An Investigation on Super- and Sub-Terminal Drops in Two Different Rain Categories and Climate Regimes

Abstract

The pressing need for accurate and reliable precipitation measurements and forecasting poses theoretical and technological problems. Remote-sensing instruments with increased coverage and sensitivity (such as space-borne and ground-based radar) are available; however, their full exploitation requires physical calibration and validation based on a deep knowledge of precipitation microphysics. This study reports a detailed analysis of the evidence of non-terminal velocities in a mid-latitude (Bologna, Italy) and a tropical location (Kolkata, India). The data from two identical disdrometers OTT-Parsivel2 were analyzed to shed light on the nature of the raindrops that fall at a velocity that is significantly higher (i.e., super-terminal drops) or lower (sub-terminal drops) than the terminal velocity expected for the raindrop sizes. The results show a significant fraction of super- and sub-terminal drops in both locations. The percentages of both super- and sub-terminal drops were higher in Kolkata. However, the difference was more notable for convective rain. The percentages of both super- and sub-terminal drops were found to be high within a drop diameter of 1 mm. The number of sub-terminal drops seemed to increase with an increase in diameter for drops larger than ~2.5 mm. The natural rain in Bologna showed stronger evidence of drop break-up in correspondence with the evolution of non-terminal velocities. Moreover, this study once again pointed toward the fact that the process of break-up cannot be neglected in natural rain of tropical or mid-latitude locations. We found that 7% and 10% of rain samples in Bologna and Kolkata seemed to be subjected to drop break-up. The results indicate that radar measurements of rain in the tropics or mid-latitude regions, relying on the Gunn–Kinzer relationship between velocity and diameter, should be verified by observations of disdrometers for a high precision QPE.

Performance of the Thies Clima 3D Stereo Disdrometer: Evaluation during Rain and Snow Events

“…In particular, the 3DS has more classes in the range of 10 to 20 m·s −1 than the LPM, so it can provide more detailed information about fall velocities. Additionally, the fraction of raindrops with small diameters and high fall velocity seems lower than detected by the LPM, which could be charged to artifact or a physical behavior as well [ 38 , 39 ] This behavior is also present in the LPM snow spectrogram, likely due to artifacts or wrong classification of snow events by the instrument. In the case of rain events, it is worth noting that no particles with diameter greater than 6.7 mm are detected by 3DS.…”

Recording Rainfall Intensity: Has an Optimum Method Been Found?

“…The magnitude of the effects may vary with the type of rainfall (stratiform or convective) or during a single event as the nature of rainfall and/or the windspeed vary. Small drops have been found to commonly exhibit super-terminal fall speeds, and large drops, subterminal speeds (Niu et al [91], Bringi et al [92], Chatterjee et al [93]). Corrections can be made if the wind speed and drop trajectories are also measured sufficiently often during rainfall, however, in the absence of such corrections, errors in mean rainfall rate as recorded by a Parsivel disdrometer through 5 min, in wind at 14 m s −1 , can amount to >−22% (Lin et al [107]).…”

“…In strong and gusty wind, there is no certainty that empirical diameter-fall speed relationships would still hold. Indeed, the behaviour of fall speeds in the range of environments where rainfall observations are required remains incompletely known, and super-terminal and sub-terminal speeds have been documented (Montero-Martinez et al [90], Niu et al [91], Bringi et al [92], Chatterjee et al [93]).…”