The Optical Spectro Pluviometer (OSP), an infrared optical device, is used to measure the rain drop size, fall velocity and intensity. The OSP, which works on the principle of measuring an optical shadow, enables independent measurement of drop size and drop velocity in real time and thus any parameter linked to rain erosivity. Simulated rain characteristics measured with the OSP are reported. Calibrations made in the laboratory using single raindrops allow an estimation of the accuracy of drop size and velocity measurements. The single drop calibration shows that drop diameters are measured with an accuracy that is at least 6% in the range 0.3-4.7 mm. Larger drops are detected but without quantification of their diameter. The accuracy in the drop fall velocity measurements varies from 1% for the lowest velocity (0.2 m s-1) to 20% for the highest velocity (10 m s-1). Simulated and natural raindrop-size distributions measured by the OSP are compared with distribution obtained by the filter paper method. Taking into account the uncertainties associated with the two methods, the raindrop-size distributions derived from the OSP and the filter paper method are in good agreement. To complete the calibration, rain intensity is derived from the raindrop-size distribution measurements and compared with values measured by a nearby rain gauge. For rain intensities less than 35 mm h-1 the OSP underestimates intensity by 12%. However, for rain intensities in excess of 35 mm h-1, the OSP underestimates intensity by 38%. The errors on the two widely used rain erosivity indices, the kinetic energy and the momentum, are estimated to be less than 4%.

• measurement and instrumentation : optical spectro pluviometer for rain drop size distribution
• measurement: Infrared
• rain : rain drop size distribution
• wind driven rain

1. The optical spectropluviometer revisited