Disdrometer Joins Tipping Bucket to Improve Precipitation Measurements

At the SGP site, the disdrometer is installed near the site's main instrument cluster, approximately 50 feet east of the Central Facility. To avoid secondary splash contamination, the disdrometer's sensor cone is surrounded by splash-resistant material. ARM file photo. — At the SGP site, the disdrometer is installed near the site’s main instrument cluster, approximately 50 feet east of the Central Facility. To avoid secondary splash contamination, the disdrometer’s sensor cone is surrounded by splash-resistant material. ARM file photo.

This spring, a pair of new distrometers began collecting data at the ARM Southern Great Plains (SGP) site and the ARM Darwin site in the Tropical Western Pacific (TWP), adding details about rain drop size distribution to the extensive collection of ARM measurements. A disdrometer is an instrument that measures the velocity and size distribution of falling hydrometeors (such as rain or hail). Accurate measurements of these properties are important for studying the evolution of water droplets in and near the melting layer—the region in the atmosphere where frozen precipitation melts and turns into liquid. Disdrometers can also be used to measure rain rate, which is currently obtained using “tipping buckets.”

This tipping bucket, used to collect rain rate data at the TWP Darwin site, was joined in March by a disdrometer for obtaining measurements of drop size distribution. ARM file photo.

Tipping bucket rain gauges are collocated with the disdrometers to verify their proper operation. Each rain gauge includes two buckets, each measuring 0.01 inches of water. When one bucket fills, it pivots to the side, bringing the second bucket into position. Each pivot closes the unit’s magnetic switch and is counted by a data logger. This momentary switch closure, or pulse, can then be used to record rainfall rate and rain amount. While useful for rainfall rates, the tip bucket method cannot provide information on drop size distribution. With the disdrometer, drops that land on the sensor cone displace a very sensitive plunger. Momentum is transferred and the plunger displacement is directly related to the drop size.

In January 2005, the ARM Cloud Properties Working Group recommended the deployment of the disdrometers. The disdrometer at TWP began operating in January, followed shortly thereafter by the one at SGP in April. Measurements from the disdrometers will be used to test retrievals of size distribution obtained from the millimeter wavelength cloud radar, as well as to test an algorithm the radar uses to calculate rain rate. Rain drop distribution and rain rate data will be useful in the upcoming field campaign, the Cloud and Land Surface Interaction Campaign, or CLASIC, set to take place at SGP in 2007.