Abstract
In an effort to better represent aerosol transport in mesoscale and global-scale models, large eddy simulations (LES) from the National Center for Atmospheric Research (NCAR) Turbulence with Particles (NTLP) code are used to develop a Markov chain random walk model that predicts aerosol particle profiles in a cloud-free marine atmospheric boundary layer (MABL). The evolution of vertical concentration profiles are simulated for a range of aerosol particle sizes and in a neutral and an unstable boundary layer. For the neutral boundary layer we find, based on the LES statistics and a specific model time step, that there exist significant correlation for particle positions, meaning that particles near the bottom of the boundary are more likely to remain near the bottom of the boundary layer than being abruptly transported to the top, and vice versa. For the unstable boundary layer, a similar time interval exhibits a weaker tendency for an aerosol particle to remain close to its current location compared to the neutral case due to the strong nonlocal convective motions. In the limit of a large time interval, particles have been mixed throughout the MABL and virtually no temporal correlation exists. We leverage this information to parameterize a Markov chain random walk model that accurately predicts the evolution of vertical concentration profiles. The new methodology has significant potential to be applied at the subgrid level for coarser-scale weather and climate models, the utility of which is shown by comparison to airborne field data and global aerosol models.
Highlights
At the ocean surface, the combination of winds and breaking waves generate sea spray aerosol droplets that are transported throughout the marine atmospheric boundary layer (MABL) (Andreas, 1998; de Leeuw et al, 2000; Veron, 2015)
We briefly summarize the parameterization of the Markov chain random walk model after statistically steady-state turbulence is achieved
If the magnitude of settling is less than that of the vertical velocity seen by the Lagrangian particles throughout their lifetime, they have a high probability of reaching the top of the boundary layer
Summary
The combination of winds and breaking waves generate sea spray aerosol droplets that are transported throughout the marine atmospheric boundary layer (MABL) (Andreas, 1998; de Leeuw et al, 2000; Veron, 2015). Sea spray aerosol particles can act as cloud condensation nuclei (Ghan et al, 1998; Lewis & Schwartz, 2004; Clarke et al, 2006), influence the propagation of electromagnetic radiation (Stolaki et al, 2015; Gerber, 1991), and interact with geochemical cycles of reactant species (Erickson et al, 1999) The impact on these processes depends on the aerosol number concentration, mass loading, chemical composition, and sea spray droplet size, which spans a wide distribution (Reid et al, 2008; Quinn et al, 2015). This representation is important along the sea surface, where aerosol particles are generated and are mostly confined (Blanchard et al, 1984; Toba, 1965)
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