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

Abstract. Shortwave-absorbing aerosols seasonally overlay extensive low-level stratocumulus clouds over the southeast Atlantic. While a lot of attention has been focused on the interactions between the low-level clouds and the overlying aerosols, no study has yet focused on the mid-level clouds that also occur over the region. The presence of mid-level clouds over the region complicates the attribution of the cloud radiation budget, as well as of space-based remote-sensing retrievals. Here we characterize the mid-level clouds over the southeast Atlantic using lidar- and radar-based satellite cloud retrievals in addition to the observations collected in September 2016 during the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) field campaign. We find that the mid-level clouds over the southeast Atlantic are relatively common, with the overwhelming majority of the cloud occurring between altitudes of 5 and 7 km and temperatures of 0 and −20 °C. These clouds occur at the top of a moist mid-tropospheric smoke aerosol layer, most frequently between August and October, closer to the southern African coast than farther offshore, and more frequently during the night than during the day. Between July and October, we find that about 64 % of the mid-level clouds have a geometric cloud thickness less than 1 km, and about 60 % have a cloud optical depth less than 4. Using the lidar-based depolarization–backscatter relationship for September 2016, we find that the mid-level clouds are liquid-only clouds with no evidence of the existence of ice. Furthermore, we also find that 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 presence of these supercooled mid-level clouds influences the regional cloud radiative budget by reducing the radiative cooling rates by about 10 K/day near the top of the more-dominant low-level clouds.