GPS zenith delay sensitivity evaluated from high-resolution numerical weather prediction simulations of the 8-9 September 2002 flash flood over southeastern France

Estimations of zenith total delays (ZTD) were obtained during postprocessing of a high‐resolution (2.4 km) nonhydrostatic atmospheric model (Méso‐NH). These estimations were used to determine their sensitivity with respect to formulations of atmospheric refractivity, the approximation of zenith hydrostatic delays (ZHD) deduced from ground pressure, and the contributions of hydrometeors. The factor κ for the conversion of zenith wet delay (ZWD) to integrated water vapor (IWV) was examined. Méso‐NH is applied here to the extreme flash flood event of 8–9 September 2002 in southeastern France. The use of the hydrostatic formulation (to infer ZHD) leads to an overestimation of up to 18 mm with respect to the vertical integration of refractivity. Delay contributions of hydrometeors simulated by the high‐resolution model reached more than 70 mm (≈11 kg/m2 IWV) in the heart of the convective cells in the case of the extreme flood event. The mean variations of IWV due to the use of different conversion factors (κ used to transform ZWD to IWV) are evaluated to be less than 0.3 kg/m2. This is less than the mean underestimation of IWV by 0.6 kg/m2 relative to the GPS‐like evaluation of IWV using the hydrostatic formulation and the ground temperature. In this study we also use GPS ZTD observations to validate three different numerical simulations of this extreme flood event. The simulation with the best fit to the GPS observations is also in best agreement with the surface rainfall measurements.