Variations in raindrop size distributions measured at Rome

Summary The knowledge of raindrop size distributions is of great interest to cloud physicists, radar meteorologists and communication engineers. Beginning from the Winter 1986–87, the Precipitation Group of the Atmospheric Physics Institute installed on the roof of the Institute building in Roma-Eur a disdrometer RD-69. The sensor is an electromechanical device with surface of 50 cm² able to measure the raindrop diameter with a precision of 5% and a resolution (delay between two drops hitting the sensor) of one millisecond. After the calibration, significant precipitations were recorded continuously, with a sample period of 1 minute, during the whole life of the storms. It can be expected that a knowledge of the drop size distributions, and in particular of the differences between distrubitions, can be used to infer details of the storm and the environment. In this work the first results of the analyses are presented. Particular attention was conferred to the variation characteristics of the precipitation type (thunderstorm and widespread) as well as to the variations of the distributions in short time intervals, that may be caused by different effects, for example the growth and decay of a precipitation cell or the movement of a cell combined with the different fall velocities of small and large drops. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome.