Mid-level clouds are frequent above the southeast Atlantic stratocumulus clouds

Abstract

Abstract. Shortwave-absorbing aerosols seasonally overlay extensive low-level stratocumulus clouds over the southeast Atlantic. While much attention has focused on the interactions between the low-level clouds and the overlying aerosols, few studies have focused on the mid-level clouds that also occur over the region. The presence of mid-level clouds over the region complicates the space-based remote-sensing retrievals of cloud properties and the evaluation of cloud radiation budgets. Here we characterize the mid-level clouds over the southeast Atlantic using lidar- and radar-based satellite cloud retrievals and observations collected in September 2016 during the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) field campaign. We find that mid-level clouds over the southeast Atlantic are relatively common, with the majority of the clouds occurring between altitudes of 5 and 7 km and at temperatures between 0 and −20 ∘C. The mid-level clouds occur at the top of a moist mid-tropospheric smoke-aerosol layer, most frequently between August and October, and closer to the southern African coast than farther offshore. They occur more frequently during the night than during the day. Between July and October, approximately 64 % of the mid-level clouds had a geometric cloud thickness less than 1 km, corresponding to a cloud optical depth of less than 4. A lidar-based depolarization–backscatter relationship for September 2016 indicates that the mid-level clouds are liquid-only clouds with no evidence of the existence of ice. In addition, a polarimeter-derived cloud droplet size distribution indicates that approximately 85 % of the September 2016 mid-level clouds had an effective radius less than 7 µm, which could further discourage the ability of the clouds to glaciate. These clouds are mostly associated with synoptically modulated mid-tropospheric moisture outflow that can be linked to the detrainment from the continental-based clouds. Overall, the supercooled mid-level clouds reduce the radiative cooling rates of the underlying low-altitude cloud tops by approximately 10 K d−1, thus influencing the regional cloud radiative budget.

Modeled and observed properties related to the direct aerosol radiative effect of biomass burning aerosol over the Southeast Atlantic

“…An exception is the gridboxes closer to the African coastline, where mid-level clouds in particular can be more frequent. This is consistent with the fact that most mid-level and high clouds in the region originate over the continent (Adebiyi et al, 2020), a phenomenon we observed directly during the field campaigns.…”

Modeling the smoky troposphere of the southeast Atlantic: a comparison to ORACLES airborne observations from September of 2016

“…16), the model cloud top heights are typically higher than those observed, except for the EAM-E3SM model. Note that mid-level clouds (Adebiyi et al, 2020) are excluded by only selecting for cloud top heights less than 4 km. The overestimated model cloud top heights are particularly noticeable to the north, near the coast.…”

“…Efforts to distinguish aerosol effects from meteorology using satellite and reanalysis data suggest that large radiative impacts can be attributed to the shortwave-absorbing aerosol (Adebiyi and Zuidema, 2018;Chand et al, 2009;de Graaf et al, 2019;Lacagnina et al, 2017;Wilcox, 2012), but ultimately models are necessary for attributing radiative impacts to the underlying processes. Recent modeling studies have emphasized both the radiative impact of aerosol-cloud microphysical interactions (Lu et al, 2018) and the effects of free-tropospheric stabilization by smoke (Amiri-Farahani et al, 2020;Gordon et al, 2018;Herbert et al, 2020;Sakaeda et al, 2011). The model process uncertainty, to some extent, reflects the paucity of in situ measurements of aerosol properties in this complex region, in which aerosols and clouds typically occur in the same vertical column, though not necessarily co-located.…”

An overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) project: aerosol–cloud–radiation interactions in the southeast Atlantic basin

“…Heating of the atmosphere by aerosol absorption can induce changes in atmospheric circulation and mixing that can either enhance or decrease cloudiness (Ackerman et al, 2000;Koch and Del Genio, 2010). Aerosol particles also serve as cloud condensation nuclei (CCN), which can enhance cloud albedo by increasing the concentration of cloud droplets and reducing their size when aerosol concentrations increase at fixed liquid water content (Twomey, 1974), and also potentially drive changes in cloud condensate or cloud cover by altering cloud lifetimes (Simpson and Wiggert, 1969;Albrecht, 1989;Ackerman et al, 2004;Wood, 2007). The magnitudes of all effects and even the sign of the latter two aerosol ef-fects are not well quantified globally (IPCC, 2013) and are expected to be geographically and seasonally heterogeneous because the total aerosol forcing is dependent upon the nature and amount of different aerosol species, cloud type and cover, and surface albedo, all of which vary on such scales.…”