L'analyse de 561 jours d'orage sur 6 années de données de l'imageur d'éclair 3D Saetta a permis d'identifier des nuages convectifs en région Corse présentant une structure électrique anormale et apparaissant par flux de sud de poussières désertiques africaines. L'explication physique des processus électriques apporte les bases pour comprendre ce que l'imageur permet de déduire sur la structure électrique des cellules orageuses. Des hypothèses microphysiques et radiatives conduisant à un faible contenu en gouttelettes d'eau surfondue à l'origine de cette électrisation anormale sont explorées en s'appuyant sur l'analyse du contexte en aérosols et des conditions météorologiques environnantes. The analysis of 561 days of thunderstorms over 6 years of data from the 3D lightning imager Saetta has allowed to identify convective clouds in Corsica region with an abnormal electrical structure and appearing by southern flow of African desert dust. The physical explanation of the electrical processes brings the basis to understand what the imager allows to deduce about the electrical structure of thunderstorm cells. Microphysical and radiative hypotheses leading to a low content of supercooled droplets at the origin of this abnormal electrification are explored by relying on the analysis of the aerosol context and the surrounding meteorological conditions.

By means of the SAETTA 3D lightning mapping array, the total lightning activity has been detected in a 240 km × 240 km square domain centred on the island of Corsica located in the West Mediterranean basin, and characterized by a maritime and mountainous environment, with a complex and relatively high relief. The study period covers the months from April to December of the 6 years from 2014 to 2019. Observations are reported with a horizontal resolution of 1 km and a vertical resolution of 0.1 km in terms of density of VHF sources emitted by lightning, and of number of lightning day, in plane- and vertical- projections. Vertical distributions of VHF sources are also provided monthly for the whole period and over the full domain. These 3D long term observations show that the number of lightning days is more important on the main relief of the island. The density of VHF sources exhibits a sharp maximum over the confluence area of the 3 main valleys in the center of the northern part of the island, characterized by a recurrent and vertically well developed lightning activity. The period from 11:00 UTC to 14:00 UTC in July, and in a lesser extent in June and August, is at the origin of this density maximum. This behavior is thus due to a diurnal convection. The whole lightning activity is characterized by 2 maxima in June and September. The first one is linked with the diurnal convection in phase with the maximum elevation of the Sun. The second one must be due to usual large-scale organized thunderstorm events of the fall season. The vertical distribution of VHF sources exhibits an increase in the number of VHF sources from April to August at all altitudes, as well as an increase in the altitude of the main upper peak (from 5.8 km to 10 km) and of the secondary lower peak (from 4.2 km to 6.7 km) of this distribution. From August to December, the opposite evolution appears, except that October is characterized by a strong anomaly with a single intense lower peak at 5.5 km, apparently due to 2 strong events corresponding to anomalously electrified thunderstorms. Several of these anomalously electrified thunderstorms (negative dipoles) are reported. They are all characterized by the movement of cloud cells spatially small and relatively undeveloped vertically propagating from southwest to northeast and associated with a strong transport of desert dust from the African continent. The analysis of the meteorological environment of some of the studied events allows concluding that in each of the studied cases elevated convection was triggered, above the relatively dry atmospheric boundary layer characterized by a strong convective inhibition. The low cloud liquid water content which is a key parameter of the non-inductive charging process is probably a good candidate to explain the anomalous electrification of these events.

The present paper is devoted to the quality assessment of two global land surface albedo products developed by Meteo France in the frame of the Copernicus Climate Change Service (C3S) and the LSA-SAF (Satellite Application Facility on Land Surface Analysis), herein called, respectively, VGT (VeGeTation) (the C3Sv1 dataset, derived from VGT sensors onboard Satellites for the Observation of the Earth, also called SPOT) and ETAL (European polar system Ten-day surface ALbedo, derived from Advanced Very High Resolution Radiometers (AVHRR) onboard METeorological OPerational (METOP) satellites). The evaluation study inter-compared these products with measurements at 33 ground stations and two independent operational products, MTAL-R/NRT (Meteosat second generation Ten-day ALbedo Reprocessed/Near Real-Time) and MODIS (MODerate resolution Imaging Spectroradiometer), over two distinct four-year periods. In accordance with the prescription from the Land Product Validation group of the joint Committee on Earth Observation Satellites (LPV/CEOS), the evaluation was addressed per land cover; furthermore, two albedo regimes were considered throughout the evaluation to distinguish between high (over 0.15) and low (below 0.15) surface albedo behaviors. First, we show that both VGT and ETAL products agree well with the measurements and the other satellite products at the ground stations. Second, when inter-compared with MODIS, the results are noteworthy for ETAL as opposed to VGT, with 11 out of 13 land cover types passing the Global Climate Observing System (GCOS) requirements for more than 80% of the sites for albedo values less than 0.15 (compared with none for VGT) and 10 out of 14 land cover types passing the GCOS requirements for more than 50% of the sites for albedo values greater than 0.15 (compared with 5 for VGT). Finally, a pixel-by-pixel analysis reveals that VGT overestimates the surface albedo as compared with MODIS by about 0.02 in absolute value for albedo values less than 0.15 and by about 22% in relative value for albedo values greater than 0.15. The root-mean-square-deviation (RMSD) in absolute value is about 0.015 for albedo values less than 0.15 and 51.5% in relative value for albedo values greater than 0.15. In contrast, the bias for ETAL when compared with MODIS remains very small. Over the four-year period, ETAL overestimates the surface albedo as compared with MODIS by 0.001 in absolute value for the regime of surface albedo less than 0.15 and by about 5.8% in relative value for albedo values greater than 0.15. The RMSD in absolute value is about 0.014 for albedo values less than 0.15 and 19.4% in relative value for albedo values greater than 0.15. Assuming that the MODIS product is a good reference, a relative bias of around 6% can be judged satisfactory for ETAL surface albedo. The lower performance of the VGT (C3Sv1) product is currently the subject of investigation. Work is ongoing to upgrade it further towards the final C3S product.

Statistical methods are usually used to provide estimations of the wet tropospheric correction (WTC), necessary to correct altimetry measurements for atmospheric path delays, using brightness temperatures measured at two or three low frequencies from a passive microwave radiometer on board the altimeter mission. Despite their overall accuracy over oceanic surfaces, uncertainties still remain in specific regions of complex atmospheric stratification. Thus, there is still a need to improve the methods currently used by taking into account the frequency-dependent information content of the observations and the atmospheric and surface variations in the surroundings of the observations. In this article we focus on the assimilation of relevant passive microwave observations to retrieve the WTC over ocean using different altimeter mission contexts (current and future, providing brightness temperature measurements at higher frequencies in addition to classical low frequencies). Data assimilation is performed using a one-dimensional variational data assimilation (1D-Var) method. The behavior of the 1D-Var is evaluated by verifying its physical consistency when using pseudo- and real observations. Several observing-system simulation experiments are run and their results are analyzed to evaluate global and regional WTC retrievals. Comparisons of 1D-Var-based TWC retrieval and reference products from classical WTC retrieval algorithms or radio-occultation data are also performed to assess the 1D-Var performances.

In 2009 and 2010 the L-band microwave Cooperative Airborne Radiometer for Ocean and Land Studies (CAROLS) campaign was performed in southwestern France to support the calibration and validation of the new Soil Moisture and Ocean Salinity (SMOS) satellite mission. The L-band Microwave Emission of the Biosphere (L-MEB) model was used to retrieve surface soil moisture (SSM) and the vegetation optical depth (VOD) from the CAROLS brightness temperature measurements. The CAROLS SSM was compared with in situ observations at 11 sites of the SMOSMANIA (Soil Moisture Observing System-Meteorological Automatic Network Integrated Application) network of Météo-France. For eight of them, significant correlations were observed (0.51 ≤ r ≤ 0.82), with standard deviation of differences ranging from 0.039 m3 m−3 to 0.141 m3 m−3. Also, the CAROLS SSM was compared with SSM values simulated by the A-gs version of the Interactions between Soil, Biosphere and Atmosphere (ISBA-A-gs) model along 20 flight lines, at a resolution of 8 km × 8 km. A significant spatial correlation between these two datasets was observed for all the flights (0.36 ≤ r ≤ 0.85). The CAROLS VOD presented significant spatial correlations with the vegetation water content (VWC) derived from the spatial distribution of vegetation types used in ISBA-A-gs and from the Leaf Area Index (LAI) simulated for low vegetation. On the other hand, the CAROLS VOD presented little temporal changes, and no temporal correlation was observed with the simulated LAI. For low vegetation, the ratio of VOD to VWC tended to decrease, from springtime to summertime. The ISBA-A-gs grid cells (8 km × 8 km) were sampled every 5 m by CAROLS observations, at a spatial resolution of about 2 km. For 83% of the grid cells, the standard deviation of the sub-grid CAROLS SSM was lower than 0.05 m3 m−3. The presence of small water bodies within the ISBA-A-gs grid cells tended to increase the CAROLS SSM spatial variability, up to 0.10 m3 m−3. Also, the grid cells characterised by a high vegetation cover heterogeneity presented higher standard deviation values, for both SSM and VOD.

Introduction Conclusions References

A renewal of interest for the radiometric L-band Sea Surface Salinity (SSS) remote sensing appeared in the 1990s and led to the Soil Moisture and Ocean Salinity (SMOS) satellite launched in November 2009 and to the Aquarius mission (launched in June 2011). However, due to low signal to noise ratio, retrieving SSS from L-band radiometry is very challenging. In order to validate and improve L-band radiative transfer model and salinity retrieval method used in SMOS data processing, the Cooperative Airborne Radiometer for Ocean and Land Studies (CAROLS) was developed. We analyze here a coastal flight (20 May 2009), in the Gulf of Biscay, characterized by strong SSS gradients (28 to 35 pss-78). Extensive in-situ measurements were gathered along the plane track. Brightness temperature (T-b) integrated over 800 ms correlates well with simulated T-b (correlation coefficients between 0.80 and 0.96; standard deviations of the difference of 0.2 K). Over the whole flight, the standard deviation of the difference between CAROLS and in-situ SSS is about 0.3 pss-78 more accurate than SSS fields derived from coastal numerical model or objective analysis. In the northern part of the flight, CAROLS and in-situ SSS agree. In the southern part, the best agreement is found when using only V-polarization measured at 30 degrees incidence angle or when using a multiparameter retrieval assuming large error on T-b (suggesting the presence of biases on H-polarization). When compared to high-resolution model SSS, the CAROLS SSS underlines the high SSS temporal variability in river plume and on continental shelf border, and the importance of using realistic river run-offs for modeling coastal SSS.

The CAROLS Cooperative Airborne Radiometer for Ocean and Land Studies L band radiometer was designed and built as a copy of the EMIRAD II radiometer constructed by the Technical University of Denmark team. It is a fully polarimetric and direct sampling correlation radiometer. CAROLS is installed on board a dedicated French ATR42 research aircraft, in conjunction with other airborne instruments. Following initial laboratory qualifications and other scientific campaigns, SMOS CAL/VAL campaigns involving 22 flights were carried out over South West France, the Valencia site and the Bay of Biscay (Atlantic Ocean) in spring 2010, in coordination with in situ field campaigns

The Soil Moisture and Ocean Salinity (SMOS) satellite mission, based on an aperture synthesis L-band radiometer was successfully launched in November 2009. In the context of a validation campaign for the SMOS mission, intensive airborne and in situ observations were performed in southwestern France for the SMOS CAL/VAL, from April to May 2009 and from April to July 2010. The CAROLS (Cooperative Airborne Radiometer for Ocean and Land Studies) bi-angular (34°–0°) and dual-polarized (V and H) L-band radiometer was designed, built and installed on board the French ATR-42 research aircraft. During springs of 2009 and 2010, soil moisture observations from the SMOSMANIA (Soil Moisture Observing System–Meteorological Automatic Network Integrated Application) network of Météo-France were complemented by airborne observations of the CAROLS L-band radiometer, following an Atlantic–Mediterranean transect in southwestern France. Additionally to the 12 stations of the SMOSMANIA soil moisture network, in situ measurements were collected in three specific sites within an area representative of a SMOS pixel. Microwave radiometer observations, acquired over southwestern France by the CAROLS instrument were analyzed in order to assess their sensitivity to surface soil moisture (wg). A combination of microwave brightness temperature (Tb) at either two polarizations or two contrasting incidence angles was used to retrieve wg through regressed empirical logarithmic equations with good results, depending on the chosen configuration. The regressions derived from the CAROLS measurements were applied to the SMOS Tb and their retrieval performance was evaluated. The retrievals of wg showed significant correlation (p-value < 0.05) with surface measurements for most of the SMOSMANIA stations (8 of 12 stations) and with additional field measurements at two specific sites, also. Root mean square errors varied from 0.03 to 0.09 m3 m− 3 (0.06 m3 m− 3 on average).

A method to identify and mitigate radio frequency interference (RFI) in microwave radiometry based on the use of a spectrum analyzer has been developed. This method has been tested with CAROLS L-band airborne radiometer data that are strongly corrupted by RFI. RFI is a major limiting factor in passive microwave remote sensing interpretation. Although the 1.400–1.427 GHz bandwidth is protected, RFI sources close to these frequencies are still capable of corrupting radiometric measurements. In order to reduce the detrimental effects of RFI on brightness temperature measurements, a new spectrum analyzer has been added to the CAROLS radiometer system. A post processing algorithm is proposed, based on selective filters within the useful bandwidth divided into sub-bands. Two discriminant analyses based on the computation of kurtosis and Euclidian distances have been compared evaluated and validated in order to accurately separate the RF interference from natural signals.

We propose in this paper to evaluate a method to retrieve soil moisture (SM) and vegetation optical thickness, in areas of unknown roughness and unknown vegetation water content in view of operational applications, by using airborne Tb measurements acquired in South-West of France. Results are compared to in situ measurements, manual and automatic ones included in SMOSmania network, in the South-West of France.

The L-band Cooperative Airborne Radiometer for Ocean and Land Studies (CAROLS) radiometer flew four times over the Gulf of Biscay between September 24 to September 28, 2007 around 20UTC. These flights were the first ones over the ocean of this new instrument. Concomitant ship campaign and drifter deployments provide in situ ground truths for sea surface salinity (between 34.6 and 35.8 pss) and temperature (between 15 degrees C and 17 degrees C). Wind speed (between 2 and 10 m/s) and direction are estimated from the QSCAT scatterometer.Observed variations of CAROLS brightness temperatures in horizontal polarization are very consistent with the ones simulated using a direct modeling of L-band radiometric signals over the ocean, demonstrating a good sensitivity of CAROLS instrument and the realism of the modeling. During circle flights during which variations of the galactic signal scattered by the sea surface was moderate (less than 0.5 K), observed azimuth variations at 15 degrees incidence angle are consistent with the galactic noise signal scattered by the sea surface plus a small signal due to rough sea asymmetry as simulated by a two-scale model. On September 28, a more than 1 K increase of Tb over 2 degrees longitude is observed, linked to an increase of wind speed from 3 to 8 m/s.

The SMOS satellite mission, launched in 2009, allows global soil moisture estimations to be made using the L-band Microwave Emission of the Biosphere (L-MEB) model, which simulates the L-band microwave emissions produced by the soil–vegetation layer. This model was calibrated using various sources of in situ and airborne data. In the present study, we propose to evaluate the L-MEB model on the basis of a large set of airborne data, recorded by the CAROLS radiometer during the course of 20 flights made over South West France (the SMOSMANIA site), and supported by simultaneous soil moisture measurements, made in 2009 and 2010. In terms of volumetric soil moisture, the retrieval accuracy achieved with the L-MEB model, with two default roughness parameters, ranges between 8% and 13%. Local calibrations of the roughness parameter, using data from the 2009 flights for different areas of the site, allowed an accuracy of approximately 5.3% to be achieved with the 2010 CAROLS data. Simultaneously we estimated the vegetation optical thickness (t) and we showed that, when roughness is locally adjusted, MODIS NDVI values are correlated (R2 = 0.36) to t. Finally, as a consequence of the significant influence of the roughness parameter on the estimated absolute values of soil moisture, we propose to evaluate the relative variability of the soil moisture, using a default soil roughness parameter. The soil moisture variations are estimated with an uncertainty of approximately 6%.

Ponencia presentada en el PIERS Progress In Electromagnetics Research Symposium celebrado en Marrakech (Marruecos) del 20 al 23 de marzo de 2011