Disdrometers

A disdrometer is an instrument used to measure the drop size distribution of precipitation and, in many designs, the fall velocity of drops. By recording at high temporal resolution the individual raindrop events, disdrometers provide detailed information on the microphysical properties of rain, which are essential for hydrology, radar rainfall estimation, and atmospheric research.

Disdrometers fall into two broad classes according to measurement principle: impact (or piezoelectric) disdrometers, and optical or imaging disdrometers. Impact disdrometers detect the momentum of raindrops when they strike a sensor and convert the resulting impulse into a signal from which drop diameter D can be inferred via calibration relations; they typically provide the drop size distribution N(D) and associated rain rate estimates. Optical disdrometers use light transmission, scattering, or shadowing of a laser or LEDs to detect each drop as it passes a sensing volume. The beam interruption or shadow length yields D, and the time between successive detections or the Doppler-like timing yields velocity v. Some optical devices also image drops to reconstruct two-dimensional trajectories.

A widely used optical disdrometer is the Parsivel, which estimates drop size and velocity from transmitted light across a moving slotted screen; another well-known design is the 2D video disdrometer (2DVD), which records drop silhouettes with two cameras to derive size and velocity from images.

From the measured DSD, researchers compute rainfall rate, liquid water content, and radar reflectivity factors. The DSD is often characterized by moments or fitted to gamma distributions. They require calibration and are sensitive to wind, evaporation, and orientation. Data are used in rainfall climatology, weather radar calibration, and urban hydrology.

Disdrometers complement tipping-bucket rain gauges by providing size- and velocity-resolved measurements, enabling better understanding of precipitation processes and microphysical parameterizations in models.