Ultraviolet radiation modelling from ground-based and satellite measurements on Reunion Island, southern tropics

Abstract

Abstract. Surface ultraviolet radiation (SUR) is not an increasing concern after the implementation of the Montreal Protocol and the recovery of the ozone layer (Morgenstern et al., 2008). However, large uncertainties remain in the prediction of future changes of SUR (Bais et al., 2015). Several studies pointed out that UV-B impacts the biosphere (Erickson et al., 2015), especially the aquatic system, which plays a central part in the biogeochemical cycle (Hader et al., 2007). It can affect phytoplankton productivity (Smith and Cullen, 1995). This influence can result in either positive or negative feedback on climate (Zepp et al., 2007).Global circulation model simulations predict an acceleration of the Brewer-Dobson circulation over the next century (Butchart, 2014), which would lead to a decrease in ozone levels in the tropics and an enhancement at higher latitudes (Hegglin and Shepherd, 2009). Reunion Island is located in the tropics (21 • S, 55 • E), in a part of the world where the amount of ozone in the ozone column is naturally low. In addition, this island is mountainous and the marine atmosphere is often clean with low aerosol concentrations. Thus, measurements show much higher SUR than at other sites at the same latitude or at midlatitudes. Ground-based measurements of SUR have been taken on Reunion Island by a Bentham DTMc300 spectroradiometer since 2009. This instrument is affiliated with the Network for the Detection of Atmospheric Composition Change (NDACC). In order to quantify the future evolution of SUR in the tropics, it is necessary to validate a model against present observations. This study is designed to be a preliminary parametric and sensitivity study of SUR modelling in the tropics.We developed a local parameterisation using the Tropospheric Ultraviolet and Visible Model (TUV; Madronich, 1993) and compared the output of TUV to multiple years of Bentham spectral measurements. This comparison started in early 2009 and continued until 2016.Only clear-sky SUR was modelled, so we needed to sort out the clear-sky measurements. We used two methods to detect cloudy conditions: the first was based on an observer's hourly report on the sky cover, while the second was based on applying Long and Ackerman (2000)'s algorithm to broadband pyranometer data to obtain the cloud fraction and then discriminating clear-sky windows on SUR measurements.Published by Copernicus Publications on behalf of the European Geosciences Union. Multiple model inputs were tested to evaluate the model sensitivity to different parameters such as total ozone column, aerosol optical properties, extraterrestrial spectrum or ozone cross section. For total column ozone, we used ground-based measurements from the SAOZ (Système d'Analyse par Observation Zénithale) spectrometer and satellite measurements from the OMI and SBUV instruments, while ozone profiles were derived from radiosoundings and the MLS ozone product. Aerosol optical properties came from a local aerosol climatology established using a Cimel photometer. Since the me...

Clear-sky ultraviolet radiation modelling using output from the Chemistry Climate Model Initiative

“…In most cases, clouds attenuate the UV signal at the surface by about 15 % to 45 % (Calbó et al, 2005). Broken cloud cover can also enhance the surface UV radiation (Lovengreen et al, 2005;Marín et al, 2017). Krzyścin and Puchalski (1998) found a 1.5 % increase in erythemal UV for a 10 % decrease in aerosol optical depth (AOD) and up to a 30 % decreases in UV erythemal can be observed due to biomass burning emissions (Lamy et al, 2018).…”

Single Scattering Albedo’s Spectral Dependence Effect on UV Irradiance

“…Ozone is the dominant attenuator of UVB irradiance and the most important atmospheric factor for its deviations [34]. Aerosol attenuation in UV spectral region is also a non-negligible factor for the irradiance reaching the surface and, in some cases, its effects have been found to be as important as those of ozone [35][36][37][38][39]. Additionally, absorbing aerosols indirectly affect irradiance's variability, by reducing the incident UV irradiance and altering lower atmosphere's photochemistry, thus affecting tropospheric ozone's concentration (up to 50 ppb has been reported) [38].…”

Inter-Comparison Campaign of Solar UVR Instruments under Clear Sky Conditions at Reunion Island (21°S, 55°E)

“…The instrument inter-comparison was performed only for clear sky conditions. Cloud fraction data were analysed in two ways to determine a clear sky threshold: 1) Lamy et al [18] showed that TUV clear sky outputs could be compared to Bentham clear sky outputs on the same site with differences of less than 5% when TUV was correctly set up. From this result we compared the TUV outputs with the appropriate setting and the Bentham over the reference period in order to determine the cloud fractions (as seen by the camera) corresponding to a difference in UVI of less than 5%.…”

“…In 2013, during a QASUME campaign [16], a BT/QASUME ratio of −5% to 0% was found [17]. A recent comparison between BT and UVI obtained by modelling showed ± 5% difference [18]. However, since 2009, there have been some gaps in the data due to technical problems and prolonged maintenance delays.…”