GPS monitoring of the tropospheric water vapor distribution and variation during the 9 September 2002 torrential precipitation episode in the Ce´vennes (southern France)

On 8–9 September 2002, torrential rainfall and flooding hit the Gard region in southern France causing extensive damages and casualties. This is an exceptional example of a so-called Cévenol episode with 24 hour cumulative rainfall up to about 600 mm at some places and more than 200 mm over a large area (5500 km2). In this work we have used GPS data to determine integrated water vapor (IWV) as well as horizontal wet gradients and residuals. Using the IWV, we have monitored the evolution of the convective system associated with the rainfall from the water vapor accumulation stage through the stagnation of the convective cell and finally to the breakup of the system. Our interpretation of the GPS meteorological parameters is supported by synoptic maps, numerical weather analyses, and rain images from meteorological radars. We have evidenced from GPS data that this heavy precipitation is associated with ongoing accumulation of water vapor, even through the raining period, but that rain stopped as soon as the weather circulation pattern changed. The evolution of this event is typical in the context of the Cévenol meteorology. Furthermore, we have shown that the horizontal wet gradients help describe the heterogeneity of the water vapor field and holds information concerning the passage of the convective system. Finally, we have noticed that the residuals, which in theory should be proportional to water vapor heterogeneity, were also highly perturbed by the precipitation itself. In our conclusions we discuss the interest of a regional GPS network for monitoring and for future studies on water vapor tomography