The pluviospectrometer: classification of falling hydrometeors via digital image processing

Characteristic properties of precipitation as intensity, raindrop size distribution, velocity of raindrops and morphology of the hydrometeors (especially snow flakes) are important parameters for the atmospheric scavenging processes. In order to measure these parameters without any disturbance, optical methods are suitable. The Pluviospectrometer is a new optical device that has been developed to classify raindrops and other hydrometeors by means of digital image processing at a low cost level. Using a commercial video system and a personal computer equipped with a frame grabber card, it is possible to measure raindrops size distributions automatically. For rain drops the considered size range reaches from 0.1 mm to 6.0 mm. There is the possibility to modify this measuring range by using different camera optics for example to analyse snowflake structures in the sub-millimetre range. To get high quality pictures of falling hydrometeors a special illumination unit has been developed on the principle of projection illumination. The image acquisition system is composed to meet the requirements given by the characteristics of the falling raindrops. The analysing image processing software measures the sizes of the objects and separates them into different size classes. At present only a few raindrop size distributions have been analysed. They show a behaviour similar to the corresponding Marshall-Palmer distributions, but droplets with size below 0.3 mm seem to be underestimated significantly by the approximation of Marshall and Palmer. A comparison of the rainfall rate calculated from the drop size distributions to values measured by an Ombrometer hasn't been satisfying in all cases considered. In principle there is the possibility for more sophisticated hydrometeor analysis with the Pluviospectrometer. Future developments may allow analysis of the velocities of fall or snowflake classification.