Abstract/Summary

Turbulent mixing processes over terrain cause local horizontal variations in water vapour from a variable vertically stratified profile. Temporal variations in water vapour distribution cause delays in phase used in the space geodetic InSAR technique. To correct for this, dynamic atmospheric models are used to simulate water vapour distribution and hence variable refractive phase delay over a small volcanic island in the humid tropics, Montserrat. Initialised by ECMWF analysis data at 16 km resolution, the Weather Research and Forecasting (WRF) model is nested to 300 m resolution. Synthetic simulations of trade window with the WRF Montserrat Model (WMM) demonstrate its ability to replicate gravity waves. WMM simulates atmospheric delay fields during the InSAR imaging of Montserrat by X-band radar (COSMO SkyMed) from two viewing geometries during December 2014. Field measurements during imaging and the recording of zenith wet delay (ZWD) by a 14-receiver GPS network are used for comparison with the radar data and atmospheric models. WMM and ZWD delay difference images appear to show two main influences: (static) topographic modulation and dynamic modulation due to the trade winds owing over and around the mountains. Mitigation of the unwanted delay field in COSMO-SkyMed interferograms using the WMM fields gives standard deviations of the residual delay field in the range 19-38 mm. Statistical calculations of pixel-wise delay estimates place model accuracy in the range 64-81%. The reasons for this level of mitigation may be truncation of initial conditions to WMM, the large amount of liquid water in the atmosphere and simulation of trade window across Montserrat.