User's Guide _______________________________________________________________________
148 __________________________________________________________________ M211107EN-C
Precipitation Intensity
The light scattering from a precipitation particle is proportional to the
volume of the particle. For rain this proportionality is quite stable
because the droplets are all quite spherical. In solid precipitation the
shape of particles varies but they are proportional to the average volume
of the particles.
The optical precipitation intensity value is calculated from the signal
change distribution data and then scaled by multiplying with Rain
intensity scale, which is an adjustable parameter.
The RAINCAP
®
data is used to calculate another estimate of
precipitation intensity. The intensity is scaled with the DRD scale
parameter. For normal operation in liquid precipitation the scaled
RAINCAP
®
intensity estimate should be higher than the optical intensity.
The water intensity value in the output messages is based on both the
optical and capacitive measurements. In liquid precipitation the optical
intensity is reported as such. When frozen precipitation is detected,
PWD32 multiplies the optical intensity with a scaling factor to get an
estimate of the water equivalent intensity. This scaling factor is
calculated from the RAINCAP
®
and optical intensities. New intensity
estimates are calculated every 15 seconds and averaged to get the 1-
minute intensity value, which is shown in the output messages.
Precipitation Accumulation
PWD32 calculates the accumulation of water (including the water
content of snow) and snow. The water sum is automatically reset when it
reaches 99.99 mm and the snow sum at 999 mm. There is also a
command for forced resetting by the operator or the system host
computer (
E
C
PW id C
C
R
).
Two methods are used in calculating the water sum. In rain, the optical
intensity values are directly scaled to sum increments and added to the
accumulated sum. In other types of precipitation an internal scaling factor
is used to get the water increment. The scaling factor is calculated from
optical and RAINCAP
®
intensities.
The snow sum is accumulated from the optical intensity when snow is
detected. The snow sum is only a coarse estimate of the accumulation.
For new snow the accumulated snow value is a reasonable measure of the
new layer of snow above the previously fallen snow.
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