Vaisala FD12P - Manual (Page 107)

Chapter 5 _______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 107
The DRD12 data is used to calculate another estimate of the
precipitation intensity. This intensity is calculated from low pass and
high pass filtered, one-second samples. The intensity is scaled with the
DRD scale parameter. For normal operation in liquid precipitation,
the scaled DRD12 intensity estimate should be higher than the optical
intensity.
In snow, the DRD12 intensity is proportional to the water content of
the snow. Due to undercatch, however, the values are often lower than
the real water content. Also, different types of snow particles have
characteristic behaviors on the warm sensing surfaces.
The precipitation intensity 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, the FD12P multiplies the optical intensity with a scaling
factor to get an estimate of the water equivalent intensity. This scaling
factor is calculated from the DRD12 and optical intensities. New
intensity estimates are calculated every 15 s and averaged to get the
one-minute intensity value, which is shown in the output messages.
Precipitation Accumulation
The FD12P calculates the accumulation of water (including the water
content of snow) and snow. The water sum is automatically reset
when it reaches 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.
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 DRD12 intensities.
The snow sum is accumulated from the optical intensity when snow is
detected. The snow sum is only a coarse estimate of the thickness of
snow. In a shorter period, the accumulated snow value is a reasonable
measure of the new snow on the old snow.