Low cloud reduction within the smoky marine boundary layer and the diurnal cycle

Abstract

<p><strong>Abstract.</strong> Previous observational studies of the southeast Atlantic emphasize an increase in the stratocumulus cloud cover when shortwave-absorbing aerosols are present in the free-troposphere. Recent field measurements at Ascension Island (8° S, 14.5° W) reveal that smoke is also often present in the marine boundary layer, most evident in August when the smoke is highly absorbing of sunlight, the boundary layer is deeper, the cloud-top inversion is weaker, and a climatologically lower cloud fraction eases penetration of the sunlight to the surface, compared to later months. In these conditions, the low cloud cover decreases further with enhanced smoke loadings, reflecting a boundary layer semi-direct effect that is a positive feedback. The low cloud cover reduction is particularly pronounced in the afternoon, although the cloud liquid water path is more strongly reduced at night. The daily-mean surface-based mixed layer is warmer by approximately 0.5 K when more smoke is present in the boundary layer, with a warming peak in the late afternoon when the cloud cover reduction is largest. After sunset, sub-cloud moisture accumulates throughout the night, increasing the moisture stratification with the cloud layer. This increase in boundary layer decoupling is consistent with reduced turbulence. A new observation is that in the sunlit morning hours, the smokier boundary layer deepens by approximately 200 m, and both the liquid water paths and cloud top heights increase. We speculate this reflects radiatively-induced vertical ascent originating from within a well-mixed smoke-filled sub-cloud layer. Overall, the reduction in daytime low cloud decreases the top-of-atmosphere all-sky albedo, despite an increase in the top-of-atmosphere direct aerosol radiation of ~ 6.5 W m<sup>2</sup> between the (most-least) smoky tercile composites. A convolving meteorological influence is also apparent near the cloud top, in that, although the free troposphere is also often smoky in August, the associated above-cloud potential temperatures are often cooler, rather than warmer, and better-mixed. The cooling weakens the inversion beyond that expected from the warming of the boundary layer and further encourages entrainment of more smoke into the already smoky boundary layer, increasing the longevity of the boundary layer smoke events. The free-tropospheric winds are also typically stronger and more easterly. More smoke appears to settle into the sub-cloud layer during the day than at night when it is smoky, speculated to reflect a deeper daytime sub-cloud layer facilitating entrainment, when the nighttime stratification does not.</p>

Open cells can decrease the mixing of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer

“…Lu et al (2018). The aerosols can also then warm the layer in which the clouds form 25 and promote decoupling of the boundary layer, suppressing moisture transport from the sea-surface to the clouds and reduce the liquid water path (a positive semi-direct effect) (Johnson et al , 2004;Hill and Dobbie , 2008;Zhang and Zuidema , 2019).This diverse and competing set of aerosol-cloud-radiation interactions has resulted in potentially large but poorly constrained effects of how biomass burning aerosols impact the climate system in the south-east Atlantic. For example, two of the most recent state-of-the art modelling studies have both shown that the net effect of the aerosol perturbations are to cool the region 30 by modulating the large-scale cloud field (Gordon et al , 2018;Lu et al , 2018).…”

“…Lu et al (2018). The aerosols can also then warm the layer in which the clouds form 25 and promote decoupling of the boundary layer, suppressing moisture transport from the sea-surface to the clouds and reduce the liquid water path (a positive semi-direct effect) (Johnson et al , 2004;Hill and Dobbie , 2008;Zhang and Zuidema , 2019).…”

On the differences in the vertical distribution of modeled aerosol optical depth over the southeast Atlantic

“…Over oceans, the PBL deepens with increasing sea surface temperatures, promoting its decoupling and deepening (Wood and Bretherton, 2004). Stratocumulus clouds often occur in the upper part of decoupled PBLs in the southeast Atlantic, and the PBL height tends to increase away from the southwest African coast before transitioning to a cumulus-dominated cloud regime (Zhang and Zuidema, 2019;Ryoo et al, 2021;Zhang and Zuidema, 2021). Models frequently define the PBL as the surface well-mixed layer, but this definition misses the upper parts of decoupled PBLs, in which most low-level clouds occur.…”