Effect of using local temperature measurements on the determination of freezing precipitation and fog

In aviation, meteorological observations are crucial to the operations of an airfield. These observations are more and more automated. In the Netherlands, there are only human observers at Schiphol airport; all the other airports use automatic observations.

One of the observables used in the aeronautical reports is the so-called present weather which includes the precipitation type. KNMI uses the Vaisala FD12P Present Weather Sensor for this purpose. Although this sensor generally works quite well there are a few issues that can be improved upon (Wauben, 2002 and Haij, de and Wauben, 2010). One of these is the precipitation type determination around 0 °C, in particular the detection of rain/snow mixtures. KNMI has performed a number of investigations into improving this issue (Bloemink and Lanzinger, 2005; Haij, de, 2007; Haij, de and Wauben, 2010).

Another issue in the determination of the precipitation type is the detection of freezing precipitation. Freezing precipitation is defined as super cooled precipitation (WMO, 2010). KNMI does not employ a dedicated sensor for the detection of icing (Wauben, 2007), instead freezing precipitation is reported when liquid precipitation occurs and the wet bulb temperature is below 0 °C. Note that this is and has been the practise for reporting freezing precipitation at manned locations of KNMI.

KNMI does not use the freezing precipitation reported by the Vaisala FD12P Present Weather Sensor, which is determined by a temperature sensor in the mast of this sensor, directly. Instead the wet-bulb temperature as derived from the ambient air temperature and the relative humidity sensor, both in a radiation screen, is used to determine whether the liquid precipitation is freezing or not.

At airports, present weather (along with visibility) is generally measured a various positions along the runway(s). Relative humidity and air temperature is generally only measured at the measurement field although recently backup sensors are installed at the other end of the runway of civil airports. So the wet-bulb temperature that is used to correct the FD12P in the determination of freezing precipitation may be located at a different site than the FD12P itself. The current investigation is set up to investigate if this difference between using a central or the local temperature is relevant to the determination of freezing precipitation.

In order to comply with the general interpretation of the WMO recommendations, KNMI has recently changed the temperature correction for the detection of freezing precipitation. The dry bulb (air) temperature is now used, rather than the wet bulb temperature. Therefore, also the difference between using the wet bulb and the dry bulb temperature will be investigated.

Freezing fog (FZFG) is defined as fog consisting predominantly of water droplets at ambient air (dry bulb) temperatures below 0 °C. This is independent on whether it is depositing rime ice or not (WMO, 2010). In this study the effect of using local temperature sensors on the determination of freezing fog will also be investigated.