Davide Ceresetti - Equipe ASP  

PhD THESIS

Organization of heavy precipitations in a mountain mediterranean region at different time and space scales.

The thesis work began in 2007, October, with the aim to explore in depth the self-organization skills of rainfall. The main application of the work concerns the comparison between the rain forecasts, given at a very low resolution, and the observations, made by means of the rain gage network and radars.

The activity is inserted into the framework of OHCMV observatory (Hydro-meteorologic Mediterranean Observatory Cevennes-Vivarais), and MEDUP, a French project on the uncertainties of precipitations in Mediterranean Regions. A strict collaboration with INRIA, Institute of Mathematics, is held.

Cevennes - Vivarais Region

As a first step a research with the aim to know the state-of-the-art on this field has been performed, analyzing the scientific production all around the world concerning three main topics:

- Theory of self-similarity of rainfall

- Methods of analysis of scale-invariant properties of rainfall

- Statistical methods for hydrology

Contemporarily, an attempt to recognize simple scale invariant properties of rainfall time series has been performed, with the help of the huge database available at OHMCV, concerning the region delimited by Mediterranean Sea, Alps and Massif Central. The database cover about 25 years of hourly data of about 150 gage stations, located in a window of 150x200 km.
In addition, interesting properties concerning the variability of rainfall at the rain gage have been found.

Power-law behaviour of rainfall series

The first results show that point rainfall behaviour is not homogeneous in the region: in the plain zone of Nimes rainfall has very high variability, while in the other zones an extreme event is difficultly seen. In particular, in the mountainous area, the rainfall intensity does not change with accumulation time.

The first conclusion we can draw, simply analyzing point rainfall data, is that we need to reject the ergodic hypothesis, since the regional behaviour is well-pronounced. Statistics are to be drawn on the single gage or, in the other case, on very close gages.

The second step of analysis concerns the space properties of heavy rainfall fields and the space-time relationship.  At this purpose, 2D rainfall fields measured by radar will be analyzed, starting from the raw data, i.e. the reflectivity. The interpolation methods, in fact, could seriously affect the original structure of the reflectivity field, and it could mask a self-organization of the storm.

Fourier Transform of 2D rainfall field

The applications of this work are mainly two:

- a better estimation of the Intensity - Duration - Frequency- Surface curves at any temporal and spatial scales, in order to be able to assess the occurrence time of any event based on robust statistics, eventually physically based.

- a calibrated scale-invariance framework for the region, by means of that it will be possible to compare the rainfall forecast with the observations, in order to qualify the forecast, mixing in a homogeneous and coherent way the rain gage network data and the radar data.

CONTENTS DOWNLOAD

Slide of PhD Thesis' Day, 2008, April 27th, Grenoble

Poster presented at 10th Plinius Conference, Cyprus, on 2008, September. Winner of conference travel award.

SCHOOL BACKGROUND

OTHER WORKS

QUALIFICATIONS