Toward elucidating the microstructure of warm rainfall: A survey

Abstract

Quantitative measurement, estimation, and prediction of precipitation remains one of the grand challenges in the hydrological and atmospheric sciences with far‐reaching implications across the natural sciences. Although the roots of current research activity in this topic go back to the beginning of the twentieth century, advances in radar technology and in numerical modeling have provided the impetus for prolific research in the area of cloud and precipitation physics over the last 50 years. As radar rainfall measurements progressively became the staple of hydrometeorological observing systems, cloud and precipitation microphysics emerged as increasingly preeminent areas of research. Here we present a synthesis of the state of the science with respect to the physical dynamics of hydrometeors and, specifically, the transient processes that affect the temporal evolution of rainfall microstructure and that are directly relevant to the quantitative interpretation of radar rainfall measurements and explicit numerical simulations. The focus of our survey is on raindrop morphodynamics (equilibrium raindrop shape and raindrop oscillations), drop‐drop interactions (bounce, coalescence, and breakup), and the dynamical evolution of raindrop size distributions in precipitating clouds.

A Robust Numerical Solution of the Stochastic Collection–Breakup Equation for Warm Rain

“…The debate concerning the existence or nonexistence of multiple peaks in the equilibrium DSD is still open. No observations under natural conditions have confirmed or denied the existence or absence of secondary peaks for the equilibrium DSD itself (Testik and Barros 2007). In numerical models, the equilibrium DSD as well as the location and existence of two or three peaks depends on the method used to enforce mass conservation and the underlying kernels.…”

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).…”

“…At 15:10 LT (Figure 8b), the dominant role of coalescence over breakup shifts to small drop sizes ranging from 0.01 to 0.05 mm in diameter, indicating intensive interactions between feeder fog droplets (10-50 µm) and seeder rain drops (>100 µm) at low-levels. Further, coalescence effectively behaves as a collection mechanism with the much larger seeder drops sweeping the small fog droplets in their path [50] as expressed by net losses in number concentrations at 15:10 LT that are substantially larger than at 14:59 LT by approximately six orders of magnitude. As expected, persistent coalescence leads to an increased production of small raindrops (0.2-0.7 mm in diameter, Figure 8d) To probe the impact of SFI on drop microphysics, the quantitative contribution of coalescence (first two terms on the right-hand side of Equation (1)) and breakup (last two terms on the right-hand side of Equation (1)) to changes in droplet number concentrations at three different heights (350-, 150-, 10-m AGL) are shown at 14:59 LT (before the activation of SFI) and at 15:10 LT (after the SFI have been going on for ten minutes) as illustrated in Figure 8.…”

“…At 15:10 LT (Figure 8b), the dominant role of coalescence over breakup shifts to small drop sizes ranging from 0.01 to 0.05 mm in diameter, indicating intensive interactions between feeder fog droplets (10-50 μm) and seeder rain drops (>100 μm) at low-levels. Further, coalescence effectively behaves as a collection mechanism with the much larger seeder drops sweeping the small fog droplets in their path [50] as expressed by net losses in number concentrations at 15:10 LT that are substantially larger than at 14:59 LT by approximately six orders of magnitude. As expected, persistent coalescence leads to an increased production of small raindrops (0.2-0.7 mm in diameter, Figure 8d) at 10 m AGL.…”

Comparison between sprinkler irrigation and natural rainfall based on droplet diameter

“…The process of precipitation droplet fragmentation is complicated. Generally it includes filamentous and sheet break up caused by the turbulent kinetic energy of the gas-liquid interface of the water jet (Testik & Barros, 2007), self-fragmentation of single large droplet (Villermaux & Bossa, 2009) or colliding and sputtering fragmentation of different sizes of droplets (Low & List, 1982), etc.…”