Raindrop Size Distributionvariability from high resolutiondisdrometer networks
2015
The characteristics of the raindrop size distribution (DSD) have been widely studied since Marshall
and Palmer (1948) introduced specific version of exponential distribution for the observed size
spectra, based on measurements of raindrops records on dyed filter papers. Across the decades,
interest in measuring and studying rain DSD has grown due to applications in cloud physics studies,
in calibration of space-borne and ground-based microwave active precipitation sensors and in soil
science and agriculture. The study of DSD and of the processes that determine it, are always been
challenging from both theoretical and experimental point of view. Moreover, the study of DSD in
natural rain is hindered by the difficulties (logistic and economic) in the management of dense
disdrometer networks.
Based on the unprecedented datasets available, this Thesis aims to contribute in characterizing, from
a microphysical point of view, the precipitation structure and the processes that generate it. In
particular, the vertical and horizontal DSD variability is analyzed, starting from the study of
collisional break-up mechanism in natural rain. The signature of collisional break-up, first evidenced
in a particular shape of Doppler power spectrum of a microwave disdrometer, is then searched and
characterized in DSD spectrum, assessing its variability with altitude. The horizontal variability of
DSD is studied both analyzing the occurrence of equilibrium DSD among the different datasets
available and evaluating the correlation of integral and non-integral DSD parameters at small scale.
In the first part of the Thesis, an overview on past and recent studies on different aspects of DSD is
given. The main mechanisms that govern the rain development are firstly summarized, then the DSD
parameterization and the DSD variability in natural rain are discussed. Finally, the description of the
characteristics of instruments and of the field campaigns considered in this work are presented.
The vertical variability of DSD has been studied thanks to the development of specific algorithms
able to detect and characterize both the collisional break-up and the equilibrium DSD. I analyzed the
signature of collisional break-up both on the Pludix Doppler power spectrum and on DSD spectrum.
The analysis is carried out developing two algorithms that detect the collisional break-up as well as
estimate the break-up diameter as function of altitude. The results show a decrease of break-up
diameter with altitude, due to the reduction of air density, that plays a critical role in the energetic
balance of the collision between two raindrops. The analysis also indicates that, regardless the
altitude, the collisional break-up occurs if the kinetic energy of the collision exceeds 12.2 μJ. The
results, together with the detailed analysis of some case study at high altitude (over the Tibetan
Plateau), show also that the dominance of the break-up process is required to reach the equilibr ium
DSD.
The study of the DSD variability was deepened focusing the analysis on the 2DVD DSD properties
to evaluate the occurrence of equilibrium DSD in natural rain. Another algorithm, based on 2DVD
characteristics, is set up to automatically detect the equilibrium DSD by using the great amount of
high quality disdrometric data available from the datasets of Ground Validation program of NASAGlobal
Precipitation Measurement mission. The results shows a good agreement between the
experimental equilibrium DSD and the equilibrium DSD obtained by theoretical models. The analysis
shows also that the equilibrium DSD is mainly reached during convective rain and its dependence on
season and latitude (no equilibrium DSD is observed at high latitude - 60°N). The occurrence of
equilibrium DSD is a rare event in natural rain (maximum 8% of selected minutes), while an increase
is observed if transition situations are considered.
The results are also analyzed to estimate the goodness of fitting the equilibrium DSD by a three
parameter gamma distribution, that is widely used to parameterize the DSD. The low correlation
between the experimental DSDs and the gamma distribution evidences that the gamma is not the best
parametric form to fit the experimental equilibrium DSD. The behavior of the rain and DSD
parameters is studied as function of break-up occurrence and shows that they can be considered an
additional indicators to screen out the situations that are not expected to reach the equilibrium DSD.
The data collected from two high-resolution disdrometric dataset are used to study the horizonta l
DSD spatial variability at small scale. The size of the measuring fields are different but comparable
with a ground radar pixel or satellite footprint and this makes the analysis of the particular interest .
The rainfall rate and other DSD parameters are analyzed using a three parameter exponential function
to estimate their correlation at small scale. The estimated correlation distance shows that the most of
the rain and DSD parameters are correlated within a radar pixel or satellite footprint (generally, the
integral DSD parameters – rainfall rate, radar reflectivity, liquid water content, etc. – are less
correlated than the non integral DSD parameters – maximum diameter, mean mass diameter, etc.).
The root mean square error evidences a very good fit of the function used with respect the
experimental data, indicating a good reliability of data.
The results presented in this Thesis, first, increase the knowledge of break-up phenomenon and its
effect on the DSD up to reach the equilibrium DSD, and can be used to improve the parameterizat ion
form for break-up and equilibrium DSD occurrence and the modeling of cloud and precipitat ion
mechanisms. Secondly, they give reliable indications about the spatial variability of the structure of
precipitation within a radar pixel and/or a satellite footprint, with an immediate application to the
interpretation of remote sensing measurements to improve precipitation retrieval from radar/satellite
measurements, especially after the launch of Dual-frequency Polarization Radar in the frame of
Global Precipitation Measurement mission.
The results obtained in this Thesis lead to the study of many other aspects that can be investigated to
better characterize the precipitation. The time evolution of the precipitation with particular emphasis
to the time necessary to the break-up to modify the DSD to reach equilibrium DSD can be investigated
by using the algorithms proposed here. A new parameterization of DSD affected by break-up and of
equilibrium DSD is necessary to improve the remote sensing of precipitation. Finally, a deeper study
of DSD spatial variability is needed to have more information about rain structures at small/medium
spatial scales, by different techniques and datasets in different season/location.
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