Zenith total delay study of a mesoscale convective system: GPS observations and fine-scale modeling

Abstract

Zenith Total Delay (ZTD) observations and model calculations are used to analyze a mesoscaleconvective system which yielded a large amount of precipitation over a short period of time inthe north-western Mediterranean. ZTD observations are derived from the GPS signal delaywhereas the ZTD model results are calculated by means of the MM5 mesoscale model.Large values of the root-mean-square (rms) differences between the ZTD derived from theobservations and the modeling are found for the maximum activity of the mesoscale convectivesystem. It appears that the average bias between observations and modeling results is slightlyaffected (20%) by the passage of the storm system which is associated to the water vaporvariability of the atmosphere.We have analyzed the ZTD differences in terms of the two components: the Zenith HydrostaticDelay (ZHD) and the Zenith Wet Delay (ZWD). The hydrostatic error is mainly caused bythe differences between the elevation of the GPS stations and the model topography and isreduced when using a more accurate topography data set. We propose a correction for thiserror assuming hydrostatic equilibrium. The remaining average ZTD difference is associated tothe ZWD and is mainly generated by inaccuracies of the mesoscale model to predict the watervapor content during the rainfall event.