During the fall 1999, the Mesoscale Alpine Programme (MAP) took place over the Alps to study, among the others, severe precipitation events. This work will focus on one event occurred all over Italy, during Oct 18-21, 1999 (IOP8). An attempt is made to assimilate GPS Precipitable Water (PW) into a mesoscale model by nudging technique. GPS PW data from a small network (six stations) are assimilated into the MM5 model; five domains two-way nested (up to 3km) are used over the area of the 3 GPS Italian stations. The early stage of this event was characterized by westerly flow over the central Mediterranean area, producing precipitation over central Italy were the Italian ground based GPS are located.

To evaluate the impact of the GPS PW on the forecast, a few experiments are performed using different assimilation techniques: Objective Analysis and Nudging.

The results are compared with the observations and with a simulation performed without any assimilation (used as reference) to correctly eveluate the impact of the GPS PW.