New insights into the nature and effects of the water vapour field on InSAR measurements over Etna
Rachel Holley(1), Geoff Wadge(1) and Zhu Min(2)
(1) University of Reading, ESSC, Harry Pitt Building, 3 Earley Gate, Reading, RG6 6AL, United Kingdom
(2) University of manchester, Simon Building, University of Manchester, Manchester, M13 9PL, United Kingdom
Etna’s high relief and coastal climate make atmospheric errors in interferograms a particular challenge. This study uses the UK Met Office’s Unified Model (UM) to produce 3D water vapour fields corresponding to ascending and descending Envisat ASAR passes over Etna, every thirty-five days, from October 2004 – October 2006. The model utilises a ‘nested mode’ to generate a high resolution numerical simulation from global model data. Since the modelling technique does not require a large set of previous data or ground-based control, it is well suited for development into an operational volcano monitoring tool. The final model produces a 300m horizontal resolution water vapour field, and the difference between the vapour fields along the line of sight for a pair of acquisition dates can be used to estimate and remove the radio wave delay in the interferogram.
In testing this method, it was possible to explore the structure of the atmospheric water vapour field, and the resulting radar wave delays, in ways not normally investigated. In particular, the model results include 3D fields of liquid and solid water which, although usually neglected in correction methods, may have a significant effect in some interferograms. We show examples of such effects. Whereas approaches which calculate radar signal delay from the precipitable water vapour measurements use an empirically-derived factor, Q (often assumed constant); the forward atmospheric model provides an atmospheric temperature field which can be used to calculate this directly. At Etna, comparisons show that using a constant value for Q can introduce errors of up to 15% in the resulting interferogram. The model may also be used to study the scales of spatial variability within the water vapour fields, and the relative contributions from the dynamic and hydrostatic components. Our work shows the importance of the land-sea breeze effect in creating dynamic spatial variability in the water vapour field in the lee of Etna during the mornings. We also look at corrected interferograms for data acquired during the study, in particular focusing on deformation signals during and after the 2004-05 eruption period.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,