climate change and projected ozone depletion in this region.

This article is devoted to the presentation of the MAP-IO observation program. This program, launched in early 2021, has enabled the observation of nearly 700 d of measurements over the Indian and Southern Ocean with the equipment of 17 meteorological and oceanographic scientific instruments on board the ship Marion Dufresne. Several observational techniques have been developed to respond to the difficulties of observations on board the ship, in particular for passive remote sensing data, as well as for quasi-autonomous data acquisition and transfer. The first measurements made it possible to draw up unprecedented climatological data of the Southern Ocean regarding the size distribution and optical thickness of aerosols, the concentration of trace gases and greenhouse gases, UV, and integrated water vapor. High-resolution observations of phytoplankton in surface waters have also shown a great variability in latitude in terms of abundance and community structure (diversity). The operational success of this program and these unique scientific results together establish a proof of concept and underline the need to transform this program into a permanent observatory. The multi-year rotations over the Indian Ocean will enable us to assess the trends and seasonal variability of phytoplankton, greenhouse gases, ozone, and marine aerosols in a sensitive and poorly documented climatic region. Without being exhaustive, MAP-IO should make it possible to better understand and assess the biological carbon pump, to study the variability of gases and aerosols in a region that is remote in relation to the main anthropogenic sources, and to monitor the transport of stratospheric ozone by the Brewer–Dobson circulation. The meteorological MAP-IO data set is publicly available at https://www.aeris-data.fr/catalogue-map-io/ (last access: 26 August 2024) (atmospheric data) and at https://doi.org/10.17882/89505 (Thyssen et al., 2022a) (phytoplankton data).

During the 2020 austral summer, the pristine atmosphere of the southwest Indian Ocean (SWIO) basin experienced significant perturbations. This study examines the variability of aerosols and carbon monoxide (CO) over this remote oceanic region and investigates the underlying processes in the upper troposphere–lower stratosphere (UT-LS). Aerosol profiles in January and February 2020 revealed a multi-layer structure in the tropical UT-LS. Numerical models – the FLEXible PARTicle dispersion model (FLEXPART) and the Modèle Isentropique de transport Mésoéchelle de l'Ozone Stratosphérique par Advection (MIMOSA) – indicated that the lower-stratospheric aerosol content was influenced by the intense and persistent stratospheric aerosol layer generated during the 2019–2020 extreme Australian bushfire events. A portion of this layer was transported eastward by prevailing easterly winds, leading to increased aerosol extinction profiles over Réunion on 27 and 28 January. Analysis of advected potential vorticity revealed isentropic transport of air masses containing Australian biomass burning aerosols from extratropical latitudes to Réunion at the 400 K isentropic level on 28 January. Interestingly, we found that biomass burning (BB) activity in eastern Africa, though weak during this season, significantly influenced (contributed up to 90 % of) the vertical distribution of CO and aerosols in the upper troposphere over the SWIO basin. Ground-based observations at Réunion confirmed the simultaneous presence of African and Australian aerosol layers. This study provides the first evidence of African BB emissions impacting the CO and aerosol distribution in the upper troposphere over the SWIO basin during the convective season.

The ultraviolet radiation index (UV index–UVI) is a dimensionless indicator that informs the intensity of ultraviolet radiation on the Earth’s surface. It makes it easier for people to assess UV levels and understand how to protect themselves from excessive Sun exposure. In Brazil, however, the information regarding UV is scarce, with low spatial and temporal coverage. Thus, continuous monitoring is conducted through satellites, although ground-based monitoring of UV is more accurate than satellite retrievals, and comparisons are necessary for validation. This paper aims to compare the levels of UV index measured on the ground and by satellite (OMI and GOME-2) over Natal, Brazil (05.78°S; 35.21°W) from 2005 to 2022. The comparison was made under clear-sky conditions using METAR cloud cover and LER data. Characterization of the diurnal and seasonal variability of the ground-based UV index levels under all and clear-sky conditions is also reported. The analysis indicates that in Natal, noontime all-sky UV index were 6.8% higher during periods of prevalent broken clouds. The two satellite sources (OMI noontime and overpass) and GOME-2 noontime are reliable sources for UV index, which show good agreement with ground-based measurements, with UVI estimated from OMI both at the overpass and noontime being less biased than GOME-2-estimated UVI. Such a process of data verification is important should these data be used for long-term trend analysis or the monitoring of UV exposure risk and possible impacts on human health.

Abstract. On 15 January 2022, the Hunga volcano (20.5° S, 175.4° E) erupted, releasing significant amounts of aerosols, water vapor (H2O) and a moderate quantity of sulfur dioxide (SO2) into the stratosphere. Due to the general stratospheric circulation of the southern hemisphere, this volcanic plume traveled westward and impacted the Indian Ocean and Reunion (21.1° S, 55.5° E) a few days after the eruption. This study aims to describe current observations of an ozone mini-hole in the first week following the eruption. The Ozone Mapping and Profiler Suite Limb Profiler (OMPS-LP) aerosol extinction profiles were used to investigate the vertical and latitudinal extension of the volcanic plume over the Indian Ocean. The volcanic aerosol plume was also observed with an aerosol lidar and a sun-photometer located at Reunion. The impact of this plume on stratospheric ozone was then investigated using the Microwave Limb Spectrometer (MLS) and Infrared Atmospheric Sounding Interferometer (IASI) ozone profiles and total ozone maps. Results show that the volcanic plume was observed over Reunion at altitudes ranging from 26.8 to 29.7 km and spanned more than 20 degrees of latitude on 22 January while over the Indian Ocean. Ozone maps reveal an ozone mini-hole structure, with a maximum Total Column Ozone (TCO) anomaly of -38.97 ± 25.39 DU from IASI on 21 January. The MLS profiles impacted by the Hunga water vapor plume show an average ozone anomaly of -0.43 ppmv with a standard deviation of 0.66 ppmv at the 14.68 hPa pressure level.

The analysis of observational data sequences in Geophysics consists of characterizing the underlying dynamics. An important preliminary step aims to analyze the variability related to the observed dynamic. The specific objectives related to this step are to remove noise, to determine the overall trend of the observational time series and to identify the relevant components contributing significantly to the original time series variability knowing that their number determines the dimensionality of the observed dynamics. Most of the observational time series have characteristics of non-stationarity and present fluctuations at all-time scales. In this context, variability analysis consists in representing time series in the time-frequency space and requires the development of specific numerical signal decomposition methods. The most commonly used techniques are adaptive and data-driven and among the most cited in the literature are the empirical mode decomposition, the empirical wavelet transform, and singular spectrum analysis. In this work, we describe all of these techniques and evaluate their ability to remove noise and to identify components corresponding to the physical processes involved in the evolution of the observed system and deduce the dimensionality of the associated dynamics. Results obtained with all of these methods on experimental total ozone columns and rainfall time series will be discussed and compared.

Abstract. Ultraviolet solar radiation that reaches the surface of the Earth has a very important role in the energy budget of our planet, although at certain amounts of exposure it can be harmful not only for human health, but animals and plants. Brazil is a tropical country receiving high intensity of UV radiation mostly of the year. UV radiation, specially UV-B is mainly absorbed in the ozone layer, changes in ozone content or ozone distribution directly affects UV radiation attenuation, so that surface UV. Ozone hole is characterized by an intense ozone depletion in the stratosphere in Antarctic region, happening during spring in the south hemisphere. There is evidence of poor-ozone air masses detachment from polar vortex and reaching mid-latitude regions, such as South of Brazil. Those air masses cause impact on the regions which they act due to low ozone content, allowing more radiation to reach the surface. UV index data from a Brewer Spectrophotometer (MKIII #167), located in south of Brazil, in the Southern Space Observatory (29.42° S, 53.87° W) was analysed with data collected between 2005 and 2017. Ground data was compared to satellite-derived from OMI/Aura, resulting in a correlation coefficient of 0.95 %. The climatology of Brewer data was done, analysing seasonal and interannual variability. Wavelet analysis showed the influence of ENSO, QBO and solar cycle on the variability of UV index. The results show a typical seasonal behaviour of UV Index, reaching extreme during summer season. Events of secondary effects of Antarctic ozone hole, identified between 2005 and 2016 (Bittencourt, 2018), were selected and the behavior of UV index and UV-B non-weighted during those events were studied. All ten events studied showed a significant increase in UV radiation at the days of events, the majority resulted in an increase of 4 % of UV index for each 1 % ozone total column decrease. One specific event was described in detail due to the major ozone depletion. Occurred in October 2016, ozone depletion was of almost 20 % while UV index increased 88 % in relation to the climatological mean, raising the concern with public health regarding excess exposure to UV radiation.

The recent expansion of Aedes a lbopictus across continents in both tropical and temperate regions and the exponential growth in the number and geographical of dengue cases spread over the past 50 years pose a significant risk to human health. Although climate change is not the only reason for these expansions, it may increase this risk on global and regional scales. Here we show that regional climatic and microclimatic variations can induce different impacts on the abundance of Ae. albopictus. We use the instructive example of Réunion Island with its varied climatic and environmental conditions and available meteorological, climatic, entomological and epidemiological data. Temperature and precipitation data representing three climate scenarios from high resolution regional climate simulations (3 km × 3 km) are input to a mosquito population dynamics model to assess the impact of climate change on the life cycle of Ae. albopictus. Our results show the joint influence of temperature and precipitation on the abundance of Ae. albopictus as a function of elevation and geographical subregion. In low-elevation areas, decreased precipitation is predicted to negatively impact the environmental carrying capacity and, consequently, the abundance of Ae. albopictus. At medium and high elevations, decreased precipitation is expected to be counterbalanced by significantly increased temperatures, leading to increases in the transitions between the different life stages of the mosquito and, consequently, to an increase in the abundance of the dengue vector in the 2070-2100 period (from 2 to 118% depending on the climate scenario).

The solar ultraviolet radiation - UV (280-400 nm) is a highly energetic component of the solar spectrum that needs to be monitored especially because of the effects on human health and on the ecosystems. The UV index (UVI) is a dimensionless indicator designed to report the intensity of UV incident on the Earth’s surface. It has five exposure categories, ranging from low to extreme, with recommended sun protections at each level. The higher the value, the greater the health risk. With only few stations reporting long-term ground-based UV measurements in several countries, which significantly restricts its extrapolations to all populated areas, a way for continuous monitoring UV on a global scale is through satellites. In this work, the monthly and seasonal variability of the incident UVI in Santarém, Pará, was analyzed. For this, a 13-year time series of daily UVI data from the OMI satellite instrument was used, as well as the 13-year UVI time series from the GOME-2 satellite instrument. According to the results, the dry period (July to December) shows higher average UVI than the rainy period (January to June) for the GOME-2 time series. The rainy period, on the other hand, in both series presents greater amplitude in the variability of the UVI.

Explosive volcanism periodically induces disturbances of the upper troposphere and low stratosphere. These injections of massive amount of aerosols, ash and gases perturb locally the physico-chemical balance of the impacted atmospheric layers, in particular the ozone concentration via heterogeneous chemistry on particles. On a larger scale some exceptional eruption can have a significant influence on the Earth radiative budget as it was the case following eruptions of El Chichon in 1982 and Mount Pinatubo in 1991. On January 15, 2022, the Hunga-Tonga volcano erupted in the Tonga archipelago (20.5°S, 175.4°W). The Plinian eruption was of a rare intensity, especially because of the depth of the underwater caldera. The first estimates indicate a power between 10 and 15 Mt TNT, probably the most powerful since the eruption of Krakatoa in 1883. This short (~ 8min) but intense explosion whose pressure wave was observed all around the globe injected about 400 kt of material into the atmosphere (to be compared to the 20 Mt injected during the Mount Pinatubo eruption). The Volcano Stratospheric Plume (VSP) quickly moved westwards and then overflew the island of La Réunion (21°S, 55°E), located at ~12000 km away from Tonga. In order to monitor the evolution of the VSP, lidar observations were performed at the Observatoire de Physique de l’Atmosphère de La Réunion (OPAR). This observatory is equipped with three lidars capable of stratospheric aerosols measurements at two wavelengths (355 nm and 532 nm). First observations were performed every night from 19 to 27 January 2022 when the first passage of the VSP occurred. The plume structures appeared to be highly variable along time, with altitudes ranging from 19 km to 36 km above the mean sea level while plume thicknesses were ranging from ~1 km to more than 3 km. Remarkable aerosol optical depth were associated with these stratospheric aerosol layers, up to 0.8 at 532 nm on January 21. The temporal evolution of the VSP structure and optical properties will be presented and discussed.

Most observational data sequences in geophysics can be interpreted as resulting from the interaction of several physical processes at several timescales and space scales. In consequence, measurement time series often have characteristics of non-linearity and non-stationarity and thereby exhibit strong fluctuations at different timescales. The application of decomposition methods is an important step in the analysis of time series variability, allowing patterns and behaviour to be extracted as components providing insight into the mechanisms producing the time series. This study introduces empirical adaptive wavelet decomposition (EAWD), a new adaptive method for decomposing non-linear and non-stationary time series into multiple empirical modes with non-overlapping spectral contents. The method takes its origin from the coupling of two widely used decomposition techniques: empirical mode decomposition (EMD) and empirical wavelet transformation (EWT). It thus combines the advantages of both methods and can be interpreted as an optimization of EMD. Here, through experimental time series applications, EAWD is shown to accurately retrieve different physically meaningful components concealed in the original signal.

Le rayonnement UV solaire a des effets bénéfiques et délétères pour la santé humaine. Les agences de santé recommandent d'éviter l'exposition lorsque le rayonnement UV est maximum : entre 2 heures avant et après midi solaire en été. Une réduction de l'exposition UV pouvant entraîner une diminution de la synthèse de vitamine D, il est utile d'estimer et de comparer les durées d'exposition induisant un érythème ou une production suffisante de vitamine D. Ces durées ont été établies à partir de mesures du rayonnement UV obtenues à l'aide de spectroradiomètres opérant en continu sur trois sites français. On constate que la durée d'exposition nécessaire à la synthèse de vitamine D est généralement inférieure à celle induisant l'érythème. De plus, les périodes durant lesquelles une exposition limitée est recommandée devraient être étendues, spécialement aux basses latitudes : en été au moins entre 3 heures avant et après le midi solaire en métropole et, à La Réunion, entre 4 heures avant et après le midi solaire. Au printemps et en automne, il faut également être prudent. Solar UV radiation induces beneficial and detrimental effects on human health. Health agencies recommend avoiding sun exposure when UV radiation is maximum: between 2 h before and after solar noon in summer. A reduction in UV exposure being susceptible to reduce vitamin D synthesis, it is useful to estimate and compare exposure durations leading to erythema and to sufficient vitamin D production. These duration times were calculated using UV radiation measurements from spectroradiometers operating continuously at 3 French sites. One observes that exposure duration requested for vitamin D synthesis is generally shorter than that inducing erythema. Moreover, the time periods where limited solar exposure is recommended should be extended, especially at low latitude locations: in summer at least between 3 h before and after solar noon at mainland sites and, at La Réunion, between 4 h before and after solar noon. In spring and autumn, prudence is also needed.

Authors : Olivier Delage, Thierry Portafaix, Hassan Bencherif, Guillaume Guimbretiere Abstract : Most observation data sequences in geophysics can be interpreted as resulting from the interaction of several physical processes at several time and space scales. As a consequence, measurements time series in geophysics have often characteristics of non-linearity and non-stationarity and thereby exhibit strong fluctuations at all time-scales and require a time-frequency representation to analyze their variability. Empirical Mode Decomposition (EMD) is a relatively new technic as part of a more general signal processing method called the Hilbert-Huang transform. This analysis method turns out to be particularly suitable for non-linear and non-stationary signals and consists in decomposing a signal in an auto adaptive way into a sum of oscillating components named IMFs (Intrinsic Mode Functions), and thereby acts as a bank of bandpass filters. The advantages of the EMD technic are to be entirely data driven and to provide the principal variability modes of the dynamics represented by the original time series. However, the main limiting factor is the frequency resolution that may give rise to the mode mixing phenomenon where the spectral contents of some IMFs overlap each other. To overcome this problem, J. Gilles proposed an alternative entitled “Empirical Wavelet Transform” (EWT) which consists in building from the segmentation of the original signal Fourier spectrum, a bank of filters. The method used is based on the idea utilized in the construction of both Littlewood-Paley and Meyer’s wavelets. The heart of the method lies in the segmentation of the Fourier spectrum based on the local maxima detection in order to obtain a set of non-overlapping segments. Because linked to the Fourier spectrum, the frequency resolution provided by EWT is higher than that provided by EMD and therefore allows to overcome the mode-mixing problem. On the other hand, if the EWT technique is able to detect the frequencies involved in the original time series fluctuations, EWT does not allow to associate the detected frequencies to a specific mode of variability as in the EMD technic. Because EMD is closer to the observation of physical phenomena than EWT, we propose here a new technic called EAWD (Empirical Adaptive Wavelet Decomposition) based on the coupling of the EMD and EWT technics by using the IMFs density spectral content to optimize the segmentation of the Fourier spectrum required by EWT. In this study, EMD and EWT technics are described, then EAWD technic is presented. Comparison of results obtained respectively by EMD, EWT and EAWD technics on time series of ozone total columns recorded at Reunion island over [1978-2019] period is discussed. This study was carried out as part of the SOLSTYCE project dedicated to the characterization and modeling of the underlying dynamics of time series issued from complex systems in atmospheric sciences