A comparison of two optical precipitation sensors with different operating principles: The PWS100 and the OAP-2DP

Precipitation measurements can be performed with a variety of techniques and instruments. Optical devices allow for rain measurements in terms of the number and size of individual drops and have been helpful to improve the knowledge of the processes involved in the development and variability of precipitation. A comparative study was conducted during natural rain events in Mexico City using two classes of optical instruments: a light beam occlusion type (OAP-2DP) and a light scatter sensor (PWS100). The sensors were collocated a few metres from each other, along with a tipping bucket rain gauge to simultaneously record rainfall intensity. The results show that both optical instruments measured larger amounts of accumulated rain compared to the rain gauge. Bias and percent bias outcomes suggest that the PWS100 records the largest rain amounts among the devices used, but statistical analyses and the study of instrumental uncertainties did not provide enough evidence to prove significant differences in rain estimation. Although the PWS100 detects larger drops than the OAP-2DP, no significant differences were found between the liquid water content estimates obtained with them. Drop size spectra and velocity distributions also showed good agreement. In particular, both instruments showed similar characteristics for drop fall speed distributions that may be attributed to effects in the hydrometeor fall behaviour produced by horizontal wind. It is concluded that the performance of these two optical instruments is very similar, and that the PWS100 provides reliable data for rain measurements.