PLUDIX
rain gage and disdrometer
has been developed, and is manufactured, by
PLUDIX
Pludix is a multifunctional instrument for monitoring and characterising atmospheric
precipitation on the ground with high sensitivity and fast response. It can be operated automatically and
has low maintenance requirements.
Instrument functions
• Monitors precipitation.
• Identifies precipitation type (rain, snow, hail, drizzle..).
• Provides hydrometeor size distribution (for drops, snowflakes,
hailstones);
this is called disdrometric function.
• Measures instantaneous rainfall rate.
• Gives the total rainfall in a given time interval.
The knowledge of hydrometeor size distribution (disdrometric function) is crucial in
investigating, or forecasting, such events and occurrences as soil erosion and percolation, landslides,
evapotranspiration, plant physiology, spring seepage etc., or in several other applications, such as in
meteorological radar calibration, in defining reflectivity vs rainfall rate relationships, etc..
Characteristics of the instrument PLUDIX
• it can operate automatically and can be connected in networks for
operation in remote areas;
• it is highly reliable.
Detection and characterization of precipitation
The detection and characterization of a precipitation and its characterization in PLUDIX is based
on the fact that each precipitation type (rain, snow, hail) has its own Doppler spectrum. So for each
precipitation type a different algorithm is selected to determine size distribution and intensity.
Disdrometric function
The instrument measures the size of the hydrometeors falling in a well defined volume (about 3
m high and 1 m wide) above it.
The hydrometeors are classified in variable size intervals (e.g. 0.3 mm for intermediate size
drops). In each size the average concentrations (m-3) and its contribution to the total precipitation
intensity (in mm h-1) is given.
Rain gage function
By integration of the disdrometric function the instrument is providing the instantaneous rainfall
rate and the total amount in a given time interval.
The instrument
The instrument is made of a sensor located in a waterproof Fiberglass container to be placed
outside, connected by a cable to the power supply/signal processing unit.
The sensor
The sensor is shown on the front cover page with a portable PC. The upper part of the container
(dome) is shaped in order to minimize dry deposition and snowcover, and can be heated.
The length of the cable connecting the sensor with the power supply/signal processing unit can be
extended to several tens of meters.
The signal processing unit
The signal processing unit communicates to the outside through a serial port RS232C, which can
be used to connect the instrument to the data transmission system (in the field) or to a PC for archiving
and visualization (in the lab).
Sensor description
The sensor is an X band continuous wave, low power (10 mW) Doppler Radar (9.5 GHz
frequency of operation). The radar electronics are hosted in a custom made microwave module tuned to
the proper sensitivity to detect the weakest precipitation.
Functioning principle
The MW beam emitted by an upward oriented antenna is backscattered by hydrometeors in free
fall. Near the ground each hydrometeor has reached an aerodynamic equilibrium and falls at a constant
terminal velocity only as a function of its size. The backscattered signal at the receiving antenna is
comprised of many components that are shifted in frequency from the transmitted signal depending on the
hydrometeors' terminal velocities (Doppler effect). The amplitude of these components is a function of the
reflectivity of the hydrometeors and their concentrations in the volume seen by the sensor.
Since a given drop at terminal velocity is not generating a constant but rather a variable doppler
echo depending on its position in space, a special algorithm has been developed which obtains the
hydrometeor size distribution from the signal spectrum and hence the parameters characterizing the
precipitation.
Software updating
There is the possibility of updating the software to cope with the evolution of the signal
processing algorithms.
Advantages
With respect to optical disdrometers:
• simplicity and lack of maintenance;
• lower cost.
With respect to electromechanical disdrometers:
• no difficulty in distinguishing simultaneously or almost simultaneously
falling drops;
With respect to rain gages
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low precipitation intensity detected;
no clogging of the funnel by leaves, feathers and other objects;
no maintenance (waterproof bell shaped container);
specific response for snow and hail;
disdrometric ability.
PLUDIX has been patented n. MI 99A 001 906.
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Figure 1
Rainfall event instrument display:
1. total received power vs time (1 step, 1 min);
2. power spectrum in the running time interval (1 min), 100 Hz/div in abscissa;
3. power spectrum in the size ranges;
4. Logarithmic presentation of drop concentration vs size. Note the agreement with the
yellow line which shows the Marshall-Palmer distribution for the given precipitation
intensity;
5. Precipitation intensity vs time (1 div, 1 min) superimposed with intensity measured
by the tipping bucket rain gage.
Figure 2
Fig2. Full duration rainfall event presentation (24 hours). In the lowest diagram the
agreement of PLUDIX rainfall intensity (in red) with conventional tipping bucket (in
black, in green the tipping events) is evident.