Mapping the Atmosphere Precipitable Water Vapor: The Experience with Envisat-asar and Terrasar-X
Mateus, Pedro¹; Nico, Giovanni²; Catalao, Joao^1
1Instituto Dom Luiz, Universidade de Lisboa, PORTUGAL; ²CNR-IAC, ITALY

In this work we present the results of a few experiments carried out to map the temporal evolution of the Precipitable Water Vapor (PWV) using ENVISAT-ASAR and TERRASAR-X SAR data.
The atmospheric propagation of microwaves introduces a phase delay in Synthetic Aperture Radar Interferometry (InSAR) images depending on the temporal variation of the wet component of the PWV. A comparison with phase delays computed using Numerical Weather Models (NWMs) and measured by GPS shows that InSAR interferograms is related to the temporal variation of the wet component of atmospheric phase delay [1], [2], [3]. Furthermore, SAR interferometry gives information on PWV with a more dense and regular spatial distribution than GPS measurements and a higher spatial resolution than NWMs. This led to the idea of using temporal series of SAR interferograms to track the temporal evolution of PWV spatial distribution [4]. Besides one or more temporal series of SAR interferograms, this methodology requires a means to calibrate InSAR phase measurements which could be for instance an independent set of atmospheric phase delay measurements furnished by a network of permanent GPS stations.
The updating frequency of PWV maps depends on the temporal baseline of strip-map acquisitions. However, the combination of different sensors and ascending/descending acquisitions allows to update PWV maps at a higher temporal frequency. Each interferogram was corrected for topographic contribution using a DEM at a scale 1:25.000 with a 20m cell size and a nominal vertical resolution of 2.5m. After correction for possible orbital and topographic errors, interferograms are calibrated using a network of 12 permanent GPS station installed in the area of Lisbon between 1997 and the beginning of 2009. The recording interval of GPS station was 30 seconds and the cut-off elevation angle was set to 20 degrees. The total zenith delay was estimated by means of the GAMIT/GLOBK software. The GPS network gave estimates of PWV every 15 minutes referred to the portion of atmosphere observed within a cone with an aperture depending on GPS cut-off angle. The availability of InSAR maps of atmosphere PWV on a routine basis can help to capture variations of the local concentrations of water vapor, not possible with current measurement techniques which give information on water vapor distribution only at a coarse scales. This property could be important for numerical forecasting procedures (e.g. those based on the 3DVAR and 4DVAR analysis). In fact, these procedures could benefit from the availability of high resolution maps of PWV to provide more accurate precipitation forecasts and open interesting perspectives for now-casting applications. The knowledge, with high spatial resolution, of the temporal evolution of PWV spatial distribution could eventually serve as an additional constraint in variational data assimilation models.

11. References

[1] P. Mateus, G. Nico, R. Tomé, J. Catalão, P. Miranda, ''Comparison of precipitable water vapor (PWV) maps derived by GPS, SAR interferometry and numerical forecasting models'', Proceedings of SPIE, 7827, 782814-1-782714-7, 2010 (doi: 10.1117/12.864733).

[2] G. Nico, R. Tomé, J. Catalão, P. Miranda, ''On the use of the WRF model for the mitigation of atmospheric effects in InSAR interferograms'', IEEE Transactions on Geoscience and Remote Sensing, 49(12), 4970-4976, 2011 (doi: 10.1109/TGRS.2011.2157511).

[4] P. Mateus, G. Nico, R. Tome, J. Catalao, P.M.A. Miranda, ''Experimental study on the atmospheric delay based on GPS, SAR interferometry, and Numerical Weather Model data'', IEEE Transactions on Geoscience and Remote Sensing, 51(1), 6-11 (doi:10.1109/TGRS.2012.2200901)

[3] P. Mateus, G. Nico, J. Catalão, ''Can spaceborne SAR interferometry be used to study the temporal evolution of PWV'', Atmospheric Research, 119(SI), 70-80, 2013 (doi: 10.1016/j.atmosres.2011.10.002).