Validation of photosynthetically active radiation by OLCI on Sentinel-3 against ground-based measurements in the central Mediterranean and possible aerosol effects

Instantaneous determinations of photosynthetically active radiation (PAR) over the sea from the Ocean and Land Color Instrument (OLCI) on Sentinel-3 are compared with in-situ measurements at the island of Lampedusa in the central Mediterranean Sea. Radiative transfer calculations show that the PAR measured at the island site is representative for open ocean conditions. Satellite data show a good agreement (5.2% positive bias, R-2 = 0.97) with in-situ data, in line with similar analyses for other satellite sensors. Larger satellite-in situ differences are found during summer, and the possible role of aerosols in degrading PAR estimate has been investigated by comparing AOD values measured at Lampedusa and derived by OLCI. The relative difference between OLCI and in-situ PAR appears to be negatively correlated with the relative differences between OLCI and in-situ AOD, suggesting that a more accurate determination of AOD, in particular, for cases with AOD > 0.2, mostly related to Saharan dust, may lead to improved satellite PAR estimates

Air–Sea Interaction in the Central Mediterranean Sea: Assessment of Reanalysis and Satellite Observations

Air–sea heat fluxes are essential climate variables, required for understanding air–sea interactions, local, regional and global climate, the hydrological cycle and atmospheric and oceanic circulation. In situ measurements of fluxes over the ocean are sparse and model reanalysis and satellite data can provide estimates at different scales. The accuracy of such estimates is therefore essential to obtain a reliable description of the occurring phenomena and changes. In this work, air–sea radiative fluxes derived from the SEVIRI sensor onboard the MSG satellite and from ERA5 reanalysis have been compared to direct high quality measurements performed over a complete annual cycle at the ENEA oceanographic observatory, near the island of Lampedusa in the Central Mediterranean Sea. Our analysis reveals that satellite derived products overestimate in situ direct observations of the downwelling short-wave (bias of 6.1 W/m2) and longwave (bias of 6.6 W/m2) irradiances. ERA5 reanalysis data show a negligible positive bias (+1.0 W/m2) for the shortwave irradiance and a large negative bias (−17 W/m2) for the longwave irradiance with respect to in situ observations. ERA5 meteorological variables, which are needed to calculate the air–sea heat flux using bulk formulae, have been compared with in situ measurements made at the oceanographic observatory. The two meteorological datasets show a very good agreement, with some underestimate of the wind speed by ERA5 for high wind conditions. We investigated the impact of different determinations of heat fluxes on the near surface sea temperature (1 m depth), as determined by calculations with a one-dimensional numerical model, the General Ocean Turbulence Model (GOTM). The sensitivity of the model to the different forcing was measured in terms of differences with respect to in situ temperature measurements made during the period under investigation. All simulations reproduced the true seasonal cycle and all high frequency variabilities. The best results on the overall seasonal cycle were obtained when using meteorological variables in the bulk formulae formulations used by the model itself. The derived overall annual net heat flux values were between +1.6 and 40.4 W/m2, depending on the used dataset. The large variability obtained with different datasets suggests that current determinations of the heat flux components and, in particular, of the longwave irradiance, need to be improved. The ENEA oceanographic observatory provides a complete, long-term, high resolution time series of high quality in situ observations. In the future, more similar sites worldwide will be needed for model and satellite validations and to improve the determination of the air–sea exchange and the understanding of related processes

ABSORPTION CROSS-SECTIONS OF OZONE IN THE 590-NM TO 610-NM REGION AT T = 230-K AND T = 299-K

Measurements of the ozone absorption cross section in the 590- to 610-nm region of the Chappuis bands have been carried out at the temperatures T = 230 K and T = 299 K. Ozone is produced with an electrical discharge and stored cryogenically. Differential absorption measurements are obtained in a slowly evolving mixture of ozone and molecular oxygen. Previous results by other authors indicate a certain disagreement on the temperature dependence of the absorption cross section. Our data at each wavelength show no significant dependence of the absorption coefficient on the temperature, within experimental errors. The statistical consistency of the results is discussed

Satellite Multi/Hyper Spectral HR Sensors for Mapping the <i>Posidonia oceanica</i> in South Mediterranean Islands

The Mediterranean basin is a hot spot of climate change where the Posidonia oceanica (L.) Delile (PO) and other seagrasses are under stress due to its effect on marine coastal habitats and the rising influence of anthropogenic activities (i.e., tourism, fishery). The PO and seabed ecosystems, in the coastal environments of Pantelleria and Lampedusa, suffer additional growing impacts from tourism in synergy with specific stress factors due to increasing vessel traffic for supplying potable water and fossil fuels for electrical power generation. Earth Observation (EO) data, provided by high resolution (HR) multi/hyperspectral operative satellite sensors of the last generation (i.e., Sentinel 2 MSI and PRISMA) have been successfully tested, using innovative calibration and sea truth collecting methods, for monitoring and mapping of PO meadows under stress, in the coastal waters of these islands, located in the Sicily Channel, to better support the sustainable management of these vulnerable ecosystems. The area of interest in Pantelleria was where the first prototype of the Italian Inertial Sea Wave Energy Converter (ISWEC) for renewable energy production was installed in 2015, and sea truth campaigns on the PO meadows were conducted. The PO of Lampedusa coastal areas, impacted by ship traffic linked to the previous factors and tropicalization effects of Italy’s southernmost climate change transitional zone, was mapped through a multi/hyper spectral EO-based approach, using training/testing data provided by side scan sonar data, previously acquired. Some advanced machine learning algorithms (MLA) were successfully evaluated with different supervised regression/classification models to map seabed and PO meadow classes and related Leaf Area Index (LAI) distributions in the areas of interest, using multi/hyperspectral data atmospherically corrected via different advanced approaches

Variability of mineral dust deposition in the western Mediterranean basin and south-east of France

International audiencePrevious studies have provided some insight into the Saharan dust deposition at a few specific locations from observations over long time periods or intensive field campaigns. However, no assessment of the dust deposition temporal variability in connection with its regional spatial distribution has been achieved so far from network observations over more than 1 year. To investigate dust deposition dynamics at the regional scale, five automatic deposition collectors named CARAGA (Collecteur Automatique de Retombées Atmosphériques insolubles à Grande Autonomie in French) have been deployed in the western Mediterranean region during 1 to 3 years depending on the station. The sites include, from south to north, Lampedusa, Majorca, Corsica, Frioul and Le Casset (southern French Alps). Deposition measurements are performed on a common weekly period at the five sites. The mean dust deposition fluxes are higher close to the northern African coasts and decrease following a south-north gradient, with values from 7.4 g m −2 year −1 in Lampe-dusa (35 • 31 N, 12 • 37 E) to 1 g m −2 year −1 in Le Casset (44 • 59 N, 6 • 28 E). The maximum deposition flux recorded is of 3.2 g m −2 wk −1 in Majorca with only two other events showing more than 1 g m −2 wk −1 in Lampedusa, and a maximum of 0.5 g m −2 wk −1 in Corsica. The maximum value of 2.1 g m −2 year −1 observed in Corsica in 2013 is much lower than existing records in the area over the 3 previous decades (11-14 g m −2 year −1). From the 537 available samples, 98 major Saharan dust deposition events have been identified in the records between 2011 and 2013. Complementary observations provided by both satellite and air mass trajectories are used to identify the dust provenance areas and the transport pathways from the Sahara to the stations for the studied period. Despite the large size of African dust plumes detected Published by Copernicus Publications on behalf of the European Geosciences Union. 8750 J. Vincent et al.: Variability of mineral dust deposition by satellites, more than 80 % of the major dust deposition events are recorded at only one station, suggesting that the dust provenance, transport and deposition processes (i.e. wet vs. dry) of dust are different and specific for the different de-position sites in the Mediterranean studied area. The results tend to indicate that wet deposition is the main form of de-position for mineral dust in the western Mediterranean basin, but the contribution of dry deposition (in the sense that no precipitation was detected at the surface) is far from being negligible, and contributes 10 to 46 % to the major dust de-position events, depending on the sampling site

ABSOLUTE DETERMINATION OF THE CROSS-SECTIONS OF OZONE IN THE WAVELENGTH REGION 339-355 NM AT TEMPERATURES 220-293-K

Absolute measurements of the ozone absorption coefficient in the Huggins bands at different temperatures have been carried out. Ozone is produced by an electrical discharge and stored cryogenically; differential absorption measurements are subsequently obtained in a slowly evolving mixture of ozone and molecular oxygen. High resolution (to 0.012 nm) measurements cover a spectral range (339–355 nm) where the ozone absorption shows a strong dependance on temperature. Results at 293 and 220 K are reported; they are particularly interesting in view of the utilization of this spectral region as a low-absorption reference channel for the observation of atmospheric ozone profiles by active probing techniques. Coherent radiation at two wavelengths, around 355 and 353 nm, respectively, can be obtained as third harmonic of the fundamental output of a Nd:YAG laser and by H2 Raman shifting of a XeCl excimer laser output

Aerosol and tropospheric ozone direct radiative impacts

This chapter describes the direct radiative effect (RE) over the Mediterranean region exerted by aerosols and tropospheric ozone. Recent results on the regional direct aerosol RE at the surface and at the top-of-the-atmosphere (TOA) are presented, together with trends in surface solar radiation. Absorption by aerosol particles within the troposphere affects heating rates and atmospheric stability, therefore playing a key role in the regional radiative impact of aerosols. In addition, the impact of aerosols on photochemistry and solar energy production over the Mediterranean region is also discussed with a focus on recent advances on the impact of the aerosol dimming on photovoltaic (PV) panels. Finally, the last section describes the regional RE of tropospheric ozone and drivers of its uncertainty

Aerosol and tropospheric ozone direct radiative impacts

This chapter describes the direct radiative effect (RE) over the Mediterranean region exerted by aerosols and tropospheric ozone. Recent results on the regional direct aerosol RE at the surface and at the top-of-the-atmosphere (TOA) are presented, together with trends in surface solar radiation. Absorption by aerosol particles within the troposphere affects heating rates and atmospheric stability, therefore playing a key role in the regional radiative impact of aerosols. In addition, the impact of aerosols on photochemistry and solar energy production over the Mediterranean region is also discussed with a focus on recent advances on the impact of the aerosol dimming on photovoltaic (PV) panels. Finally, the last section describes the regional RE of tropospheric ozone and drivers of its uncertainty