4D-VAR assimilation of GPS-derived ZTD: a Case Study

L. Cucurull, A. Rius, F. Vandenberghe and M. de Pondeca

Abstract:	The impact of GPS Zenith Total Delay (ZTD) mea- surements on mesoscale precipitation forecasts is stud- ied. GPS ZTD observations from a permanent network of ve geodetic GPS receivers are continuously assimilated into the Penn State/NCAR Mesoscale Model (MM5) us- ing the adjoint technique. This 4-Dimensional (space and time) Variational (4D-VAR) data assimilation experiment focuses on a strong precipitation event that occurs on September 14, 1999 over the NE cost of the Iberian Penin- sula. The GPS receiver network consists of 5 Trimble 400SSI GPS receivers with baselines ranging in length from about 100 to 350 km and a maximum altitude dierence between the highest and lowest GPS sites of about 2400 m. The geographical locations of the GPS sites almost sample quite equally the model domain, which roughly extends from 0 to 4 and from 40 to 43. We have used the GPS precise orbits and clocks as well as consistent earth- rotation parameters provided by the International GPS Service (IGS) to estimate the ZTD stochastic process at the GPS sites. Comparison between observed 3-h accumulated rain- fall on one hand and model forecast with and without assimilation on the other hand show a dramatic impact of the assimilation of ZTD on model precipitation forecast around the GPS receiver location. This is particularly ef- fective at the end of the assimilation time window for the coastal GPS site. These early results show that at least ve GPS receivers can have a signicant impact on regional precipitation forecast, provided the observations are assimilated with an algorithm that respects their temporal distribution. This tends for the deployment of more and denser GPS networks.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	1099 - 1108
Cite this article:	Cucurull, L., Rius, A., Vandenberghe, F., de Pondeca, M., "4D-VAR assimilation of GPS-derived ZTD: a Case Study," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 1099-1108.
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