The NSSL 2D-video-disdrometer (2DVD, developed by Joanneum Research of Graz, Austria), was purchased in the summer of 1999. It consists of three interdependent units: 1) a sensor unit (SU), which samples the data, 2) an outdoor electronics unit (OEU), which monitors the operation of the SU and stores the initial data packets, and 3) an indoor user terminal (IUT), which receives data from the OEU, processes the image data to determine drop size, shape, and fallspeed, and displays the information in real time. A picture of the SU with two of its side panels removed is shown in Fig. 1. The instrument is approximately 1 m high and has a 25 cm ( 25 cm orifice at the top (though the actual measurement area is a somewhat smaller nested area, discussed below). Just below the orifice, an inclined surface slopes outward towards the side of the SU which, at its maximum width, has a rectangular dimension of 50 cm ( 50 cm. A velcro fabric that is designed to prevent splashing drops from entering the measurement area covers the inclined surface. The SU is fully self-contained, water tight, and has sides that are easily removed in order that lamps may be replaced or calibration work performed. A small hole at the base of the opening is connected to a pipe that funnels rain water through the SU and out onto the ground.

The disdrometer is designed to obtain two (hereafter referred to as A and B), orthogonal side images of each raindrop. Two lamps that are nested within the sensor unit provide light sources that are first redirected by mirrors and then passed through slit plates that are located on the sides of the orifice. This creates A (upper) and B (lower) planes of light that transect the approximate 10 cm ( 10 cm measurement area with a vertical separation of ~6.2 mm. Additional slit plates and mirrors located on the opposite side of the orifice then redirect the planes downward onto two line scan cameras that are located near the base of the instrument. The line scan cameras sample each plane at a rate of ~29 microseconds and a horizontal resolution of ~200 microns. Therefore, as a raindrop falls through the measurement area, several line scans of each image are recorded from two sides and two different heights. The line scan data from each camera are then instantaneously transmitted by cable to the OEU where they are packaged into 3 second files and transmitted by a radio frequency link (in our setup) to the IUT, which is located in an office approximately 100 m from the disdrometer site. The IUT software matches the A and B images, determines drop shape, size, oblateness, and fallspeed for each drop, and displays all of the above (along with a rain rate and DSD) in real time.