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Affiner la recherche Interroger des sources externesSensitivity of GPS tropospheric estimates to mesoscale convective systems in West Africa / Samuel Nahmani in Atmospheric chemistry and physics, vol 19 n° 14 (July 2019)
Titre : Sensitivity of GPS tropospheric estimates to mesoscale convective systems in West Africa Type de document : Article/Communication Auteurs : Samuel Nahmani Année de publication : 2019 Projets : VEGA (LEFE/INSU) / Bock, Olivier Article en page(s) : pp 9541 - 9561 Note générale : Bibliographie
This work was supported by CNES through the TOSCA committee (grant no. 4359). It was also supported by the Centre National de la Recherche Scientifique (grant LEFE/INSU projet VEGA).Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Géodésie spatiale
[Termes descripteurs IGN] Afrique occidentale
[Termes descripteurs IGN] analyse de sensibilité
[Termes descripteurs IGN] bande C
[Termes descripteurs IGN] convection
[Termes descripteurs IGN] données GPS
[Termes descripteurs IGN] GAMIT
[Termes descripteurs IGN] GIPSY-OASIS
[Termes descripteurs IGN] gradient de troposphère
[Termes descripteurs IGN] meso échelle
[Termes descripteurs IGN] modèle atmosphérique
[Termes descripteurs IGN] Niger
[Termes descripteurs IGN] propagation troposphérique
[Termes descripteurs IGN] résidu
[Termes descripteurs IGN] retard troposphérique zénithal
[Termes descripteurs IGN] signal GPSRésumé : (Auteur) This study analyzes the characteristics of GPS tropospheric estimates (zenith wet delays – ZWDs, gradients, and post-fit phase residuals) during the passage of mesoscale convective systems (MCSs) and evaluates their sensitivity to the research-level GPS data processing strategy implemented. Here, we focus on MCS events observed during the monsoon season of West Africa. This region is particularly well suited for the study of these events due to the high frequency of MCS occurrences in the contrasting climatic environments between the Guinean coast and the Sahel. This contrast is well sampled with data generated by six African Monsoon Multidisciplinary Analysis (AMMA) GPS stations. Tropospheric estimates for a 3-year period (2006–2008), processed with both the GAMIT and GIPSY-OASIS software packages, were analyzed and intercompared. First, the case of a MCS that passed over Niamey, Niger, on 11 August 2006 demonstrates a strong impact of the MCS on GPS estimates and post-fit residuals when the GPS signals propagate through the convective cells as detected on reflectivity maps from the MIT C-band Doppler radar. The estimates are also capable of detecting changes in the structure and dynamics of the MCS. However, the sensitivity is different depending on the tropospheric modeling approach adopted in the software. With GIPSY-OASIS, the high temporal sampling (5 min) of ZWDs and gradients is well suited for detecting the small-scale, short-lived, convective cells, while the post-fit residuals remain quite small. With GAMIT, the lower temporal sampling of the estimated parameters (hourly for ZWDs and daily for gradients) is not sufficient to capture the rapid delay variations associated with the passage of the MCS, but the post-fit phase residuals clearly reflect the presence of a strong refractivity anomaly. The results are generalized with a composite analysis of 414 MCS events observed over the 3-year period at the six GPS stations with the GIPSY-OASIS estimates. A systematic peak is found in the ZWDs coincident with the cold pool crossing time associated with the MCSs. The tropospheric gradients reflect the path of the MCS propagation (generally from east to west). This study concludes that ZWDs, gradients, and post-fit phase residuals provide relevant and complementary information on MCSs passing over or in the vicinity of a GPS station. Numéro de notice : A2019-572 Affiliation des auteurs : UMR IPGP-Géod+Ext (2020- ) Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.5194/acp-19-9541-2019 date de publication en ligne : 29/07/2019 En ligne : https://doi.org/10.5194/acp-19-9541-2019 Format de la ressource électronique : URL Article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=94444
in Atmospheric chemistry and physics > vol 19 n° 14 (July 2019) . - pp 9541 - 9561[article]Documents numériques
en open access
Sensitivity of GPS tropospheric... - pdf éditeurAdobe Acrobat PDF
Titre : Enhanced MODIS atmospheric total water vapour content trends in response to Arctic amplification Type de document : Article/Communication Auteurs : Dunya Alraddawi, Auteur ; Philippe Keckhut, Auteur ; Alain Sarkissian, Auteur ; Olivier Bock Année de publication : 2017 Projets : VEGA (LEFE/INSU) / Bock, Olivier Note générale : bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Applications de géodésie spatiale
[Termes descripteurs IGN] Arctique
[Termes descripteurs IGN] Arctique, océan
[Termes descripteurs IGN] changement climatique
[Termes descripteurs IGN] Groenland
[Termes descripteurs IGN] image Aqua-MODIS
[Termes descripteurs IGN] image Terra-MODIS
[Termes descripteurs IGN] modèle de simulation
[Termes descripteurs IGN] série temporelle
[Termes descripteurs IGN] Svalbard
[Termes descripteurs IGN] température de surface de la mer
[Termes descripteurs IGN] teneur en vapeur d'eauRésumé : (auteur) In order to assess the strength of the water vapour feedback within Arctic climate change, 15 years of the total column-integrated density of water vapour (TCWV) from the moderate resolution imaging spectrometer (MODIS) are analysed. Arctic TCWV distribution, trends, and anomalies for the 2001–2015 period, broken down into seasons and months, are analysed. Enhanced local spring TCWV trends above the terrestrial Arctic regions are discussed in relation to land snow cover and vegetation changes. Upward TCWV trends above the oceanic areas are discussed in lien with sea ice extent and sea surface temperature changes. Increased winter TCWV (up to 40%) south of the Svalbard archipelago are observed; these trends are probably driven by a local warming and sea ice extent decline. Similarly, the Barents/Kara regions underwent wet trends (up to 40%), also associated with winter/fall local sea ice loss. Positive late summer TCWV trends above the western Greenland and Beaufort seas (about 20%) result from enhanced upper ocean warming and thereby a local coastal decline in ice extent. The Mackenzie and Siberia enhanced TCWV trends (about 25%) during spring are found to be associated with coincident decreased snow cover and increased vegetation, as a result of the earlier melt onset. Results show drier summers in the Eurasia and western Alaska regions, thought to be affected by changes in albedo from changing vegetation. Other TCWV anomalies are also presented and discussed in relation to the dramatic decline in sea ice extent and the exceptional rise in sea surface temperature. Numéro de notice : A2017-858 Affiliation des auteurs : LAREG+Ext (avant 2012) Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.3390/atmos8120241 date de publication en ligne : 02/12/2017 En ligne : https://doi.org/10.3390/atmos8120241 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=89835
in Atmosphere > vol 8 n° 12 (December 2017)[article]
Titre : Study and mitigation of calibration factor instabilities in a water vapor Raman lidar Type de document : Article/Communication Auteurs : Leslie David, Auteur ; Olivier Bock Année de publication : 2017 Projets : VEGA (LEFE/INSU) / Bock, Olivier Article en page(s) : pp 2745 - 2758 Note générale : bibliographie
This work was developed in the framework of project VEGA and supported by the French national program LEFE/INSU.Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Atmosphère
[Termes descripteurs IGN] azote
[Termes descripteurs IGN] étalonnage d'instrument
[Termes descripteurs IGN] fibre optique
[Termes descripteurs IGN] lidar Raman
[Termes descripteurs IGN] retard troposphérique
[Termes descripteurs IGN] vapeur d'eauRésumé : (auteur) We have investigated calibration variations in the Rameau water vapor Raman lidar. This lidar system was developed by the Institut National de l'Information Géographique et Forestière (IGN) together with the Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS). It aims at calibrating Global Navigation Satellite System (GNSS) measurements for tropospheric wet delays and sounding the water vapor variability in the lower troposphere. The Rameau system demonstrated good capacity in retrieving water vapor mixing ratio (WVMR) profiles accurately in several campaigns. However, systematic short-term and long-term variations in the lidar calibration factor pointed to persistent instabilities. A careful testing of each subsystem independently revealed that these instabilities are mainly induced by mode fluctuations in the optic fiber used to couple the telescope to the detection subsystem and by the spatial nonuniformity of the photomultiplier photocathodes. Laboratory tests that replicate and quantify these instability sources are presented. A redesign of the detection subsystem is presented, which, combined with careful alignment procedures, is shown to significantly reduce the instabilities. Outdoor measurements were performed over a period of 5 months to check the stability of the modified lidar system. The calibration changes in the detection subsystem were monitored with lidar profile measurements using a common nitrogen filter in both Raman channels. A short-term stability of 2–3 % and a long-term drift of 2–3 % per month are demonstrated. Compared to the earlier Development of Methodologies for Water Vapour Measurement (DEMEVAP) campaign, this is a 3-fold improvement in the long-term stability of the detection subsystem. The overall water vapor calibration factors were determined and monitored with capacitive humidity sensor measurements and with GPS zenith wet delay (ZWD) data. The changes in the water vapor calibration factors are shown to be fairly consistent with the changes in the nitrogen calibration factors. The nitrogen calibration results can be used to correct the overall calibration factors without the need for additional water vapor measurements to within 1 % per month. Numéro de notice : A2017-868 Affiliation des auteurs : LAREG+LOEMI+Ext Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.5194/amt-10-2745-2017 date de publication en ligne : 31/07/2017 En ligne : https://doi.org/10.5194/amt-10-2745-2017 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=89882
in Atmospheric measurement techniques > vol 10 n° 7 (July 2017) . - pp 2745 - 2758[article]Documents numériques
en open access
Study and mitigation of calibration ... - pdf éditeurAdobe Acrobat PDF
