Instrument : Video Disdrometer (VDIS)
A disdrometer is an instrument used to measure the drop size distribution and velocity of falling hydrometeors. The 2-dimensional video-disdrometer (vdis) is comprised of video cameras capable of observing individual hydrometeors from views perpendicular to each other. Two CCD line scan cameras are directed towards the measurement area. Objects passing thru the measurement area—which is determined by the cross-section of the two optical paths as seen from above—obstruct the light and are detected as shadows by the cameras. Each camera contains a small embedded computer which is responsible for handling the data capture process, the analysis of the data and its conversion and compression into a format suitable for further processing. Subsequently the data are transferred to the computer used for instrument control and final analysis. In order to identify individual precipitation particles by matching their views as seen by each of the cameras, it is necessary to synchronize the shutter and capture control of both cameras. To this end, both cameras use a synchronous line trigger signal. In order to reconstruct observables like falling velocity, oblateness, etc. from the data streams of the two cameras, the two optical paths are displaced vertically by a distance of typically about 6mm. Measuring this distance and adjusting the background illumination are the two major calibration and maintenance tasks necessary for successful operation of the device.
The following measurements are those considered scientifically relevant.
- Hydrometeor Geometry
- Hydrometeor Size Distribution
- Hydrometeor fall velocity
- Hydrometeor size
- Southern Great Plains (SGP)
- Central Facility, Lamont, OK (C1)
- Tropical Western Pacific (TWP)
- Central Facility, Darwin, Australia (C3)
- Central Facility, Manus I., PNG (C1)
- ARM Mobile Facility (GAN)
- Gan Airport, Gan Island, Maldives (M1)
Mary Jane Bartholomew