Broadening of Cloud Droplet Spectra through Eddy Hopping: Turbulent Adiabatic Parcel Simulations

Abstract

This paper investigates spectral broadening of droplet size distributions through a mechanism referred to as the eddy hopping. The key idea, suggested a quarter century ago, is that droplets arriving at a given location within a turbulent cloud follow different trajectories and thus experience different growth histories and that this leads to a significant spectral broadening. In this study, the adiabatic parcel model with superdroplets is used to contrast droplet growth with and without turbulence. Turbulence inside the parcel is described by two parameters: (i) the dissipation rate of the turbulent kinetic energy ε and (ii) the linear extent of the parcel L. As expected, an adiabatic parcel without turbulence produces extremely narrow droplet spectra. In the turbulent parcel, a stochastic scheme is used to account for vertical velocity fluctuations that lead to local supersaturation fluctuations for each superdroplet. These fluctuations mimic the impact of droplets hopping turbulent eddies in a natural cloud. For L smaller than a few meters, noticeable spectral broadening is possible only for strong turbulence—say, ε > 100 cm2 s−3. For L typical for grid lengths of large-eddy simulation (LES) models (say, L between 10 and 100 m), the impact is significant even with relatively modest turbulence intensities. The impact increases with both L and ε. The representation of eddy hopping developed in this paper can be included in a straightforward way in the subgrid-scale scheme of a Lagrangian LES cloud model and may lead to a significant acceleration of simulated rain development through collision–coalescence.

Bridging the condensation–collision size gap: a direct numerical simulation of continuous droplet growth in turbulent clouds

“…Sardina et al (2015) also used a similar model to Lanotte et al (2009) but extended the simulation time to 20 min to be comparable to the formation time of rain revealed in real observations. They found that the variance of the droplet size distribution was mainly determined by the large-scale flow, i.e., the large-hopping effect suggested by Grabowski and Wang (2013) and studied by Grabowski and Abade (2017). Nevertheless, it should be noted that their conclusion was based on the simplified assumption that both the mean updraft speed and the mean supersaturation were zero.…”

Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds

“…Since fluctuations of temperature and the water mixing ratio are affected by turbulence, the supersaturation fluctuations are inevitably subjected to turbulence. Naturally, condensational growth due to supersaturation fluctuations became the focus (Sedunov, 1965;Kabanov and Mazin, 1970;Cooper, 1989;Srivastava, 1989;Korolev, 1995;Khvorostyanov and Curry, 1999;Sardina et al, 2015;Grabowski and Abade, 2017). The supersaturation fluctuations are particularly important for understanding the condensational growth of cloud droplets in stratiform clouds, where the updraft velocity of the parcel is almost zero (Hudson and Svensson, 1995;Korolev, 1995).…”

Search for Microphysical Signatures of Stochastic Condensation in Marine Boundary Layer Clouds Using Airborne Digital Holography

“…Various mechanisms such as entrainment with environmental air (Baker et al, 1980;Paluch & Knight, 1986;Telford & Chai, 1980;Warner, 1969), stochastic condensation (Khvorostyanov & Curry, 1999;Mazin & Smirnoff, 1969), and particle-turbulence interactions (Grabowski & Wang, 2013;Shaw, 2003) have been considered possible mechanisms to explain this broadening. Recently, numerical studies (Field et al, 2014;Grabowski & Abade, 2017;Paoli & Shariff, 2009;Sardina et al, 2015;Siewert et al, 2017) and observations of supersaturation variability (Ditas et al, 2012;Siebert & Shaw, 2017) have reignited the debate that stochastic condensation may play an important role in broadening of the size distribution. These build on the work by Cooper (1989) showing that stochastic condensation may still produce broadening despite the Journal of Geophysical Research: Atmospheres 10.1029/2018JD029033 limitations pointed out by Bartlett and Jonas (1972).…”