LDIS

A laser disdrometer (LDIS) measures the drop size spectra and fall velocity of hydrometeors during precipitation events. The passage of hydrometeors through a horizontal area positioned between an eye-safe laser and an array detector results in blockage of the laser beam (a shadow). The size of this blockage is proportional to the size of the falling drop or particle. For liquid drops (rain), a well-known relationship between size and fall speed can be used to convert the size to mass, and therefore the water equivalent. For frozen particles (e.g., hail, snow, graupel), the relationship between size and fall speed is more complex, so the conversion to mass is less reliable. The instrument assumes the particles and drops are moving through the beam on vertical trajectories, but even with wind screening (e.g., double Alter shields), this assumption may be violated (diagonal trajectories), and some error in precipitation rate and total rain/snowfall could accrue.

In addition to drop/particle size and fall speed, precipitation type is classified.

ARM also deploys video disdrometers (VDIS).

• Hydrometeor size
• Hydrometeor fall velocity
• Hydrometeor Size Distribution
• Liquid water content
• Precipitation
• Snow depth