Revisiting Low and List (1982): Evaluation of Raindrop Collision Parameterizations Using Laboratory Observations and Modeling

Abstract

Raindrop collision and breakup is a stochastic process that affects the evolution of drop size distributions (DSDs) in precipitating clouds. Low and List have remained the obligatory reference on this matter for almost three decades. Based on a limited number of drop sizes (10), Low and List proposed generalized parameterizations of collisional breakup across the raindrop spectra that are standard building blocks for numerical models of rainfall microphysics. Here, recent laboratory experiments of drop collision at NASA’s Wallops Island Facility (NWIF) using updated high-speed imaging technology with the objective of assessing the generality of Low and List are reported. The experimental fragment size distributions (FSDs) for the collision of selected drop pairs were evaluated against explicit simulations using a dynamical microphysics model (Prat and Barros, with parameterizations based on Low and List updated by McFarquhar). One-to-one comparison of the FSDs shows similar distributions; however, the model was found to underestimate the fragment numbers observed in the smallest diameter range (e.g., D < 0.2 mm), and to overestimate the number of fragments produced when small drops (diameter DS ≥ 1mm) and large drops (diameter DL ≥ 3mm) collide. This effect is particularly large for fragments in the 0.5–1.0-mm range, and more so for filament breakup (the most frequent type of breakup observed in laboratory conditions), reflecting up to 30% uncertainty in the left-hand side of the FSD (i.e., the submillimeter range). For coalescence, the NWIF experiments confirmed the drop collision energy cutoff (ET) estimated by Low and List (i.e., ET > 5.0 μJ). Finally, the digital imagery of the laboratory experiments was analyzed to determine the characteristic time necessary to reach stability in relevant statistical properties. The results indicate that the temporal separation between particle (i.e., single hydrometeor) and population behavior, that is, the characteristic time scale to reach homogeneity in the NWIF raindrop populations, is 160 ms, which provides a lower bound to the governing time scale in population-based microphysical models.

Toward a Physical Characterization of Raindrop Collision Outcome Regimes

“…McTaggartCowan and List (1975) classified collision-induced drop breakups into four different categories: neck (also referred to as filament), sheet, disk, and bag breakups. Photographic presentations of these breakup patterns are detailed in the literature (e.g., Testik and Barros 2007;Testik and Young 2008;Barros et al 2008;Testik 2009). Because drop breakup is a key aspect of this study, a brief qualitative description of the dynamic evolution of breakup process supported with sequential high-speed images 4 ms apart is given below to ensure the completeness of this communication.…”

“…This study has been the building block for numerical simulations of raindrop size distribution evolution studies over the last three decades (e.g., List and McFarquhar 1990;McFarquhar 2004;Prat and Barros 2007a,b). Barros et al (2008) conducted experiments, similar to Low and List (1982a), focusing on the evaluation of fragment size distributions (FSDs) from different types of breakup. The experimental FSDs for the collision of selected drop pairs were evaluated against explicit simulations using a dynamical microphysics model.…”

“…The relative importance of each of these mechanisms in governing drop breakup occurrences and ultimately shaping the raindrop size distributions observed in nature has long been a subject of debate. Lack of accurate quantitative information on the amplitudes of the electric fields required for electrostatic drop disruption and the question of whether typical electrical fields in thunderclouds are sufficient to cause such disruptions (see the discussion given by Jones et al 2010) leave drop collision and aerodynamic instability mechanisms at the center of this debate [see exchange regarding Villermaux and Bossa (2009) and Barros et al (2010)]. Comprehensive experiments from List and Whelpdale (1969), List et al (1970), and Low and List (1982a) to Barros et al (2008) show that drop collisions play a major role in shaping the raindrop size distribution through coalescence and breakup processes, whether the aerodynamic instability mechanism has a sizable contribution.…”

Understanding How Low-Level Clouds and Fog Modify the Diurnal Cycle of Orographic Precipitation Using In Situ and Satellite Observations

“…From left to right, they are the production of droplets resulting from coalescence of smaller drops, the removal of droplets resulting from coalescence with other droplets, the gain of droplets due to breakup of larger drops, and the loss of droplets due to their breakup. One distinct aspect of the model is the explicit incorporation of bounce and distinct modes of breakup (neck/filament, sheet, crown and disk) [15,18,50,51] using a We-p parameterization of regimes of collision outcomes after Testik et al [52] and Prat et al [21], where We is the Weber number and p is the ratio of the small to the large diameter of two colliding raindrops (see Figure S1 in the supplementary material).…”

4-D-VAR assimilation of disdrometer data and radar spectral reflectivities for raindrop size distribution and vertical wind retrievals

“…More generally the observation of vertical DSD profiles is fundamental to address many questions regarding the numerous processes involved during a rain event. A large number of investigations have been conducted in order to model the vertical F. Mercier et al: Vertical variability of DSD evolution of the DSD during rain for different meteorological situations (List and McFarquhar, 1990;Brown Jr, 1993;Hu, 1995;Prat and Barros, 2007a;Prat and Barros, 2007b;Barros et al, 2008;Mcfarquhar, 2010). The parameterization of the different phenomena occurring during droplet fall is a complex task due to the great number of processes involved.…”