Determining radio wave delay by non-hydrostatic atmospheric modelling of water vapour over mountains

Abstract

Meteorological modelling of tropospheric water vapour over and around Mount Etna, Sicily is used to determine the atmospheric delay on radio waves used in synthetic aperture radar interferometry (InSAR). The model, NH3D, is used to simulate a non-hydrostatic three-dimensional representation of the troposphere over and around Mount Etna. It has radiative lateral boundary conditions, a bottom boundary derived from a 30″ DEM of Sicily and an `absorption' layer above 10 km to act as a non-reflective upper boundary. The simulations have a horizontal spatial resolution of 1.7 km over a domain of 180 km and 30 vertical levels. Water vapour is treated as a passive tracer and the model diagnostics include three-dimensional fields of specific humidity, wind speed and the column integrated water vapour (IWV). The IWV results from the NH3D models are converted to equivalent delays in radio wave propagation at the time of radar imaging by the ERS-2 satellite. Initialisation of the model uses data from radiosonde launches or numerical weather prediction (NWP) model output. Results are sensitive to the source and timing of these initialisations, particularly when frontal systems are involved. Six water vapour delay difference fields are simulated and compared to the differential interferogram for the 6th September–11th October 2000. The field initialised by NWP data closest in space and time to the radar acquisitions best represents the interferogram. The NWP initialisations will generally give more opportunity to get the initial conditions close to the optimum.