Assessment of Rainfall Instrumentations and Wind Effects on Raindrop Collision

This study evaluates rainfall instrumentations in terms of rainfall intensity and raindrop fall speed measurement, and investigates wind induced effects on raindrop collision. OTT Pluvio2 weighing precipitation gauge and OTT Parsivel2 disdrometer are assessed for real-time rainfall intensity (RI) and raindrops fall speed measurements, respectively. Moreover, wind induced effects on microphysical processes of raindrop collision are explored to reveal the drop size distribution (DSD) evolution from cloud to ground level. Laboratory experiments considering both static and dynamic RI revealed that the lower threshold for the OTT Pluvio2's RI measurements should be revised. Moreover, results from dynamic in situ scenarios express the potentiality of large errors in Pluvio2 RI measurements due to time delay in bucket weight measurement and deficient of the Pluvio2's internal algorithm. An assessment on OTT Parsivel2 fall speed measurement revealed that Parsivel2 disregarded super terminal raindrops at lower diameter bin till 0.8125 mm and tends to bring down the mean fall speed towards sub terminal zone without any physical evidence. Moreover, it exhibited a bell-shaped sub terminal deviation in Parsivel2 fall speed measurements above 1 mm diameter raindrops with peak deviation around 1.625 mm diameter while no such patterns are noticed in High-speed Optical Disdrometer (HOD) measurements of identical rain events. A comparison study on Parsivel2 measured DSD along with DSDs using the terminal speed and HOD measured fall speed revealed that systematic underestimation of Parsivel2 fall speed measurement increased the equivalent volume drops concentrations. Meanwhile, 128 raindrop collisions are identified from 3-year field campaigns by visual inspection of HOD collected raindrops images during total 158 hours of rain occurrences. Comparison in between field observed and theoretical collision rates revealed that, field observed collision rates showed an increasing trend with increasing rain rate as well as increasing wind speed. Moreover, a regression equation is developed considering both rain rate and wind speed measurements that is well fitted with the field observed collision rates. Findings of this study express the necessity to modify the theoretical collision rate calculation considering the ambient conditions as it is used in various numerical investigations in microphysics of precipitation to determine the controlling role of raindrop collisions in DSD evolution.