Assessment of different raindrop size measuring techniques: Inter-comparison of Doppler radar, impact and optical disdrometer

• Raindrop size distribution is important for microphysical understanding of rain.
• Different instruments measure drop size distribution differently.
• A study on inter-comparison of different instruments is presented.
• Results indicate similar performance of instruments up to 30 mm/h only.
• Large and small drops are not sensed by all the instruments satisfactorily.

The performances of three instruments namely, Joss–Waldvogel disdrometer, laser precipitation monitor and micro rain radar, are assessed in terms of their ability to measure rain related parameters and to better understand the dependency of measured parameters on the working principles of the instruments. Twenty one rain events of year 2013 pertaining to South-West monsoon are considered for this study. The raindrop size distributions measured by the three instruments show good agreement only for medium sized (1–3 mm) raindrops and for medium rain rates (up to 30 mm/h). However, the mutual agreements are not very good in case of large (> 5 mm) and very small raindrops (< 0.5 mm) as well as for very high rain rates (> 30 mm/h) and very high radar reflectivity factor (> 40 dB). The radar reflectivity–rain rate relation is also studied using linear regression method which shows distinct differences for these instruments, indicating the high sensitivity of Z–R relation on the instrumental measuring principle.