Droplet Characterization and Generation

Studies of cloud microphysics require in situ measurements of droplet size distribution and droplet concentration. Deployment platforms are shifting from aircraft towards unmanned aerial systems including tethered balloons and small UAV’s. There is a need in new inexpensive, fast and compact instruments for in situ cloud droplet/drizzle measurement, suitable for deployment on small unmanned aerial platforms.

Atmospheric measurement instrumentation needs to be calibrated in the field. Mesa Photonics’ Portable Monodisperse Droplet Generator (PMDG) is a compact battery-powered instrument that generates a jet of fixed-size monodisperse droplets. The PMDG is ideal for field calibration of in situ cloud probes and precipitation measurement instruments.

Cloud Droplet Measurement System (CDMS)

The CDMS implements a compact optical imaging technology for in situ measurement of cloud droplet size distribution function and droplet number concentration. The method is based on analysis of scattered light images acquired at 90-degree scattering angle from multiple individual droplets illuminated by a sheet-shaped, pulsed, spectrally broadband (“white”) light source. White light illumination mitigates the ambiguity of the inverse Mie scattering problem of obtaining the particle size from the scattered light intensity. The droplet/drizzle diameter measurement range is 10-1000 micrometers; the droplet number density is up to several hundred per cubic centimeter. The CDMS has dimensions of 12 in. x 12 in. x 4 in.; it weighs less than 7 pounds (including battery) and draws less than 30 W of electrical power. The instrument is equipped with a wireless telecommunication system that broadcasts the data to the ground station. The data can also be stored on a local hard drive. The main application is in situ characterization of droplets in atmospheric clouds. The preferred deployment platform is tethered balloon system (TBS).

Technical Specifications (preliminary)

• Droplet diameter measurement range: 10–1000 µm
• Droplet number density measurement range: up to several hundred per cm³ (preliminary)
• Effective measurement rate: 4 Hz
• Sample volume: 0.32 cm³
• Effective sampling rate: 1.28 cm³/s
• Remote station (payload) dimensions, weight and power draw: 12″ × 12″ × 4″, < 7 lbs, and < 30 W, respectively
• Remote station to ground station telecommunication: 5 GHz duplex wireless telecommunication system
• Field calibration: one-point calibration using Mesa Photonics’ portable monodisperse droplet generator

Portable Monodisperse Droplet Generator (PMDG)

Portable Monodisperse Droplet Generator (PMDG) is a compact battery-powered instrument that generates a jet of fixed-sized monodisperse droplets. The principle of operation is based on disintegration of a cylindrical liquid jet caused by periodic disturbance applied to the jet, which makes it dynamically unstable under the action of surface tension — this effect is commonly referred to as Savart–Plateau–Rayleigh instability of a liquid column. The PMDG is ideal for field calibration of atmospheric measurement instrumentation such as in situ cloud probes and precipitation measurement instruments. The PMDG has recently been used to field calibrate the Mesa Photonics’ tethered-balloon-based Cloud Droplet Measurement System (CDMS). The current version of PMDG produces a jet of monodisperse droplets 193 µm in diameter; the instrument weighs 25 lbs and has dimensions of 21″ x 13″ x 7.5″. The power draw is 12 W; the battery operation time is about 5 h.

Technical Specifications (preliminary)

• Droplet diameter (fixed): 193 µm
• Dimensions, weight and power draw: 21″ x 13″ x 5″, 25 lbs, and 12 W, respectively
• Powering options: (1) embedded rechargeable battery (about 5 h battery operation time and (2) AC/DC power supply