Simulating Aerosol Lifecycle Impacts on the Subtropical Stratocumulus‐to‐Cumulus Transition Using Large‐Eddy Simulations

Abstract

Observed stratocumulus to cumulus transitions (SCTs) and their sensitivity to aerosols are studied using a large‐eddy simulation (LES) model that simulates the aerosol lifecycle, including aerosol sources and sinks. To initialize, force, and evaluate the LES, we used a combination of reanalysis, satellite, and aircraft data from the 2015 Cloud System Evolution in the Trades field campaign over the Northeast Pacific. The simulations follow two Lagrangian trajectories from initially overcast stratocumulus (Sc) to the tropical shallow cumulus region near Hawaii. The first trajectory is characterized by an initially clean, well‐mixed Sc‐topped marine boundary layer (MBL), then continuous MBL deepening and precipitation onset followed by a clear SCT and a consistent reduction of aerosols that ultimately leads to an ultra‐clean layer in the upper MBL. The second trajectory is characterized by an initially polluted and decoupled MBL, weak precipitation, and a late SCT. Overall, the LES simulates the observed general MBL features. Sensitivity studies with different aerosol initial and boundary conditions reveal aerosol‐induced changes in the transition, and albedo changes are decomposed into the Twomey effect and adjustments of cloud liquid water path and cloud fraction. Impacts on precipitation play a key role in the sensitivity to aerosols: for the first case, runs with enhanced aerosols exhibit distinct changes in microphysics and macrophysics such as enhanced cloud droplet number concentration, reduced precipitation, and delayed SCT. Cloud adjustments are dominant in this case. For the second case, enhancing aerosols does not affect cloud macrophysical properties significantly, and the Twomey effect dominates.

Surface‐Atmosphere Decoupling Prolongs Cloud Lifetime Under Warm Advection Due To Reduced Entrainment Drying

“…Also, our study is based on the ASTEX case where the drizzle is weak, so in the regime where precipitation is ubiquitous, like those shown in Zhu et al (2022), suppression of precipitation under warm-advection conditions may have a greater impact on prolonging LWP than the reduction in entrainment drying. Finally, the effects of aerosols on cloud evolution are potentially important (Erfani et al, 2022), which were not included here. These limitations could be addressed in the future by running realistic LES simulations when Lagrangian observations of stratocumulus decks experiencing warm-air advection are available.…”

“…We address the question using an idealized large-eddy simulation (LES) approach by decreasing the SST over a doubly periodic LES domain to mimic the influence of warm-air advection. This simulation strategy is inspired by the conventional LES approach of studying the subtropical stratocumulus-to-cumulus (Sc-to-Cu) transition (Bretherton et al, 1999;de Roode et al, 2016;Erfani et al, 2022;McGibbon & Bretherton, 2017;Sandu et al, 2010;van der Dussen et al, 2013van der Dussen et al, , 2016, where the domain-averaged SST is increased to represent the role of cold-air advection. Comparing modeled MBLCs under warm-and cold-advection conditions enables singling out the control of T adv on the clouds.…”

Process Modeling of Aerosol‐Cloud Interaction in Summertime Precipitating Shallow Cumulus Over the Western North Atlantic

“…Erfani et al. (2022) confirmed the delayed stratocumulus‐to‐cumulus‐transition due to aerosol‐cloud‐precipitation interactions for initially clean MBL and Twomey effect for initially polluted MBL using LES with a prognostic aerosol model, where aerosol life cycle with sources and sinks of aerosols included. However, studies on ACI of the strongly non‐stationary precipitating cumuli are scarce.…”

“…We use prescribed aerosol size distribution instead of the prognostic one as in Erfani et al. (2022) because a prognostic aerosol model requires accurate information about particle and gas emissions to reproduce the observed aerosol size distributions. All simulations start at 06:00 UTC and end at 21:00 UTC with a fixed time step of 1 s. Initial profiles of temperature, humidity, and winds for all simulations are obtained from the corresponding ERA5 profiles averaged over the targeted case domain at 06:00 UTC.…”

Physical science research needed to evaluate the viability and risks of marine cloud brightening

“…Clouds with a high susceptibility to seeding would need to exist frequently enough, and with large enough spatial coverage, for local cooling to be globally relevant. Given the dependence of cloud brightening susceptibility on meteorological factors, background aerosol conditions, and meteorological-aerosol covariability, a clear understanding of how susceptibility varies over the diurnal and seasonal cycles in concert with meteorological and aerosol conditions would be essential ( 33 , 72 – 74 ).…”