Dr Jacopo Grazioli's PhD public defense on July 31, 2015
© 2015 EPFL
Professor A. Berne, thesis director, and the whole LTE team, congratulate Dr J. Grazioli for his thesis on "Polarimetric weather radar: from signal processing to microphysical retrievals.
Accurate modelling of liquid, solid and mixed-phase precipitation requires a thorough understanding
of phenomena occurring at various spatial and temporal scales. At the smallest
scales, precipitation microphysics defines all the processes occurring at the level where precipitation
is a discrete process. The knowledge of these microphysical processes originates
from the interpretation of snowfall and rainfall measurements collected with various sensors.
Direct sampling, performed with in-situ instruments, provides data of superior quality.
However, the development of remote sensing (and dual-polarization radar in particular) offers
a noteworthy alternative: large domains can in fact be sampled in real time and with
a single instrument. The drawback is obviously the fact that radars measure precipitation
indirectly. Only through appropriate interpretation radar data can be translated into physical
mechanisms of precipitation.
This thesis contributes to the effort to decode polarimetric radar measurements into microphysical
processes or microphysical quantities that characterize precipitation. The first part
of the work is devoted to radar data processing. In particular, it focuses on how to obtain
high resolution estimates of the specific differential phase shift, a very important polarimetric
variable with significant meteorological importance. Then, hydrometeor classification, i.e. the
first qualitative microphysical aspect that may come to mind, is tackled and two hydrometeor
classification methods are proposed. One is designed for polarimetric radars and one for an
in-situ instrument: the two-dimensional video disdrometer. These methods illustrate the
potential that supervised and unsupervised techniques can have for the interpretation of
meteorological measurements.
The combination of in-situ measurements and polarimetric data (including hydrometeor
classification) is exploited in the last part of the thesis, devoted to the microphysics of snowfall
and in particular of rimed precipitation. Riming is shown to be an important factor leading
to significant accumulation of snowfall in the alpine environment. Additionally, the vertical
structure of rimed precipitation is examined and interpreted.