Campbell Scientific PWS100 Present Weather Sensor - Manual (Page 93)

Section 8. Functional Description
Signal to Pedestal Ratio Analysis
0
100
200
300
400
500
600
1 1.5 2 2.5 3 3.5 4 4.5 5
Signal to Pedestal Ratio
P
ar
tic
le C
ount
(S
now
)
0
10
20
30
40
50
60
70
80
90
100
P
ar
tic
le C
ount
(Li
qui
d)
Snowflakes
Snow Grains
Drizzle
Rain
FIGURE 8-6. Signal to pedestal ratio values for different
precipitation types.
The above data was collected during the sensor development. Results from
production instruments will differ. In particular the optics do not allow
accurate measurement of the pedestal ratio for true drizzle (particles <0.5 mm
diameter).
Auxiliary data including values of temperature, relative humidity and derived
wet bulb temperature are used in the analysis of particles if a CS215-PWS is
connected to the PWS100. There is a complex relationship between these
parameters and the types of particles that are formed; however since the
PWS100 only measures point values, and the evolution of particle formation is
not known, the true nature of the particle type can only be estimated from such
parameters.
Processing of these data values is done by a series of fuzzy logic tables, some
of which provide certain pure logical differentiators such as the size bounds for
drizzle and rain, snow grains and snowflakes and ice pellets and hail. Through
the use of the parameters determined from the PWS100 and CS215-PWS the
fuzzy logic tables provide a means of determining particle type. The general
parameters thus analyzed in this fuzzy logic processing are size versus velocity,
signal peak to signal pedestal ratio, temperature, relative humidity and wet bulb
temperature.
Each particle type from drizzle, freezing drizzle, rain, freezing rain, snow
grains, snowflakes, ice pellets, hail and graupel have a set of fuzzy logic tables
covering the above parameters. The size versus velocity tables are based on
values derived from previous studies either by Campbell Scientific Ltd. or
other research institutes and organizations (see Ellis R A et al, 2006, New
Laser Technology to Determine Present Weather Parameters, Meas. Sci.
Technol., 17 1715-1722). The fuzzy part of these tables gives a reduced fuzzy
logic score off of the defined size versus velocity curve. These values then
overlap on the various tables such that a particle could be assigned a
8-7