@misc{KnerrTrachteGareletal., author = {Knerr, Isabel and Trachte, Katja and Garel, Emilie and Huneau, Fr{\´e}d{\´e}ric and Santoni, S{\´e}bastien and Bendix, J{\"o}rg}, title = {Partitioning of Large-Scale and Local-Scale Precipitation Events by Means of Spatio-Temporal Precipitation Regimes on Corsica}, series = {Atmosphere}, volume = {11}, journal = {Atmosphere}, number = {4}, doi = {10.3390/atmos11040417}, pages = {19}, abstract = {The island of Corsica in the western Mediterranean is characterized by a pronounced topography in which local breeze systems develop in the diurnal cycle. In interaction with the large-scale synoptic situation, various precipitation events occur, which are classified in this study with regard to their duration and intensity. For this purpose, the island was grouped into five precipitation regimes using a cluster analysis, namely the western coastal area, the central mountainous region, the southern coastal area, the northeast coastal area, and the eastern coastal area. Based on principal component analysis using mean sea level pressure (mslp) obtained from ERA5 reanalysis (the fifth generation of the European Centre for Medium-Range Weather Forecasts, ECMWF), six spatial patterns were identified which explain 98\% of the large-scale synoptic situation, while the diurnal breeze systems within the regimes characterize local drivers. It is shown that on radiation days with weak large-scale pressure gradients, pronounced local circulations in mountainous regions are coupled with sea breezes, leading to a higher number of short and intense precipitation events. Meridional circulation patterns lead to more intensive precipitation events on the eastern part of the island (30\% intensive events with meridional patterns on the east side compared to 11\% on the west side). On the west side of Corsica, however, coastal precipitation events are seldom and less intense than further inland, which can be attributed to the influence of the topography in frontal passages.}, language = {en} } @misc{JuhlkeSueltenfussTrachteetal., author = {Juhlke, Tobias R. and S{\"u}ltenfuß, J{\"u}rgen and Trachte, Katja and Huneau, Fr{\´e}d{\´e}ric and Garel, Emilie and Santoni, S{\´e}bastien and Barth, Johannes A. C. and Geldern, Robert van}, title = {Tritium as a hydrological tracer in Mediterranean precipitation events}, series = {Atmospheric Chemistry and Physics}, volume = {20}, journal = {Atmospheric Chemistry and Physics}, number = {6}, doi = {10.5194/acp-20-3555-2020}, pages = {3555 -- 3568}, abstract = {Climate models are in need of improved constraints for water vapor transport in the atmosphere, and tritium can serve as a powerful tracer in the hydrological cycle. Although the general principles of tritium distribution and transfer processes within and between the various hydrological compartments are known, variation on short timescales and aspects of altitude dependence are still under debate. To address questions regarding tritium sources, sinks, and transfer processes, the sampling of individual precipitation events in Corte on the island of Corsica in the Mediterranean Sea was performed between April 2017 and April 2018. Tritium concentrations of 46 event samples were compared to their moisture origin and corresponding air mass history. Air mass back-trajectories were generated from the novel high-resolution ERA5 dataset from the ECMWF (European Centre for Medium-Range Weather Forecasts). Geographical source regions with similar tritium concentrations were predefined using generally known tritium distribution patterns, such as the "continental effect", and from data records derived at long-term measurement stations of tritium in precipitation across the working area. Our model-derived source region tritium concentrations agreed well with annual mean station values. Moisture that originated from continental Europe and the Atlantic Ocean was most distinct regarding tritium concentrations with values up to 8.8 TU (tritium units) and near 0 TU, respectively. The seasonality of tritium values ranged from 1.6 TU in January to 10.1 TU in May, and they exhibited well-known elevated concentrations in spring and early summer due to increased stratosphere-troposphere exchange. However, this pattern was interrupted by extreme events. The average altitude of trajectories was correlated with the tritium concentrations in precipitation, especially in spring and early summer and if outlier values of extreme tritium concentrations were excluded. However, in combination with the trajectory information, these outlier values proved to be valuable for improving the comprehension of tritium movement in the atmosphere. Our work shows how event-based tritium research can advance the understanding of its distribution in the atmosphere}, language = {en} } @misc{JuhlkeGeldernBarthetal., author = {Juhlke, Tobias R. and Geldern, Robert van and Barth, Johannes A. C. and Bendix, J{\"o}rg and Br{\"a}uning, Achim and Garel, Emilie and H{\"a}usser, Martin and Huneau, Fr{\´e}d{\´e}ric and Knerr, Isabel and Santoni, S{\´e}bastien and Szymczak, Sonja and Trachte, Katja}, title = {Temporal offset between precipitation and water uptake of Mediterranean pine trees varies with elevation and season}, series = {Science of the Total Environment}, volume = {755}, journal = {Science of the Total Environment}, number = {2}, doi = {10.1016/j.scitotenv.2020.142539}, pages = {12}, abstract = {For climate models that use paleo-environment data to predict future climate change, tree-ring isotope variations are one important archive for the reconstruction of paleo-hydrological conditions. Due to the rather complicated pathway of water, starting from precipitation until its uptake by trees and the final incorporation of its components into tree-ring cellulose, a closer inspection of seasonal variations of tree water uptake is important. In this study, branch and needle samples of two pine species (Pinus pinaster and Pinus nigra subsp. laricio) and several water compartments (precipitation, creek, soil) were sampled over a two-year period and analyzed for the temporal variations of their oxygen and hydrogen stable isotope ratios (δ18O and δ2H) at five sites over an elevation gradient from sea level to around 1600 m a.s.l. on the Mediterranean island of Corsica (France). A new model was established to disentangle temporal relationships of source water uptake of trees. It uses a calculation method that incorporates the two processes mostly expected to affect source water composition: mixing of waters and evaporation. The model results showed that the temporal offset from precipitation to water uptake is not constant and varies with elevation and season. Overall, seasonal source water origin was shown to be dominated by precipitation from autumn and spring. While autumn precipitation was a more important water source for trees growing at mid- (~800-1000 m a.s.l) and high-elevation (~1600 m a.s.l.) sites, trees at coastal sites mostly took up water from late winter and spring. These findings show that predicted decreases in precipitation amounts during the wet season in the Mediterranean can have strong impacts on water availability for pine trees, especially at higher elevations.}, language = {en} } @misc{SzymczakHaeusserGareletal., author = {Szymczak, Sonja and H{\"a}usser, Martin and Garel, Emilie and Santoni, S{\´e}bastien and Huneau, Fr{\´e}d{\´e}ric and Knerr, Isabel and Trachte, Katja and Bendix, J{\"o}rg}, title = {How Do Mediterranean Pine Trees Respond to Drought and Precipitation Events along an Elevation Gradient}, series = {Forests}, volume = {7}, journal = {Forests}, number = {11}, doi = {10.3390/f11070758}, pages = {17}, abstract = {Drought is a major factor limiting tree growth and plant vitality. In the Mediterranean region,the length and intensity of drought stress strongly varies with altitude and site conditions. We usedelectronic dendrometers to analyze the response of two native pine species to drought and precipitationevents. The five study sites were located along an elevation gradient on the Mediterranean islandof Corsica (France). Positive stem increment in the raw dendrometer measurements was separatedinto radial stem growth and stem swelling/shrinkage in order to determine which part of the trees'response to climate signals can be attributed to growth. Precipitation events of at least 5 mm anddry periods of at least seven consecutive days without precipitation were determined over a periodof two years. Seasonal dynamics of stem circumference changes were highly variable among thefive study sites. At higher elevations, seasonal tree growth showed patterns characteristic for coldenvironments, while low-elevation sites showed bimodal growth patterns characteristic of droughtprone areas. The response to precipitation events was uniform and occurred within the first six hoursafter the beginning of a precipitation event. The majority of stem circumference increases were causedby radial growth, not by stem swelling due to water uptake. Growth-induced stem circumferenceincrease occurred at three of the five sites even during dry periods, which could be attributed tostored water reserves within the trees or the soils. Trees at sites with soils of low water-holdingcapacity were most vulnerable to dry periods.}, language = {en} } @misc{SzymczakBarthBendixetal., author = {Szymczak, Sonja and Barth, Johannes A. C. and Bendix, J{\"o}rg and Huneau, Fr{\´e}d{\´e}ric and Garel, Emilie and H{\"a}usser, Martin and Juhlke, Tobias R. and Knerr, Isabel and Santoni, S{\´e}bastien and Mayr, Christoph C. and Trachte, Katja and Geldern, Robert van and Br{\"a}uning, Achim}, title = {First indications of seasonal and spatial variations of water sources in pine trees along an elevation gradient in a Mediterranean ecosystem derived from δ18O}, series = {Chemical Geology}, volume = {2020}, journal = {Chemical Geology}, number = {549}, doi = {10.1016/j.chemgeo.2020.119695}, abstract = {Water availability is the most important factor for the vitality of forest ecosystems, especially in dry environments. The Mediterranean region is one of the hotspots of future climate change; therefore, data on the water cycle are urgently needed. We measured oxygen isotope compositions in creek water, precipitation, stem water, needle water, and tree-ring cellulose over one growing season to establish the relationship between isotope compositions in different compartments along a fractionation pathway. We analyzed plant material from pine trees (Pinus nigra J.F. Arn subsp. laricio (Poiret) Maire var. Corsicana Hyl. and Pinus pinaster Aiton) at five locations along an elevation gradient from sea level to 1600 m asl. We traced back the oxygen isotope composition from source to sink in tree-ring cellulose in order to identify the water sources used by the trees, and to quantify the extent of isotope fractionation processes. Our results showed that the trees used different water sources over the course of the growing season, ranging from winter snow meltwater to summer precipitation at higher sites and deep soil water reservoirs at coastal sites. Needle water enrichment was higher at higher elevation sites than at coastal locations, highlighting the importance of site-specific climate conditions on the isotopic composition values in tree material. Water availability seems to be most restricted at the highest site, making these trees most vulnerable to climate change.}, language = {en} } @misc{KnerrTrachteEglietal., author = {Knerr, Isabel and Trachte, Katja and Egli, Sebastian and Barth, Johannes A. C. and Br{\"a}uning, Achim and Garel, Emilie and H{\"a}usser, Martin and Huneau, Fr{\´e}d{\´e}ric and Juhlke, Tobias R. and Santoni, S{\´e}bastien and Szymczak, Sonja and Geldern, Robert van and Bendix, J{\"o}rg}, title = {Fog - low stratus (FLS) regimes on Corsica with wind and PBLH as key drivers}, series = {Atmospheric Research}, volume = {261}, journal = {Atmospheric Research}, issn = {0169-8095}, doi = {10.1016/j.atmosres.2021.105731}, abstract = {The French Mediterranean island of Corsica is already today confronted with a clear tendency towards water shortage, leading not only to socio-economical, but also to ecological problems. A potential, but not very widespread source of water is the presence of near-ground clouds, mostly fog. In this study, we investigate fog-low stratus (FLS) frequencies in Corsica, derived from a data set of Meteosat Second Generation SEVIRI, whereby a distinction between fog and low stratus is hardly feasible using remote sensing data. The FLS frequency was studied with respect to its interaction with distinct locally-generated wind and its dependence on the planetary boundary layer height (PBLH) obtained by ERA5 reanalysis (the fifth generation of the European Centre for Medium-Range Weather Forecasts, ECMWF). Results show that radiation FLS is formed in coastal areas at sunrise, with low PBLH. On the other hand, in the interior of the island at sunset, a maximum of advection FLS is formed, fostered by locally-generated and related transport of moisture. On the east side of the island, FLS frequency is lower throughout the year due to frequent lee situations. This situation is reinforced by reduced synoptic moisture transport by westerly winds, so that westerly exposed slopes benefit from moisture input by FLS formation.}, language = {en} }