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Sensitivity of GPS tropospheric estimates to mesoscale convective systems in West Africa / Samuel Nahmani in Atmospheric chemistry and physics, vol 19 n° 14 (July 2019)  

Public [article]  inAtmospheric chemistry and physics > vol 19 n° 14 (July 2019) . - pp 9541 - 9561 Titre : Sensitivity of GPS tropospheric estimates to mesoscale convective systems in West Africa Type de document : Article/Communication Auteurs : Samuel Nahmani , Auteur ; Olivier Bock , Auteur ; Françoise Guichard, Auteur Année de publication : 2019 Projets : VEGAN / Bock, Olivier, TOSCA / Bock, Olivier Article en page(s) : pp 9541 - 9561 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Géodésie spatiale [Termes IGN] Afrique occidentale [Termes IGN] analyse de sensibilité [Termes IGN] bande C [Termes IGN] convection [Termes IGN] données GPS [Termes IGN] GAMIT [Termes IGN] GIPSY-OASIS [Termes IGN] gradient de troposphère [Termes IGN] meso échelle [Termes IGN] modèle atmosphérique [Termes IGN] Niger [Termes IGN] propagation troposphérique [Termes IGN] résidu [Termes IGN] retard troposphérique zénithal [Termes IGN] signal GPS Résumé : (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 : Géodésie+Ext (mi2018-2019) 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 [article]

Consistency and representativeness of integrated water vapour from ground-based GPS observations and ERA-Interim reanalysis / Olivier Bock in Atmospheric chemistry and physics, vol 19 n° 14 (July 2019)  

Public [article]  inAtmospheric chemistry and physics > vol 19 n° 14 (July 2019) . - 17 p. Titre : Consistency and representativeness of integrated water vapour from ground-based GPS observations and ERA-Interim reanalysis Type de document : Article/Communication Auteurs : Olivier Bock , Auteur ; Ana-Claudia Bernardes Parracho , Auteur Année de publication : 2019 Projets : GNSS4SWEC / Bock, Olivier Article en page(s) : 17 p. Note générale : bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Applications de géodésie spatiale [Termes IGN] analyse comparative [Termes IGN] analyse diachronique [Termes IGN] données GPS [Termes IGN] erreur [Termes IGN] humidité de l'air [Termes IGN] interpolation spatiale [Termes IGN] retard troposphérique zénithal [Termes IGN] teneur intégrée en vapeur d'eau [Termes IGN] variation diurne [Termes IGN] variation saisonnière Résumé : (auteur) This study examines the consistency and representativeness differences of daily integrated water vapour (IWV) data from ERA-Interim reanalysis and GPS observations at 120 global sites over a 16-year period (1995–2010). Various comparison statistics are analysed as a function of geographic, topographic, and climatic features. A small (±1 kg m−2) bias is found in the reanalysis across latitudes (moist in northern and southern midlatitudes and dry in the tropics). The standard deviation of daily IWV differences is generally below 2 kg m−2 but peaks in the northern and southern storm-track regions. In general, the larger IWV differences are explained by increased representativeness errors, when GPS observations capture some small-scale variability that is not resolved by the reanalysis. A representativeness error statistic is proposed which measures the spatiotemporal variability in the vicinity of the GPS sites, based on reanalysis data at the four surrounding grid points. It allows to predict the standard deviation of daily IWV differences with a correlation of 0.73. In general, representativeness differences can be reduced by temporal averaging and spatial interpolation from the four surrounding grid points. A small number of outlying cases (15 sites) which do not follow the general tendencies are further examined. It is found that their special topographic and climatic features strongly enhance the representativeness errors (e.g. steep topography, coastlines, and strong seasonal cycle in monsoon regions). Discarding these sites significantly improves the global ERA-Interim and GPS comparison results. The selection of sites a priori, based on the representativeness error statistic, is able to detect 11 out of the 15 sites and improve the comparison results by 20 % to 30 %. Numéro de notice : A2019-615 Affiliation des auteurs : Géodésie+Ext (mi2018-2019) Autre URL associée : vers HAL Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.5194/acp-19-9453-2019 Date de publication en ligne : 24/07/2019 En ligne : https://hal.science/hal-02277609v1 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=95311 [article]