Impacts of solar-absorbing aerosol layers on the transition of stratocumulus to trade cumulus clouds

Abstract

<p><strong>Abstract.</strong> The effects of an initially overlying layer of solar-absorbing aerosol on the transition of stratocumulus to trade cumulus clouds are examined using large-eddy simulations. The transition of lightly drizzling cloud is generally hastened, resulting mainly from increased cloud droplet number concentration (<i>N<sub>c</sub></i>) induced by entrained aerosol. The increased <i>N<sub>c</sub></i> slows sedimentation of cloud droplets and shortens their relaxation time for diffusional growth, both of which accelerate entrainment of overlying air and thereby stratocumulus breakup. However, the decrease in albedo from cloud breakup is more than offset by redistributing cloud water over a greater number of droplets, such that the diurnal-average shortwave forcing at the top of atmosphere is negative. The negative radiative forcing is enhanced by sizable longwave contributions, which result from the greater cloud breakup and a reduced boundary layer height associated with aerosol heating. A perturbation of moisture instead of aerosol aloft leads to greater liquid water path and a more gradual transition. Adding absorbing aerosol to that atmosphere results in substantial reductions in LWP and cloud cover that lead to positive shortwave and negative longwave forcings on average canceling each other. Only for heavily drizzling clouds is the breakup delayed, as inhibition of precipitation overcomes cloud water loss from enhanced entrainment. Considering these simulations as an imperfect proxy for biomass burning plumes influencing Namibian stratocumulus, we expect regional indirect plus semi-direct forcings to be substantially negative to negligible at the top of atmosphere, with its magnitude sensitive to background and perturbation properties.</p>

Observed Impact of Smoke Aerosols on the Stratocumulus-to-Cumulus Transition in the Equatorial Atlantic

“…Also, cloud top heights decrease 20-100m, consistent with Deaconu et al, 2019. Boundary layer deepening reflected by increases in cloud top height (Fig 1c) within the GoG allow for smoke to mix and reduce low cloud cover between 0°-5°N, agreeing with hastened SCT from model results [Zhou et al, 2017]. Anomalous 850hPa winds (Fig 2b) do not reflect a pattern that would control both aerosol and cloud structures.…”

“…Confidential manuscript submitted to replace this text with name of AGU journal Using a large eddy simulation (LES), Zhou et al, [2017] find a hastened SCT linked to increased cloud droplet number concentration (due to smoke aerosols) that leads to faster evaporation of cloud water which enhances entrainment. Their complementary experiments that included additional moisture aloft, which is believed to accompany biomass burning plumes relative to surrounding air [Adebiyi et al, 2015], revealed a more hastened SCT.…”

“…Both studies find evidence of aerosols impacting the SCT but through contrasting mechanisms and end results. Yamaguchi et al, [2015] attribute a delay of the SCT to increased LTS due to aerosol heating above the cloud layer, while Zhou et al, [2017] find a hastened transition due to increased cloud droplet number concentration and cloud water evaporation.…”

“…Confidential manuscript submitted to replace this text with name of AGU journal [Yamaguchi et al, 2015;Zhou et al, 2017] incorporate observed data from the northeastern subtropical Pacific and knowledge of smoke aerosols from active fires interacting with stratocumulus clouds off the coast of Southern Africa. Both studies find evidence of aerosols impacting the SCT but through contrasting mechanisms and end results.…”

The Influence of Elevated Smoke Layers on Stratocumulus Clouds Over the SE Atlantic in the NASA Goddard Earth Observing System (GEOS) Model

“…Previous studies on aerosol‐cloud interactions (Koch & Del Genio, ; McFarquhar & Wang, ; Penner et al, ) have emphasized the crucial role of aerosol vertical distribution in the estimation of aerosol effects on clouds. LES‐based studies (e.g.,Yamaguchi et al, ; Zhou et al, ) have attempted to test model cloud responses to the variation in location of a fixed aerosol layer within and above the MBL. These studies followed the approach of Johnson et al () but included the simulation of aerosol indirect effects as well.…”