A meteorological overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign over the southeast 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: June 2022

Abstract. Part 1 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 this 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), strengthens throughout the month in concert with a warming continental heat low, with the strongest winds occurring around 23 September advecting both aerosol and moisture; 2) The mid-tropospheric black carbon mixing ratio (BC) mixes and is entrained into the boundary layer over the SE Atlantic Ocean; 3) Dry convection is found over land south of about 10° S, with moist convection 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 flight regions (0-10° E, 5-25° S)), is also moderately correlated with a low daily-mean boundary layer height (BLH, r = 0.37 – 0.52), which is defined as the altitude of the maximum vertical gradient of moisture. For the August 2017 deployment, the primary meteorological characteristics are: 1) the AEJ-S at a lower altitude (~ 3 km, ~700 hPa) further north (5–7° S) are not established until August 20 with another upper-level 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 enhanced daily-mean low-CF tends to be associated with high daily-mean LTS (r = 0.16 – 0.57) and shallow to the moderate daily-mean BLH (r = 0.11 – 0.25), but its correlation is looser than in September 2016; 4) a layer of dry convection to 700 hPa is found over the Namibian-Kalahari dryland, with the moist plume originating from the dry convection over the land advected over the southeast Atlantic by the AEJ-S. For the October 2018 deployment, the key meteorological characteristics are: 1) the AEJ-S develops around 600 hPa, driven by the Kalahari heat low, but it diminishes as continental convection and precipitation march southward at the end of the month; 2) the advection of mid-tropospheric BC (~ 4 km, 600 hPa), RH, and water vapor are strongly modulated by the AEJ-S around 8–10° S at the early October, but this decreases as mid-latitude frontal systems develop and the AEJ-S weakens around mid- to late October as the season changes; 3) the AEJ-S – low-level jet (LLJ) over the coastal Namibian region relationship is strongest among all deployment months and the daily-mean low-CF is largely reduced by the strong daily-mean LLJ, especially to its south; 4) The relationship of the daily-mean low-CF with the daily-mean BLH and LTS are insignificant compared to the other two deployment months, partially due to the variability associated with the passage of the mid-latitude disturbance.