Toulouse, France
Toulouse, France

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Cretaux J.-F.,French National Center for Space Studies | Berge-Nguyen M.,French National Center for Space Studies | Calmant S.,IRD LEGOS | Romanovski V.V.,IWPH | And 10 more authors.
Advances in Space Research | Year: 2013

This study presents the results of calibration/validation (C/V) of Envisat satellite radar altimeter over Lake Issykkul located in Kyrgyzstan, which was chosen as a dedicated radar altimetry C/V site in 2004. The objectives are to estimate the absolute altimeter bias of Envisat and its orbit based on cross-over analysis with TOPEX/Poseidon (T/P), Jason-1 and Jason-2 over the ocean. We have used a new method of GPS data processing in a kinematic mode, developed at the Groupe de Recherche de Geodesie Spatiale (GRGS), which allows us to calculate the position of the GPS antenna without needing a GPS reference station. The C/V is conducted using various equipments: a local GPS network, a moving GPS antenna along the satellites tracks over Lake Issykkul, In Situ level gauges and weather stations. The absolute bias obtained for Envisat from field campaigns conducted in 2009 and 2010 is between 62.1 and 63.4 ± 3.7 cm, using the Ice-1 retracking algorithm, and between 46.9 and 51.2 cm with the ocean retracking algorithm. These results differ by about 10 cm from previous studies, principally due to improvement of the C/V procedure. Apart from the new algorithm for GPS data processing and the orbit error reduction, more attention has been paid to the GPS antenna height calculation, and we have reduced the errors induced by seiche over Lake Issykkul. This has been assured using cruise data along the Envisat satellite track at the exact date of the pass of the satellite for the two campaigns. The calculation of the Envisat radar altimeter bias with respect to the GPS levelling is essential to allow the continuity of multi-mission data on the same orbit, with the expected launch of SARAL/Altika mission in 2012. Implications for hydrology in particular, will be to produce long term homogeneous and reliable time series of lake levels worldwide. © 2012 COSPAR. Published by Elsevier Ltd. All rights reserved.

Chambon P.,NASA | Roca R.,OMP LEGOS | Jobard I.,Ecole Polytechnique - Palaiseau | Capderou M.,Ecole Polytechnique - Palaiseau
IEEE Geoscience and Remote Sensing Letters | Year: 2013

The availability of rainfall-related measurements from space has greatly increased from the late 1980s with the Defense Meteorological Satellite Program and the launch of the Tropical Rainfall Measuring Mission in 1997 to the forthcoming Global Precipitation Measurement (GPM) program (GPM mission) whose core satellite is to be launched in 2014. The rainfall observing systems have become a constellation enhancing the frequency of measurements all over the globe. In this letter, the Megha-Tropiques TAPEER-BRAIN level-4 rainfall product is considered to explore what impacts the configuration of a microwave imager constellation has on accumulated rainfall and associated sampling error estimates at one-degree/one-day resolution in the tropics. One of the main findings of this letter is that sun-synchronous satellites providing observations separated of time intervals close to rainfall autocorrelation periods result only in small improvements of TAPEER-BRAIN quantitative precipitation estimations (i.e., rain and error estimations). By comparison, it is shown that the GPM constellation of satellites, particularly with satellites on low-inclination 'equatorial' orbits, has a high contribution to the improvements of rain and error estimates. The methodology developed in this letter could be also useful to explore the sensitivity of rainfall estimates at finer space and timescales. © 2012 IEEE.

Beucher F.,Meteo - France | Lafore J.-P.,OMP LEGOS | Karbou F.,Meteo - France | Roca R.,OMP LEGOS
Quarterly Journal of the Royal Meteorological Society | Year: 2014

In this article, we evaluate the predictions of the French cloud-resolving model AROME using a set of high-resolution (5 km) simulations that focus on the well documented African Monsoon Multidisciplinary Analysis (AMMA) period of 23-28 July 2006 over a large domain (0-22°N, 15°W-20°E). The model skill is assessed against independent Global Positioning System observations of precipitable water and in terms of quantitative precipitation forecasts. As the rain-gauge network is sparse over West Africa, the simulated precipitation fields were compared with data from satellite-based precipitation products (TRMM-3B42). We show that initial and boundary conditions significantly improve the AROME forecasts when the large-scale forcing model (ARPEGE) assimilates surface-sensitive observations from microwave remote-sensing sensors over land surface. The daily mean AROME precipitation shows a spatial distribution in good agreement with the satellite precipitation estimates. The intertropical convergence zone is correctly reproduced in terms of shape and location but its intensity is broadly overestimated by about 25%. The AROME model is shown to be able to reproduce all regimes, from light rain to the biggest Mesoscale Convective Systems (MCSs). The observations made at the Niamey AMMA supersite allow a detailed evaluation. Near the Niamey AMMA supersite, we show that AROME is able to represent most of the key features of the West African monsoon from the diurnal to synoptic scales. The life cycle of two successive sequences of MCSs associated with an African easterly wave and a deep monsoon burst are well captured by AROME. Finally, a tracking approach based on the 1 h accumulated precipitation is applied to both Global Satellite Mapping of Precipitation (GSMaP) satellite estimates and to AROME and ARPEGE forecasts, allowing a good characterization of each MCS and statistics. Contrary to ARPEGE, the AROME MCSs trajectories and lifetimes, and the diurnal cycles of their initiation and dissipation, are in agreement with the GSMaP tracking and previous MCS statistics. © 2013 Royal Meteorological Society.

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