Vaisala FD12P - Manual (Page 108)

User's Guide _______________________________________________________________________
108 _____________________________________________________ M210296en-A
Present Weather
Precipitation Types
The ratio of the optical intensity and the DRD12 intensity estimates is
the key factor used in the precipitation type decision (see section
Precipitation Intensity on page 106). Some filtering is used in
calculating intensity ratio to get the parameter that is used in the type
finding procedure. The Weather delay parameter is also used to
eliminate unrealistically quick changes in the precipitation type. The
sensor surface temperature (TS) is used in selecting the default
precipitation type. Above Warm limit (+8 °C), the "default"
precipitation type is rain. Below -5 °C it is snow and between -5 °C
and Warm limit (+8 °C) it is unknown (P or 40).
In some areas, a more flexible snow reporting temperature limit is
required. The Warm limit, which is the upper limit for reporting snow,
is a user-definable parameter with the nominal value of +8
°C. The
parameter can be defined between +4 °C and +8 °C. Changing the
limit temperature affects the discrimination of the following
parameters:
- Snow and hail if the ratio of optical and DRD intensity is over the
Snow limit value.
- Rain&snow and Rain when the intensity ratio is between one and
Snow limit.
If this temperature limit is set higher (towards +8 °C), the FD12P
reports snow and rain&snow more often (but not above this limit
temperature).
If the temperature limit value is set lower (towards +4 °C), the default
precipitation rain is reported more often.
The precipitation code P (unknown type) is selected in cases where
the precipitation intensity (optical) is very low and the intensity ratio
appears to indicate liquid precipitation below or solid precipitation
above 0
°C.