Investigations into the improvement of automated precipitation type observations at KNMI

The Royal Netherlands Meteorological Institute (KNMI) employs the Vaisala FD12P present weather sensor for automated observations of visibility, precipitation type and duration in the national meteorological observation network. The precipitation type output of this sensor is used fully automated in all synoptical and aeronautical reports, except for two international airports where an observer is still present.

Several weaknesses of the FD12P precipitation type have been recognized since its introduction in November 2002, particularly concerning detection of mixed precipitation around 0 ºC, hail detection, false alarms in dense fog and the detection of very light precipitation events. Therefore an investigation into the performance of other sensors for the observation of precipitation type was initiated.

A field test with the Thies LPM, Ott Parsivel, Lufft R2S and Vaisala WXT520 sensors started in De Bilt in September 2008. Data was analysed on 1-minute basis for special cases and hourly weather codes from all instruments were evaluated by meteorologists. Based on the results gathered during the two winters of the test, it was concluded that the Thies LPM optical disdrometer has added value and is partially able to solve the shortcomings of the FD12P.

The evaluation of the combination of FD12P and LPM will be continued at the two airports with observers. In this paper the operational experiences of the automated precipitation type observations by the FD12P present weather sensor will be presented. Furthermore, the results of the field trial in De Bilt and a brief analysis of the wind effect on LPM measurements will be reported.