Several networks of Global Positioning System receiving stations over Europe send their data to several processing centers to generate atmospheric Zenith Total Delay (ZTD) observations. Thanks to the efforts of the Targeting Optimal Use of Global Positioning System Humidity measurements in meteorology project, these observations combining surface pressure and total precipitable water information in the atmosphere have been delivered to the operational meteorological centers in near real‐time since 2004. This paper presents forecast impact trials of such ZTD observations in a global Four‐Dimensional Variational (4DVAR) assimilation and forecasting system. The implementation of the ZTD assimilation in the 4DVAR system is described, including a preprocessing developed specifically for the ZTD data. The preprocessing involves a time averaging procedure of the observations in order to ensure consistency with the resolution of the 4DVAR, a bias correction, and a station selection based on χ2 tests of the normality of the observation minus first‐guess differences. Three forecast trials were conducted: winter, spring, and summer 2005. These trials cover various meteorological conditions and a total of about 10 weeks of assimilation. All three trials suggest a positive impact of the ZTD data in helping constrain the synoptic circulation in 1 to 4 day forecasts. In the spring and the summer trials, the impact of the ZTD data also shows positively on the prediction of precipitation patterns as indicated by improved Quantitative Precipitation Forecast scores for total precipitation forecasts over France between +12 and +36 hours. We also assess in this paper ZTD observation and background errors.

AB STRACTFol low ing sev eral years of ex per i men ta tion with the GPS ra dio occultation tech nique, the 6-sat el lite FORMOSAT-3/ COS MIC (F3C) mis sion was launched mid-2006 and has been col lect ing data since then. In this pa per we pres ent early find ings of re search per formed at Météo-France re gard ing the use of these data for as sim i la tion in nu mer i cal weather pre dic tion. Ben e fit ing from the dense global cov er age al lowed by F3C re frac tion-in duced ob ser va tions, we first as sess the qual ity of these data at four lev els: bend ing an gle, refractivity, refractivity lapse rate, and tem per a ture. We com pare them with cal cu la tions from Météo-France nu mer i cal weather fore casts. Learn ing from these var i ous lev els of data we de vise qual ity con trol pro ce dures that rely on the refractivity lapse rate. Ap ply ing a re cent meth od ol ogy de vel oped in data as sim i la tion we cal cu late ob ser va tion bend ing an gle er ror vari ances for our as sim i la tion sys tem. Us ing these new qual ity con trol pro ce dures and ob ser va tion er ror es ti mates we run an as sim i la tion and fore cast ex per i ment with Météo-France's op er a tional global 4DVAR data as sim i la tion sys tem used as a ref er ence. Our re sults in di cate a very clear pos i tive im pact of the as sim i la tion of F3C bend ing an gle data in the South ern hemi sphere for the pre dic tion of geopotential heights and winds. We also ob serve an improvement in wind forecast skill in the Northern hemisphere, albeit such an improvement is smaller than in the Southern hemisphere.Key words: Data as sim i la tion, GPS ra dio occultation, Ob ser va tion er rors, Refractivity lapse rate, Qual ity con trol, Mon i tor ing Sci., 20, 101-113, doi: 10.3319/TAO.2008.01.21.02(F3C) IN TRO DUC TIONIn 1995, The GPS ra dio occultation tech nique dem onstrated its abil ity to col lect low cost ob ser va tions of the Earth's at mo sphere with a pas sive in stru ment in volv ing neither op tics nor mov ing parts (Ware et al. 1996). The technique re quires plac ing an ad vanced ra dio equip ment (i.e., a GPS re ceiver) onboard a sat el lite whose or bit de ter mi na tion needs to be pre cise. With this setup the track ing data of the highly sta ble sig nals trans mit ted by the con stel la tion of GPS sat el lites can be in verted to yield ionospheric and at mospheric information.The out line of the pa per is as fol lows. In sec tion 2 we pres ent the FORMOSAT-3/COS MIC (F3C) ob ser va tions. We dis cuss the char ac ter is tics of one week of F3C data for tem per a ture, refractivity, refractivity lapse rate, and bending an gle in sec tion 3. Sec tion 4 in tro duces Météo-France's op er a tional as sim i la tion and fore cast sys tem ARPEGE (the French ac ro nym for Ac tion de Re cher che Pe tite Echelle et Grande Echelle, i.e., Re search Pro ject on Small and Large Scales). That sec tion also de tails the ob ser va tion op er a tors used to sim u late F3C data. Sec tion 5 pres ents a com par i son of F3C data with ...

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A B S T R A C T The impact of assimilating GPS zenith total delays (ZTD) in the convective scale model AROME is assessed over a 1-month period in summer 2013. The experimental set-up is similar to the current operational usage at Me´te´o-France where the observing system has been expanded in July 2013 in a three-dimensional variational (3D-Var) data assimilation scheme with a 3-hour cycling. Three experiments are performed. In a baseline experiment the GPS ZTD provided through the E-GVAP programme are withdrawn from the observing system (NOGPS). In a second experiment, GPS ZTD from E-GVAP are included in the observing system, representing the operational configuration at Me´te´o-France (EGVAP). The last experiment is similar to EGVAP but new ZTD observations processed by the University of Luxembourg are also assimilated on top of all other observations (UL01). In the first stage, it has been verified through a systematic comparison with model counterparts that the quality of ZTD data processed by the University of Luxembourg is similar to the one provided by other analysis centres from the E-GVAP programme. After a number of quality controls, it has been possible to assimilate around 90 additional observations on top of around 600 stations from E-GVAP every 3 hours. Despite the small fraction of observations assimilated in AROME that ZTD represent ( B2%), it is shown that they systematically improve the atmospheric humidity short-range forecasts by a comparison with other observing systems informative about water vapour (radiosoundings, satellite radiances, surface networks) even though it is by small amounts. When examining objective precipitation scores over France, the improvement brought by the UL01 stations on top of E-GVAP is systematic for all daily precipitation thresholds. Examination of several case studies reveals the ability of the ZTD observations to modify the intensity and location of precipitating areas in accordance with previous studies. The addition of ZTD from UL01 is also found to be beneficial, by improving rainfall patterns. Planned improvements to the AROME forecasting and assimilation systems with higher horizontal resolution and hourly cycling of 3D-Var assimilation will be of benefit to ZTD observations.