ASOS does not presently report drizzle unless the observation is augmented by a human observer. When drizzle is occurring, ASOS reports either rain, undetermined precipitation (UP), or no precipitation. However, there is a need for observations of drizzle, particularly when temperatures are near or below freezing. The National Weather Service (NWS) currently has plans to upgrade ASOS at some time in the future with an "enhanced" precipitation identifier that has the capability of detecting drizzle.

An analysis of the raw data that is produced by the current ASOS precipitation identifier (LEDWI) indicates that this sensor is already capable of detecting drizzle, provided that information in the sensor's "Particle" channel is used. An algorithm has been developed that takes into consideration the Particle channel information, in combination with information from the sensor's "Low" channel, and data from the temperature, dewpoint, and ceilometer sensors. The algorithm is currently being tested at the NWS Research and Development site near Sterling, VA. The results of these tests will be presented and discussed.

If the algorithm proves to be acceptable to the NWS it could accelerate the process of getting drizzle and freezing drizzle reports from unmanned ASOS stations, and it has the potential of saving the NWS a considerable amount of development time and money. It also has the potential of improving the detection of icing conditions aloft by increasing the number of drizzle and freezing drizzle reports that go into the Integrated Icing Detection Algorithm (IIDA).