A meteorological overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign over the southeastern Atlantic during 2016–2018: Part 2 – Daily and synoptic characteristics
Authors: Ryoo Ju-Mee , Pfister Leonhard , Ueyama Rei , Zuidema Paquita , Wood Robert , Chang Ian , Redemann Jens 
Published: November 2022

Abstract. Part 1 (Ryoo et al., 2021) provided a climatological overview of the ObsErvation of Aerosols above CLouds and their intEractionS (ORACLES) period and assessed the representativeness of the deployment years. In part 2, more detailed meteorological analyses support the interpretation of the airborne measurements for aerosol transport and its interaction with low clouds over the southeastern (SE) Atlantic Ocean during the September 2016, August 2017, and October 2018 deployments at a daily and synoptic scale. The key meteorological characteristics during the September 2016 deployment are (1) the southern African easterly jet (AEJ-S), centered at around 600 hPa (∼4 km), which strengthens throughout the month in concert with a warming continental heat low, with the strongest winds occurring around 23 September. These advect both aerosol and moisture in the free troposphere. (2) Mid-tropospheric black
carbon (BC) is entrained at times into the boundary layer, and (3) convection over land is dry south of about 10∘ S and moist north of 10∘ S. The daily mean low-cloud fraction (low CF) is well
correlated with the daily mean high low tropospheric stability (LTS, r=0.44–0.73 over the flight domain; 0–10∘ E, 5–25∘ S)
and moderately correlated with the daily mean boundary layer height (BLH,
r=0.37–0.52), defined as the altitude of the maximum vertical gradient
of moisture. For the August 2017 deployment, the primary meteorological characteristics are that (1) the AEJ-S is at a lower altitude (∼3 km; ∼700 hPa) and further north (5–7∘ S) than in
September 2016 and only becomes established by 20 August, with a separate
easterly jet present aloft above 500 hPa (∼5.5 km) before
that, (2) the mid-tropospheric BC–RH coupling strengthens after the AEJ-S
develops, at around 3 km, (3) the daily mean low CF is less closely
correlated with the daily mean LTS (r=0.16–0.57) and BLH (r=0.11–0.25) than in September 2016, and (4) dry convection reaches 700 hPa over the Namibian–Kalahari dryland (∼15–25∘ S,
∼18–24∘ E), generating the moist plume subsequently
advected over the southeastern Atlantic by the AEJ-S. For the October 2018 deployment, the key meteorological characteristics are that (1) the AEJ-S
develops around 600 hPa, driven by the Kalahari heat low (∼10–25∘ S, ∼12–24∘ E), but it diminishes
over time as moist continental convection moves southward, (2) the offshore
advection of mid-tropospheric BC (∼4 km, 600 hPa) and water
vapor are strongly modulated by the AEJ-S around 8–10∘ S in early October, but this decreases as mid-latitude frontal systems develop
and the AEJ-S weakens around mid-to-late October, (3) the AEJ-S–low-level
jet (LLJ) over the coastal Namibian region relationship is the strongest
among all deployment months, and the daily mean low CF is largely reduced by the strong daily mean LLJ, especially to the cloud deck's south, and (4) the relationship of the daily mean low CF with the daily mean BLH and LTS are insignificant compared to the other 2 deployment months, partially due to the variability introduced by the passage of the mid-latitude disturbance.