A Technique for Improving the Detection of Drizzle and Freezing Drizzle on ASOS Automated Weather Observing Stations

Drizzle, and in particular freezing drizzle, is a form of precipitation that is not well observed on ASOS (Automated Surface Observing System) stations. It is typically reported as rain, snow, or "unknown" precipitation (UP), unless the observation is supplemented by a human observer. Under certain conditions (such as when combined with warm cloud-top temperatures aloft) drizzle may be an important indicator of in-flight icing conditions near the station. Improving the detection of drizzle at automated weather stations, therefore, may result in better forecasts of in-flight icing conditions near these stations.

The reason generally given for the inability of ASOS stations to report drizzle is that the water droplets are simply too small to be observed by the ASOS precipitation identifier: LEDWI (Light-Emitting Diode Weather Identifier). Drizzle is defined as water droplets smaller than 0.5 mm in diameter, while the LEDWI sensor was designed to observe somewhat larger precipitation particles (> 1 mm diameters). An examination of the raw output data from LEDWI sensors at ASOS stations shows, however, that there is a distinct signal that appears in these data when drizzle is occurring that can be used to detect the presence of the drizzle. The purpose of this paper is to describe the output data that are produced by the LEDWI sensor, and to show a graphical technique for displaying these data that identifies drizzle as distinct from rain or snow. By combining these results with other data measured at ASOS stations (ceiling, visibility, temperature, etc.), we will show that it is possible to improve the detection of drizzle at unmanned ASOS stations, and at the same time, reduce the number "unknown" and misidentified precipitation reports.