Towards kilometer-scale ocean-atmosphere-wave coupled forecast: a case study on a Mediterranean heavy precipitation event

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Sauvage, C., Brossier, C. L., & Bouin, M.-N. Towards kilometer-scale ocean-atmosphere-wave coupled forecast: a case study on a Mediterranean heavy precipitation event. Atmospheric Chemistry and Physics, 21(15), (2021): 11857–11887, https://doi.org/10.5194/acp-21-11857-2021.The western Mediterranean Sea area is frequently affected in autumn by heavy precipitation events (HPEs). These severe meteorological episodes, characterized by strong offshore low-level winds and heavy rain in a short period of time, can lead to severe flooding and wave-submersion events. This study aims to progress towards an integrated short-range forecast system via coupled modeling for a better representation of the processes at the air–sea interface. In order to identify and quantify the coupling impacts, coupled ocean–atmosphere–wave simulations were performed for a HPE that occurred between 12 and 14 October 2016 in the south of France. The experiment using the coupled AROME-NEMO-WaveWatchIII system was notably compared to atmosphere-only, coupled atmosphere–wave and ocean–atmosphere simulations. The results showed that the HPE fine-scale forecast is sensitive to both couplings: the interactive coupling with the ocean leads to significant changes in the heat and moisture supply of the HPE that intensify the convective systems, while coupling with a wave model mainly leads to changes in the low-level dynamics, affecting the location of the convergence that triggers convection over the sea. Result analysis of this first case study with the AROME-NEMO-WaveWatchIII system does not clearly show major changes in the forecasts with coupling and highlights some attention points to follow (ocean initialization notably). Nonetheless, it illustrates the higher realism and potential benefits of kilometer-scale coupled numerical weather prediction systems, in particular in the case of severe weather events over the sea and/or in coastal areas, and shows their affordability to confidently progress towards operational coupled forecasts.This research has been supported by the Institut national des sciences de l'Univers (grant no. MISTRALS/HyMeX/ST-TIP) and the Région Occitanie Pyrénées-Méditerranée (grant no. n∘LS 149109)

Abrupt warming and salinification of intermediate waters interplays with decline of deep convection in the Northwestern Mediterranean Sea

The Mediterranean Sea is a hotspot for climate change, and recent studies have reported its intense warming and salinification. In this study, we use an outstanding dataset relying mostly on glider endurance lines but also on other platforms to track these trends in the northwestern Mediterranean where deep convection occurs. Thanks to a high spatial coverage and a high temporal resolution over the period 2007–2017, we observed the warming (+0.06 ∘C year−1) and salinification (+0.012 year−1) of Levantine Intermediate Water (LIW) in the Ligurian Sea. These rates are similar to those reported closer to its formation area in the Eastern Mediterranean Sea. Further downstream, in the Gulf of Lion, the intermediate heat and salt content were exported to the deep layers from 2009 to 2013 thanks to deep convection processes. In 2014, a LIW step of +0.3 ∘C and +0.08 in salinity could be observed concomitant with a weak winter convection. Warmer and more saline LIW subsequently accumulated in the northwestern basin in the absence of intense deep convective winters until 2018. Deep stratification below the LIW thus increased, which, together with the air–sea heat fluxes intensity, constrained the depth of convection. A key prognostic indicator of the intensity of deep convective events appears to be the convection depth of the previous year

HyMeX: A 10-Year Multidisciplinary Program on the Mediterranean Water Cycle

Drobinski, P. ... et. al.-- 20 pages, 10 figures, 1 table, supplement material http://journals.ametsoc.org/doi/suppl/10.1175/BAMS-D-12-00244.1HyMeX strives to improve our understanding of the Mediterranean water cycle, its variability from the weather-scale events to the seasonal and interannual scales, and its characteristics over one decade (2010–20), with a special focus on hydrometeorological extremes and the associated social and economic vulnerability of the Mediterranean territoriesHyMeX was developed by an international group of scientists and is currently funded by a large number of agencies. It has been the beneficiary of financial contributions from CNRS; Météo-France; CNES; IRSTEA; INRA; ANR; Collectivité Territoriale de Corse; KIT; CNR; Université de Toulouse; Grenoble Universités; EUMETSAT; EUMETNET; AEMet; Université Blaise Pascal, Clermont Ferrand; Université de la Méditerranée (Aix-Marseille II); Université Montpellier 2; CETEMPS; Italian Civil Protection Department; Université Paris- Sud 11; IGN; EPFL; NASA; New Mexico Tech; IFSTTAR; Mercator Ocean; NOAA; ENEA; TU Delft; CEA; ONERA; IMEDEA; SOCIB; ETH; MeteoCat; Consorzio LAMMA; IRD; National Observatory of Athens; Ministerio de Ciencia e Innovación; CIMA; BRGM; Wageningen University and Research Center; Department of Geophysics, University of Zagreb; Institute of Oceanography and Fisheries, Split, Croatia; INGV; OGS; Maroc Météo; DHMZ; ARPA Piemonte; ARPA-SIMC Emilia-Romagna; ARPA Calabria; ARPA Friuli Venezia Giulia; ARPA Liguria; ISPRA; University of Connecticut; Università degli Studi dell'Aquila; Università di Bologna; Università degli Studi di Torino; Università degli Studi della Basilicata; Università La Sapienza di Roma; Università degli Studi di Padova; Università del Salento; Universitat de Barcelona; Universitat de les Illes Balears; Universidad de Castilla-La Mancha; Universidad Complutense de Madrid; MeteoSwiss; and DLR. It also received support from the European Community's Seventh Framework Programme (e.g., PERSEUS, CLIM-RUN)Peer reviewe

Land motion estimates from GPS at tide gauges: a geophysical evaluation

International audienceSpace geodesy applications have mainly been limited to horizontal deformations due to a number of restrictions on the vertical component accuracy. Monitoring vertical land motion is nonetheless of crucial interest in observations of long-term sea level change or postglacial rebound measurements. Here, we present a global vertical velocity field obtained with more than 200 permanent GPS stations, most of them colocated with tide gauges (TGs). We used a state of the art, homogeneous processing strategy to ensure that the reference frame was stable throughout the observation period of almost 10 yr. We associate realistic uncertainties to our vertical rates, taking into account the time-correlation noise in the time-series. The results are compared with two independent geophysical vertical velocity fields: (1) vertical velocity estimates using long-term TG records and (2) postglacial model predictions from the ICE-5G (VM2) adjustment. The quantitative agreement of the GPS vertical velocities with the ‘internal estimates’ of vertical displacements using the TG record is very good, with a mean difference of −0.13 ± 1.64 mm yr−1 on more than 100 sites. For 84 per cent of the GPS stations considered, the vertical velocity is confirmed by the TG estimate to within 2 mm yr−1. The overall agreement with the glacial isostatic adjustment (GIA) model is good, with discrepancy patterns related either to a local misfit of the model or to active tectonics. For 72 per cent of the sites considered, the predictions of the GIA model agree with the GPS results to within two standard deviations. Most of the GPS velocities showing discrepancies with respect to the predictions of the GIA model are, however, consistent with previously published space geodesy results. We, in turn, confirm the value of 1.8 ± 0.5 mm yr−1 for the 20th century average global sea level rise, and conclude that GPS is now a robust tool for vertical land motion monitoring which is accurate at least at 1 mm yr−1

Impact of a medicane on the oceanic surface layer from a coupled, kilometre-scale simulation

A kilometre-scale coupled ocean–atmosphere numerical simulation is used to study the impact of the 7 November 2014 medicane on the oceanic upper layer. The processes at play are elucidated through analyses of the tendency terms for temperature and salinity in the oceanic mixed layer. While comparable by its maximum wind speed to a Category 1 tropical cyclone, the medicane results in a substantially weaker cooling. As in weak to moderate tropical cyclones, the dominant contribution to the surface cooling is the surface heat fluxes with secondary effects from the turbulent mixing and lateral advection. Upper-layer salinity decreases due to heavy precipitation that overcompensates the salinizing effect of evaporation and turbulent mixing. The upper-layer evolution is marked by several features believed to be typical of Mediterranean cyclones. First, strong, convective rain occurring at the beginning of the event builds a marked salinity barrier layer. As a consequence, the action of surface forcing is favoured and the turbulent mixing dampened with a net increase in the surface cooling as a result. Second, due to colder surface temperature and weaker stratification, a cyclonic eddy is marked by a weaker cooling opposite to what is usually observed in tropical cyclones. Third, the strong dynamics of the Strait of Sicily enhance the role of the lateral advection in the cooling and warming processes of the mixed layer

Geocentric sea-level trend estimates from GPS analyses at relevant tide gauges world-wide

11 páginas, 4 figuras, 1 tablaThe problem of correcting the tide gauge records for the vertical land motion upon which the gauges are settled has only been partially solved. At best, the analyses so far have included model corrections for one of the many processes that can affect the land stability, namely the Glacial-Isostatic Adjustment (GIA). An alternative approach is to measure (rather than to model) the rates of vertical land motion at the tide gauges by means of space geodesy. A dedicated GPS processing strategy is implemented to correct the tide gauges records, and thus to obtain a GPS-corrected set of ‘absolute’ or geocentric sea-level trends. The results show a reduced dispersion of the estimated sea-level trends after application of the GPS corrections. They reveal that the reference frame implementation is now achieved within the millimetre accuracy on a weekly basis. Regardless of the application, whether local or global, we have shown that GPS data analysis has reached the maturity to provide useful information to separate land motion from oceanic processes recorded by the tide gauges or to correct these latter. For comparison purposes, we computed the global average of sea-level change according to Douglas [Douglas, B.C., 2001. Sealevel change in the era of the recording tide gauge. Int. Geophys. Ser., 75, pp. 37–64.] rules, whose estimate is 1.84 ± 0.35 mm/yr after correction for the GIA effect [Peltier, W.R., 2001. Global glacial isostatic adjustment and modern instrumental records of relative sealevel history. Int. Geophys. Ser., 75, pp. 65–95.]. We obtain a value of 1.31 ± 0.30 mm/yr, a value which appears to resolve the ‘sea level enigma’ [Munk, W., 2002. Twentieth century sea level: an enigma. Proc. Natl. Acad. Sci. U.S.A., 99(10), pp. 6550–6555].The work was partly funded by GRGS, which provided a post-doctoral fellowship for B. Martin Miguez. Special thanks are due to Pierre Exertier (GRGS/OCA) and Bernard Simon (GRGS/SHOM). The ULR GPS analysis centre was initially funded by Conseil Général desDeux Sèvres and Région Poitou-Charentes.Peer reviewe

Surface processes in the 7 November 2014 medicane from air–sea coupled high-resolution numerical modelling

A medicane, or Mediterranean cyclone with characteristics similar to tropical cyclones, is simulated using a kilometre-scale ocean–atmosphere coupled modelling platform. A first phase leads to strong convective precipitation, with high potential vorticity anomalies aloft due to an upper-level trough. Then, the deepening and tropical transition of the cyclone result from a synergy of baroclinic and diabatic processes. Heavy precipitation results from uplift of conditionally unstable air masses due to low-level convergence at sea. This convergence is enhanced by cold pools, generated either by rain evaporation or by advection of continental air masses from northern Africa. Back trajectories show that air–sea heat exchanges moisten the low-level inflow towards the cyclone centre. However, the impact of ocean–atmosphere coupling on the cyclone track, intensity and life cycle is very weak. This is due to a sea-surface cooling 1 order of magnitude weaker than for tropical cyclones, even in the area of strong enthalpy fluxes. Surface currents have no impact. Analysing the surface enthalpy fluxes shows that evaporation is controlled mainly by the sea-surface temperature and wind. Humidity and temperature at the first level play a role during the development phase only. In contrast, the sensible heat transfer depends mainly on the temperature at the first level throughout the medicane lifetime. This study shows that the tropical transition, in this case, is dependent on processes widespread in the Mediterranean Basin, like advection of continental air, rain evaporation and formation of cold pools, and dry-air intrusion

La mesure de la hauteur d'eau par GNSS au Vanuatu

Les participants au programme GEODEVA.Les subductions sont à l'origine de séismes et tsunamis dévastateurs ; mieux comprendre le potentiel de chacune à générer des séismes, en particulier en caractérisant l'extension et la position de la zone bloquée au niveau de l'inter-plaque est donc un intérêt majeur. Un des outils approprié pour cette question est la quantification de la déformation de surface

Méthodologies en traitement de données GPS pour les sciences de l'environnement (contributions à l'étude de la mousson en Afrique de l'Ouest)

Le Global Positioning System présente aujourd hui un grand intérêt dans le domaine de la météorologie et des Sciences de l Environnement. Cette thèse s intéresse plus particulièrement à l étude du cycle de l eau en Afrique de l Ouest, dans le cadre des projets AMMA (Analyse Multidisciplinaire de la Mousson Africaine) et GHYRAF (Gravité et Hydrologie en Afrique). Dans une première partie, nous analysons la précision des calculs GPS en Afrique. Les Contenus Intégrés en Vapeur d Eau sont particulièrement importants pour appréhender les processus atmosphériques clefs de la mousson couvrant des périodes subdiurnes à pluriannuelles. Nous prêtons alors une grande attention aux sources d erreurs et à la stratégie de traitement des données GPS. Un rappel des principaux éléments théoriques nous permet d identifier les sources d erreur majeures susceptibles d affecter ces estimations et d'en quantifier les effets. Dans une deuxième partie, nous nous intéressons aux estimations de position des stations dans le but de quantifier les déformations de la croûte terrestre induites par l hydrologie continentale. Nous comparons les estimations GPS à des estimations de surcharge calculées à partir de produits de modèles géophysiques et de données de gravimétrie spatiale GRACE. Les trois jeux de données considérés sont en bon accord concernant l amplitude annuelle des déformations verticales. Un signal GPS additionnel est toutefois détecté sur les hauteurs des stations GPS AMMA. Il est fortement corrélé avec l'hydrologie locale. Nous expliquons son origine par des mécanismes géotechniques impliquant une séquence de retrait/gonflement des argiles combinée à des effets hydrologiques locaux.Global Positioning System is now very useful for meteorology and environmental sciences. This thesis focuses on the study of the water cycle in West Africa, as part of AMMA (African Monsoon Multidisciplinary Analyses) and GHYRAF (Gravity and Hydrology in Africa) projects. In the first part, we analyze the precision of GPS solutions in Africa. The Integrated Water Vapour are especially important in understanding the key atmospheric processes of the monsoon from subdiurne to multiyear periods. We pay much attention to sources of errors and the strategy of GPS data processing. A recall of the main theoretical elements allows us to identify the major sources of error that may affect the GPS estimates and we quantify the sensitivity of IWV for each of these sources of error. In the second part, we focus on estimates of station positions in order to quantify crustal deformation caused by continental hydrology. We compare the GPS solutions to estimates calculated from geophysical data and from GRACE space gravimetric data. Through this study, we also evaluate the quality of representation of the seasonal variation of soil moisture in West Africa with hydrological models and GRACE products. The three data sets are rather matching with the annual range of vertical deformations. An additional GPS signal is however detected, on the heights of AMMA GPS stations. The additional GPS signal is strongly correlated with the flooding of the Niger River to nearby stations or piezometric variations of the upper aquifer in Ouagadougou. It would be explained by hydrogeological mechanisms involving a sequence of shrinkage /swelling clays combined with local hydrological effects.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Terrestrial reference frame implementation in global GPS analysis at TIGA ULR consortium

International audienceMonitoring vertical land motion at tide gauges is mandatory if absolute or climate related signals in mean sea level are to be derived from coastal tide gauge records. Different glacial isostatic adjustment models provide very different predictions at the required level of a few tenth of millimetres per year accuracy, while other processes that may affect the vertical stability of tide gauges are still more difficult to predict. An alternative approach is to measure the rates of vertical land motions at the tide gauges. Continuous GPS recordings are presently the most practical and accurate technical solution to address such an issue. The most adequate way to handle the GPS processing seems to be a global scale approach in consistency with the size of the issues: geocentric reference frame realization, long-term stability, global climate change. The laboratories CLDG (Centre Littoral De Géophysique) and LAREG (Laboratoire de Recherches en Géodésie) contribute to the Tide Gauge Benchmark Monitoring (TIGA) IGS pilot project since October 2002 by routinely processing a global set of about 140 GPS sites within the ULR analysis centre consortium. The set comprises about 122 TG-CGPS stations that are analysed using a free network approach. Time series of more than six years are now available. The accuracy of the vertical component is very sensitive to the reference frame definition and realization. We therefore performed some tests in order to quantify the impact of various analysis options on the stability of our realization. We compared the stability of our global solutions with respect to ITRF2000 solution and to its IGS realization called IGS00, while changing the number of reference stations and their geometry. The results indicate that the more reference stations are used the better the alignment to ITRF is performed. A large global distribution of the reference frame stations seems to mitigate individual station problems. The reference frame implementation is achieved within several millimetres accuracy on a weekly basis. The tests indicate that the choice of IGS00 or ITRF2000 datum was not a relevant issue to perform the alignment to ITRF at this level of accuracy