ALBERT

Description: We tested the applicability of the BIT (branched and isoprenoid tetraether) index as a proxy for palaeoflood events in the river-dominated continental margin of the Gulf of Lions (NW Mediterranean). We compared the concentrations of branched glycerol dialkyl glycerol tetraethers (br GDGTs) and crenarchaeol in suspended particulate matter (SPM) collected downstream in the Rhône River, as well as in surface sediments and a ca. 8m piston core from the Rhône prodelta. The core covered the last 400 yr, with four distinct intervals recording the river influence under natural and man-induced shifts in four main channels of the river mouth (Bras de Fer, Grand Rhône, Pégoulier and Roustan). The results indicate that there are mixed sources of br GDGTs and crenarchaeol in the prodelta, complicating application of the BIT index as an indicator of continental organic carbon input and thus as a palaeoflood proxy. However, the sedimentary BIT record for the period when ontinental material was delivered by the river more directly to the core site (Roustan phase; 1892 to the present) mimics the historical paleoflood record. This shows the potential of the BIT index as a palaeoflood proxy, provided that the delivery route of the continental material by rivers to the core sites remains constant over time. The study also highlights the idea that shifts in river channels should be taken into account for the use of the BIT index as a palaeoflood proxy.

Description: The first results of a campaign of intensive observation of precipitation in Dumont d'Urville, Antarctica, are presented. Several instruments collected data from November 2015 to February 2016 or longer, including a polarimetric radar (MXPol), a Micro Rain Radar (MRR), a weighing gauge (Pluvio2), and a Multi-Angle Snowflake Camera (MASC). These instruments collected the first ground-based measurements of precipitation in the region of Adélie Land (Terre Adélie), including precipitation microphysics. Microphysical observations during the austral summer 2015/2016 showed that, close to the ground level, aggregates are the dominant hydrometeor type, together with small ice particles (mostly originating from blowing snow), and that riming is a recurring process. Eleven percent of the measured particles were fully developed graupel, and aggregates had a mean riming degree of about 30%. Spurious precipitation in the Pluvio2 measurements in windy conditions, leading to phantom accumulations, is observed and partly removed through synergistic use of MRR data. The yearly accumulated precipitation of snow (300m above ground), obtained by means of a local conversion relation of MRR data, trained on the Pluvio2 measurement of the summer period, is estimated to be 815mm of water equivalent, with a confidence interval ranging between 739.5 and 989mm. Data obtained in previous research from satellite-borne radars, and the ERA-Interim reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF) provide lower yearly totals: 655mm for ERA-Interim and 679mm for the climatological data over DDU. ERA-Interim overestimates the occurrence of low-intensity precipitation events especially in summer, but it compensates for them by underestimating the snowfall amounts carried by the most intense events. Overall, this paper provides insightful examples of the added values of precipitation monitoring in Antarctica with a synergistic use of in situ and remote sensing measurements.

Description: Antarctic precipitation is the main positive component in the surface mass balance of the Antarctic ice sheet, thus it is closely related to the evolution of the sea level worldwide. The lack of observations, at both surface and the vertical structure, have hindered the understanding of this important component. Recently a study of the vertical structure of the precipitation in Antarctica have been carried out using micro rain radar (MRR) observations (Durán-Alarcón et al., 2019, TC) at two different sites: Dumont d'Urville (DDU) and Princess Elisabeth (PE) stations. The present collection consists in 2-years of vertical profiles of effective reflectivity (Ze), mean Doppler velocity (W), spectral width (SW) and snowfall rate (S) derived from a K-band vertically-pointing micro rain radar (MRR), obtained at DDU in the framework of the Antarctic Precipitation Remote Sensing from Surface and Space project (APRES3). The observation range of the profiles is between 300 m and 3 km above ground level, with 100m and 1h of vertical and temporal resolutions, respectively. Vertical profiles were separated into surface precipitation and virga (i.e., precipitation that completely sublimes before reaching the surface) to evaluate the impact of virga on the structure of the vertical profiles. The strong katabatic winds blowing at DDU induce a decrease in Ze near to the ground due to the sublimation of the snowfall particles, and the W and SW increases as the height decreases. It was observed that virga is a frequent phenomenon at DDU, since more than a third (36%) of the profiles of precipitation observed with MRR corresponded to virga cases (more details in Durán-Alarcón et al., 2019, TC). This unique dataset of Antarctic precipitation observations in the low troposphere represents a great opportunity to better understand the current numerical models and satellite observations.