Algorithm Description; Visibility; Detecting Precipitation; Figure 6 Raincap Surface Signal (Heavy Rain Beginning) - Vaisala PWD22 User Manual

Chapter 3________________________________________________________ Functional Description

Algorithm Description

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Figure 6 RAINCAP

Visibility

Optical signal analysis calculates the difference of the measurement
signal and offset averages for the visibility algorithm. The difference
value (frequency) is given as a parameter to a calibrated transfer
function. The transfer function converts frequency into visibility
(MOR). The transfer function has been defined with an accurate
transmissometer (Vaisala MITRAS) as a reference.
The instantaneous (15 s) visibility values are averaged to get
a 1-minute and 10-minute average output values. The averages are
calculated from extinction coefficient values to emulate human
observations better. The extinction coefficient (σ) is defined as
follows:
σ (1/km) = 3000/MOR (m)

Detecting Precipitation

Detecting the precipitation onset is based on measuring the signal
peaks caused by precipitation droplets. The peak amplitudes that are
detected during 10 minutes are summed. When the sum exceeds a
threshold value, the sensor will indicate precipitation. The threshold
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Surface Signal (Heavy Rain Beginning)
9807-035