¶ 2D Video Disdrometer (VDIS)
¶ Instrument Description
A disdrometer is a state-of-the-art instrument used to measure the drop size distribution and velocity of falling hydrometeors (ie, the size, shape, and speed of falling raindrops). These observations provide insight into precipitation processes and cloud lifecycles.
The 2-dimensional video-disdrometer (vdis) contains two cameras that view an area in the middle of the device. The light paths for each camera are shown by the purple and orange shaded areas in the figure below. What is NOT immediately apparent in the figure is the vertical displacement between the two cameras. This vertical displacement (usually by about 6mm) allows for the VDIS to reconstruct observables like falling velocity, oblateness, etc. from the datastreams of the two cameras. Measuring this distance and adjusting the background illumination are the two major calibrations and maintenance tasks necessary to successfully operate the VDIS.
For more information see the VDIS Instrument web page.
¶ Additional Information
The instrument software reports the size and shape of drops that it feels has successfully tracked from one camera to the next. Accurate determination of the drops' fall velocity is critically dependent on the calibration measurements of the spacing between the camera's field of view.