The Walvis Bay airport in western Namibia was originally considered the ideal location for ORACLES due to its proximity to the ocean and cloud deck, runway length and hangar size for the ER-2, and due to its use during the SAFARI-2K campaign by the University of Washington CV-580 aircraft. The runway had been extended to ∼3350 m, but certification of the extension was still in progress as ORACLES began. It was our intention to deploy only one aircraft in the first year and grow the activity after acclimating to the locality and airspace. However, during the ORACLES-2016 deployment planning, the UK CLARIFY team announced plans to fly its airborne assets in 2016 as well. In response, leadership re-ordered ORACLES deployments to bring both aircraft to Walvis Bay in 2016, so as to maximize their impact in a concerted effort with the international partners.

In the event that the facilities were found to be inadequate or unready or unavailable, alternate airfields were investigated. Specifically, Upington (FAUP), South Africa, and São Tomé (FPST) were pursued with due diligence until country approval was obtained from Namibia. Upington had no usable hangar but an extremely long runway, little competing traffic, and the benefit of a long-standing collaborative relationship with the USA and NASA. São Tomé's runway length and hangars could not accommodate the ER-2 requirements, but officials were very enthusiastic about a NASA collaboration. Both locations could support a P-3-only deployment. A temporary hangar in Upington, South Africa, or Windhoek, Namibia, might have supported the ER-2. Ascension Island also had no hangar and posed significant constraints to the commercial import/transport of people and equipment. In later years, Ascension Island was subject to runway construction, but it did serve as an overnight transit stop for the P-3 in 2016 and 2017 and as the target for suitcase (overnight stay) flights in 2017.

ORACLES experiment requirements dictated deployment of up to 80 people (110 in 2016) for three 5-week periods, centered on the months of September 2016, August 2017, and October 2018.

The revised ORACLES project implementation plan called for the operation of two aircraft in 2016 and the operation of only one aircraft in 2017 and 2018. The choice to deploy the ER-2 aircraft in 1 year only was solely based on funding considerations, as its operations are considerably more complicated and costly.

The ORACLES platforms and instruments were selected to efficiently and quantitatively address the science questions outlined above through measurements of radiative fluxes, derivation of heating rates, observations of aerosol and cloud microphysical and radiative properties, atmospheric thermodynamics, and chemistry. The desired flight plans were driven by expected aerosol–cloud features and their interactions within the region, by regional model forecasts, and by recent (same-flight or previous-flight) observations. When the P-3 was the sole NASA aircraft deployed (August 2017 and October 2018), Research Scanning Polarimeter (RSP) and High Spectral Resolution Lidar (HSRL-2) instruments (Table A4) were added to its payload to capture relevant science data by flying above, within, and below aerosol layers and clouds. When both the P-3 and ER-2 were present (September 2016), the ER-2 served in a remote-sensing role, obviating the need for the P-3 to fly above both cloud and aerosol layers.

It was known from the planning stage that the range of the P-3 aircraft was dependent on payload and flight pattern flown; spirals and low-altitude flight reduce flight time. Based on DISCOVER-AQ's successful inclusion of spirals from 1000 to 5000 ft (305 to 1525 m) during 8 h flights, ORACLES flights were expected to be similar in duration and character. From Walvis Bay, Namibia, this covered the target science zone (5 to 35∘ S, westward from the coast to approximately the prime meridian), transit to Ascension Island for suitcase flights and for coordination with the UK's CLARIFY team, and transit to São Tomé Island, located near the northern edge of the climatological BB plume, with margins for headwinds, profiles, and low-altitude flight.

During the 2016 deployment, it became obvious that there was merit in P-3 flights that extended beyond the originally estimated 8 h duration. The NASA P-3 crew accommodated this request as far as crew rest considerations permitted; the average flight duration for local P-3 science flights (excluding transits) in 2016 was 8.3 h (see Sect. 4.4). In 2017 and 2018, while flying from São Tomé, a larger P-3 flight crew was able to average 9.1 and 8.3 flight hours respectively for similar flights, with the 2018 average affected by increased safety margins due to expected inclement weather.

For ORACLES, ER-2 flights were envisioned to be up to 8 h in duration, similar to the prior Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) campaign. Due to the 2 h pre-flight hands-off period and pilot 12 h duty day, there were concerns that weather delays (e.g., low ceilings) would result in flight duration limitations and therefore limitations in the final geographical coverage. The ER-2 deployed only once for ORACLES, to Walvis Bay, Namibia, during September 2016. The payload consisted of the Enhanced MODIS Airborne Simulator (eMAS), Airborne Multiangle SpectroPolarimetric Imager (AirMSPI), RSP, and HSRL-2 (Table B2) for various aspects of cloud composition, aerosol properties, and the overall cloud/aerosol morphology, as well as the Solar Spectral Flux Radiometer (SSFR) for radiative flux measurements. The aircraft operated as expected, and the average flight duration from Walvis Bay was 8.1 h. Especially during the second half of the campaign, the ER-2 pilots were extremely accommodating, frequently extending individual flight duration to 9 h.

In this section, we discuss the choice of instrumentation for each platform and deployment. Depending on ORACLES deployment year, the P-3 carried 8 to 11 instruments or instrument suites, with the following included for all deployments: cloud suite (UND/OU); phase Doppler interferometer (PDI); Hawaii Group for Environmental Aerosol Research (HiGEAR) in situ measurement suite for aerosols; SSFR/CG-4 for radiative fluxes; Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) for aerosol optical depth, cloud, and sky radiances; Airborne Third Generation Precipitation Radar (APR-3) for cloud and precipitation observations; Research Scanning Polarimeter (RSP); CO Measurements and Analysis (COMA) for CO, CO2, and H2O mixing ratios; CCN spectrometer for cloud condensation nuclei; and Water Isotope System for Precipitation and Entrainment Research (WISPER) for water isotope measurements. In certain years there were targeted additions/deletions, listed as follows.

The HSRL-2 was added to the P-3 payload for deployments without the ER-2, i.e., 2017 and 2018.

Tables B1 to B3 in Appendix B provide full payload tables, including instrument names, instrument descriptions, primary measurements, and derived geophysical observables for each instrument.

ORACLES conducted a 2-week “dry-run” activity from 14–28 September 2015, prior to its first deployment in 2016. Project meteorologists, platform scientists, pilots, and the leadership team met by phone and Cisco Webex to examine daily weather forecasts, chemical weather predictions, and flight conditions. In response to these forecasts, detailed flight plans were developed for the upcoming 1 or 2 possible flight days, using a flight planning tool that was specifically designed for multi-aircraft flight operations (LeBlanc, 2018). Longer-range forecasts up to 5 d were used to plan for extended flight strategies relative to overarching flight objectives. Satellite data, primarily Meteosat-10 Spinning Enhanced Visible and Infrared Imager (SEVIRI) visible imagery and Cloud-Aerosol-Transport System (CATS) and CALIPSO vertical feature mask and attenuated backscatter profiles, were used to evaluate the likely success of a given flight plan for a given day. The process familiarized the science team with items that impacted real flight planning: aircraft limitations, staff fatigue limits (e.g., down days, crew rest), aviation authority coordination timelines, the availability and latency of meteorological forecasts, and the use of the flight planning tool. This practice time made ORACLES actual deployments more efficient, although the complexity and scope of actual chemical and meteorological forecasts in the field ended up being well beyond the scope of the dry-run exercise. This increased complexity was undoubtedly the result of lessons learned during the dry-run exercise itself.

The forecasting effort for ORACLES deployments in the field entailed both meteorological and chemical weather predictions. The meteorological forecasting effort for the ORACLES mission consisted of three components: (i) forecasting for flight planning, (ii) nowcasting during flights for real-time flight direction, and (iii) forecasting local weather for flight operations. Each of the three ORACLES deployments featured daily planning meetings. On non-flight days, the flight planning team met at 08:00 local time to discuss the weather and chemical weather forecasts for a period of up to 5 d, with special emphasis on the upcoming 1 or 2 flight days. On flight days, the team would assemble at 05:00 local time to assess whether the latest forecasts warranted any changes to flight plans made the day prior. Also on flight days, the forecast team would provide in-flight nowcasting that often led to significant adjustments of flight plans, usually to respond to actual cloud conditions that materialized on a given flight day or in response to changing local conditions.

Clouds were the primary focus of the meteorological forecasting effort for both flight planning and nowcasting. Low clouds (i.e., stratocumulus at the inversion) were of primary scientific interest for their interaction with the African smoke plume. However, middle and high clouds were also important since the presence of these clouds complicated the radiation measurements of some instruments (e.g., 4STAR, SSFR). Verification studies prior to the ORACLES deployments showed that the European Centre for Medium Range Weather Forecasts (ECMWF; Pappenberger et al., 2008; ECMWF Newsletter, 2012; Ye et al., 2014) and United Kingdom Meteorological Office (UKMO; Ran et al., 2018) global forecast models provided the best performance for cloud forecasts. ECMWF digital data were available at 0.125∘ longitude × latitude resolution and included the primary meteorological variables (relative humidity and horizontal winds at 925, 850, 800, 700, 600, 500, 400, 300, 150, and 100 hPa levels; 1000–500 hPa layer thickness; surface wind speed; mean sea level pressure; boundary layer height; precipitation; convective available potential energy) as well as 3-D ice and liquid water mass. We also used the 2-D ECMWF products of cloud fraction and cloud base for the low, middle, and high clouds, which were found to be adequate for our forecasting requirements. The ECMWF cloud forecasts were supplemented by low, middle, and high cloud distribution forecasts from the UKMO global forecast model. In order to forecast the overall circulation over the southeast Atlantic, with an emphasis on wind and relative humidity distributions from the surface to 500 hPa, we used a suite of forecast products from ECMWF, UKMO, Global Forecasting System (GFS) from the National Centers for Environmental Prediction (Environmental Modeling Center, 2003), and the NASA Goddard Earth Observing System, Version 5 (GEOS-5) model (Molod et al., 2012).

Our primary nowcasting tool during the flights was the geostationary satellite imagery from the SEVIRI instrument aboard Meteosat-10 and Meteosat-11. Raw imagery from the infrared and visible channels was useful for establishing the evolution and distribution of clouds during flight. Satellite cloud properties described by Minnis et al. (2008, 2020) were calculated from SEVIRI raw radiances by NASA Langley, including cloud altitude, water path, and effective radius, and were also used in real-time flight direction. For forecasting local weather for flight operations, particularly during the October 2018 deployment in São Tomé, we primarily relied on satellite imagery over the past 12–24 h for a short-term (i.e., a day or less) outlook of heavy precipitation at the airport, as precipitation forecasts from the models were largely unreliable.

Chemical forecasts were done using both global and regional models, with the regional models providing a lot of the detail required for flight planning on a daily basis. We used three global systems: GEOS5 (https://gmao.gsfc.nasa.gov/GEOS/, last access: May 2020), the Copernicus Atmosphere Monitoring Service (CAMS, https://atmosphere.copernicus.eu/, last access: May 2020), and a bespoke three-component aerosol (carbonaceous, mineral dust, and industrial pollution) modeling system developed by the UK Met Office for their CLARIFY deployment in 2017. Five-day aerosol forecasts provided the expected spatial and vertical location of the main smoke coming from the African continent. These models were also useful in identifying times when the smoke was expected to be mixed with dust aerosols, especially during the 2016 deployment from Walvis Bay.

For regional model forecasts, two configurations of the Weather Research and Forecasting Model (WRF; Skamarock et al., 2008) were employed. One of them used WRF coupled to chemistry (WRF-Chem, Grell et al., 2005) using the physics package from the Community Atmosphere Model version 5 (CAM5, Ma et al., 2014), run by a team from the University of Iowa (WRF-CAM5, PI: Gregory Carmichael). This model provided daily 72 h aerosol forecasts for similar purposes as for the global models by using a full chemistry suite with hundreds of chemical species considered. WRF was also configured using an aerosol-aware microphysics (AAM) scheme (Saide et al., 2016) maintained by a team from NCAR/UCLA (PI: Pablo E. Saide). The Weather Research and Forecasting aerosol-aware microphysics (WRF-AAM) model provided forecasts for lead times of up to 4 d at 12 km resolution. The system included a near-real-time emission constraint using satellite-based aerosol optical depth (Saide et al., 2016), which to our knowledge corresponds to the first near-real-time system to perform such tasks. Two simulations were performed per forecasting cycle turning smoke emissions on and off in the model. Since WRF-AAM resolves aerosol–cloud–radiation interactions, these simulations allowed assessment of the effects of smoke on weather in forecasting mode by taking the difference between the two forecasts. The forecasts also included tracers tagged to each day of smoke emissions from the African continent, which were used to provide a distribution of smoke age based on the tracer concentrations and the days since emissions. Statistics such as mean and mode were extracted from the age distribution and used for flight planning to target plumes with different ages to explore the temporal evolution of aerosol properties.

Another task performed during the planning meetings was near-real-time evaluation of the forecasts. These were focused on assessing forecast performance in predicting clouds and the aerosol plume location, and relied mostly on the latest SEVIRI cloud retrievals, and clear-sky and above-cloud AOD from MODIS. This exercise allowed the team to track forecast failures and successes and provided a sense of reliability when making decisions based on forecasts.

The ORACLES investigation concept featured a combination of routine flights to facilitate comparisons with climate models and to ensure sampling of a wide range of aerosol loadings and cloud conditions, with other flights addressing “targets of opportunity”. The “routine” flights all took place along a fixed latitude–longitude line with sampling at a range of altitudes and remote sensing of the full column. In 2016, the routine flight track was along a diagonal with endpoints of 20∘ S/10∘ E and 10∘ S/0∘ E; in 2017 and 2018 the routine flight track extended from the Equator to ∼15∘ S along 5∘ E. See Fig. 11 in Sect. 4.4 for a complete set of flight tracks.

In situ observations of aerosol microphysical and optical properties during the routine flights were envisioned to map the evolution of BB aerosol radiative properties during transport. HSRL-2 (High Spectral Resolution Lidar) observations from the P-3 or ER-2 helped map the spatial extent of the layers, while SSFR (Solar Spectral Flux Radiometer) and 4STAR (Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research) observations provided additional insights into the in situ-derived aerosol properties via optical and radiative closure experiments. Measurements to address seasonal variations in direct aerosol radiative effects and their controlling factors were derived from the routine flights. The routine flight requirements were derived based on the assumption that the statistics of important observed aerosol and cloud properties, given sampling and measurement uncertainties, are sufficiently constrained to distinguish between climate model estimates. For this, we assumed that the variability in aerosol properties at model-relevant scales (100 km2) can be extrapolated from the analysis of Shinozuka and Redemann (2011) to be less than 20 % and that such variability is well below the inter-model differences on such scales.

Another motivation for the routine flights was to ensure sampling of a wide range of aerosol loadings and cloud conditions. The five to six envisioned routine flights (equaling ∼40–50 flight hours per deployment) that comprise the ORACLES threshold science objectives were intended to yield aerosol and cloud data in about 200 100 km2 climate model grid boxes. Prior to the start of the campaign, we investigated probability density functions (PDFs) of MODIS daily 1×1∘ averaged AOD between 10–20∘ S and 5∘ W–5∘ E for September 2001 derived from the then-available dark target algorithm. We randomly subsampled the roughly 3000 1×1∘ MODIS AOD boxes with the planned 200 airborne observations and found that the resulting PDFs were a good representation of the parent population of MODIS AOD. We concluded that the number of threshold science flight hours was adequate to compile probability density functions of aerosol properties that allow assessments of climate model differences at these spatial scales. This was confirmed by analysis after the first deployment (Shinozuka et al., 2020).

About half of the flight hours in each campaign focused on targets of opportunity, as detailed in Tables A1–A3 in Appendix A. These flights targeted specific science goals (e.g., capturing a range of aerosol ages, or contrasting conditions in terms of aerosol–cloud interactions). Flight patterns (e.g., “radiation walls”, square spirals) were optimized to leverage the measurement capabilities of the range of instruments on board the P-3 and to allow for later comparison of different methods of measuring a common parameter (e.g., aerosol SSA). During the 2017 and 2018 deployments, the target-of-opportunity flights were planned to be near the routine flight track whenever possible to improve sampling statistics.

The planning for the 2016 field deployment in Namibia started in early 2015, with multiple site visits to the Walvis Bay airport, logistics and hotel providers, Namibian science partners, and representatives of various Namibian government organizations. This planning started early because the unexpected change to deploy both ORACLES aircraft in 2016 brought along a significant set of challenges due to the large contingency of scientific and aircraft support equipment needed, with this being the first of the three ORACLES deployments. The ORACLES team gratefully acknowledges the help provided by the Honorable Thomas F. Daughton, US Ambassador to Namibia from 2014 to 2017, and the support by the US embassy staff led by John Kowalski. The US embassy proved instrumental in receiving flight permissions and in arranging the student program in August 2016. The ORACLES team also received invaluable feedback and support from the Namibia University of Science and Technology (NUST), led by its rector, Tjama Tjivikua, and Dean Lameck Mwewa. In addition to NUST, the Gobabeb Training and Research Center led by Gillian Maggs-Kolling, the University of Namibia represented by Martin Hipondoka and Michael Backes as well as North-West University (South Africa) represented by Stuart Piketh provided information and logistics support throughout the 2016 campaign. As we describe in Sect. 3.5 below, these contacts were the springboard for the outreach efforts that led to the deployment of seven graduate students in the 2016 field campaign, including five students from NUST and the University of Namibia. In their totality, we hope that the efforts expended by the Namibian government and the Namibian and South African science community, as well as the reciprocating efforts by the ORACLES science team, can be considered a transformational effort in the context of science diplomacy (Annegarn and Swap, 2012), at least in so far as the experience for the individual students that participated in the outreach efforts are concerned.

During the field campaign held in Namibia in 2016, the gathering of science data not only benefited the scientists directly involved in the project; through an outreach program the science was extended to the Namibian population (and to some extent the broader southern African region). The outreach effort was multi-tiered and aimed to inform the public, develop young scientists, and encourage children to enter into STEM fields of study. Outreach activities included public lectures, interviews with local radio and newspapers, and open days at the airfield. In addition, ORACLES scientists traveled to northern Namibia for several days to participate in the Ongwediva Annual Trade Fair (OATF) together with students, staff, and faculty from the University of Namibia (UNAM), the Namibia University of Science and Technology (NUST), and the Gobabeb Research and Training Center. The OATF showcased collaborative environmental research from participating research institutes and was attended by Ongwediva-area students, business leaders, and local dignitaries.

In addition to these broader public engagement outreach activities, a targeted science development program was initiated with support from the US Embassy in Namibia and NUST. This 3-week full immersion outreach program was developed to provide promising local and regional young scientists with an opportunity to experience different components of a large complex airborne research field campaign. In total, seven post-graduate students (master's and PhD level), from Namibia (five students) and South Africa (two students), participated in the student guest program (Fig. 4). Student guests were exposed to the planning, modeling, and instrumentation used within the ORACLES field campaign. In addition to these broad field campaign skills, they received a solid foundation in basic atmospheric science through tutorials from the participating campaign scientists, some introductory programming tutorials, and an opportunity to interact with scientists aligned with their field of research. Within the duration of the program they also all had an opportunity to join a science flight. Further regional expansion of this student guest program was planned for the 2017 and 2018 field campaigns in collaboration with the CLARIFY and AEROCLO-sA campaigns, but with the move of the ORACLES field campaigns to São Tomé this expansion outreach effort could not be implemented.

To understand the challenges of implementing an outreach program as part of a scientific project like ORACLES in São Tomé and Príncipe (STP), one needs to know a little about the history of this young country of just over 200 000 inhabitants. Previously uninhabited, the STP Archipelago was colonized by Portugal throughout the 16th century, when it served as a warehouse for the slave trade and established itself as a producer of sugarcane, coffee, and cocoa. STP independence from Portugal came in 1975, keeping Portuguese as its official language, although minority groups also speak at least four other dialects.

During an initial exploratory visit in 2015, built on a previous NSF-sponsored site visit, the ORACLES team contacted the Instituto Nacional de Meteorologia (INM) to establish collaborations. The INM operates at STP airport facilities and showed great initial enthusiasm for ORACLES deployments from STP. During the 2 years of ORACLES operations in 2017 and 2018, INM kindly issued daily weather reports tailored to ORACLES needs. There were several visits from the ORACLES team to INM, and Aristómenes Amadeu do Nascimento of INM attended various ORACLES weather briefings.

The only public university in STP and the most important one, the University of STP (USTP), was established only in 2014. The creation of USTP came to address fundamental and emergency problems of the country, which included training of personnel for the health and education sectors, agriculture, and food production. When the ORACLES team deployed to STP in 2017, USTP had, in its current format, only 2 years of existence, still consolidating its vocations and priorities. Nevertheless, the institution represented by Aires Bruzaca (dean), João Pontífice (vice dean), and Manuel do Sacramento Ramos Penhor was enthusiastic about establishing scientific collaborations with NASA.

The ORACLES team organized a series of seminars about ORACLES scientific objectives for the USTP and INM communities. The seminar themes also included the AERONET (Aerosol Robotic Network) and Pandora NASA projects, global networks of spectrometers designed to retrieve, respectively, aerosol optical depth and microphysical parameters (Holben et al., 1998; Dubovik and King, 2000), and total columns of ozone and other trace gases in the atmosphere from direct-Sun measurements (Herman et al., 2009, 2015; Tzortziou et al., 2012). All lectures were presented by ORACLES science team members in Portuguese to address potential language barriers.

A Pandora Spectrometer Instrument (PSI) and an AERONET instrument were brought to STP as part of the ORACLES deployment. The main goal, especially for the PSI deployment, was to assess whether mutual goodwill, interest, and capabilities exist for NASA, USTP, and INM to collaborate scientifically long term. The team was successful in training professors of the USTP to operate the PSI and the AERONET instruments (Fig. 5), and this resulted in additional aerosol measurements beyond the campaign periods. Moreover, it laid the foundation to have STP as one of the sites of the Pandora network, with an official agreement between NASA and the USTP signed in 2018.

The ORACLES team found in STP a community open to and eager for the establishment of a fruitful scientific cooperation. Our experience points out that involvement with the local community is of extreme importance, not only for the dissemination of scientific knowledge but also to facilitate engagement between the young scientists from both communities. Collaboration with local scientific communities during field deployments such as ORACLES has the dual benefits of enhancing local scientific capabilities in under-resourced areas of the world and producing tangible benefits for this and future missions in the region.

An important line of inquiry and indeed a major motivation for the ORACLES project was the question of how frequently Sc clouds in the SE Atlantic MBL are in physical contact with the BB plume emanating from the southern African continent. The only prior work in the area using in situ aircraft measurements (Haywood et al., 2003) found frequent separation between cloud and aerosol close to the coast of Namibia and Angola but potential interactions between cloud and aerosol in the vicinity of Ascension Island. However, the statistical relevance of those findings is impossible to establish given the scarcity of the available in situ data. For a large number of coincident CALIOP and MODIS aerosol and cloud retrievals in the SE Atlantic region, Costantino and Bréon (2013) found that more than half of the vertical profiles they studied indicated “well-separated” aerosol and cloud layers, i.e., an “aerosol-free gap” above cloud top. They went on to attribute the differences in cloud effective radii between separated and unseparated cases to the probable paucity of aerosols in the MBL in the separated cases. ORACLES observations show that the separation of aerosol and cloud layers in an instantaneous profile is a poor indicator for the concentration of aerosol in the MBL, because there are many other pathways for the aerosol to reach the MBL at a given location. However, the physical contact of the BB plume and Sc cloud tops is indicative of active entrainment of BB aerosol into the MBL and is often associated with significant BB aerosol in the MBL. The relatively low signal-to-noise ratio (SNR) in CALIOP vertical profiles after traversing a BB plume with significant optical attenuation, also alluded to in Sect. 2.1, raises the question of how accurate the CALIOP-indicated frequent separation between the BB plume and Sc cloud tops (shown in Fig. 2) really is.

To help address this question, we investigate here how frequently the HSRL-2 aerosol extinction profiles in the three ORACLES deployments indicate separation or lack thereof between the BB plume base and the Sc cloud tops. The left panel of Fig. 13 shows mean aerosol extinction profiles at 532 nm for each ORACLES deployment year, along with mean cloud top heights as horizontal solid lines. Because ORACLES-2016 had a somewhat different geographic focus from ORACLES-2017 and ORACLES-2018, Fig. 13 only shows data for the geographic box bounded by 5–15∘ S and 2.5–7.5∘ E, where there are data present in all 3 flight years.

The mean extinction profiles show that the BB plume in ORACLES-2017 (August) has a local maximum of aerosol extinction at altitudes between 2–3 km, while the ORACLES-2016 (September) and ORACLES-2018 (October) mean profiles have maximum aerosol loading between 3.0 and 4.5 km in altitude, albeit with somewhat lower values. In all 3 years, there is a relative minimum in aerosol loading near the location of the mean cloud tops. This suggests that the air mass just above cloud top may indeed be significantly older, more processed, and scavenged than the FT above and possibly even the BL below, which may have been subject to entrainment of FT air upstream of the BL flow, to the south of the bounding box.

The separation between the BB plume base and the cloud tops for each year is indicated in the right hand panel in Fig. 13. It shows the cumulative distribution of HSRL-derived aerosol backscattering coefficient at 532 nm in the 300 m layer just above cloud top for the same geographic region as the left panel. The cumulative distribution of aerosol backscatter values near the cloud top varies sigmoidally, without any discontinuity that would indicate an unambiguous lower edge to the aerosol layer. Instead, it is apparent that the assessment of an aerosol-free gap depends entirely on the definition of the aerosol loading. For example, the August 2017 and October 2018 deployments featured a relatively low fraction of profiles with less than 0.25 M m-1 Sr-1 aerosol backscatter coefficient in the first 300 m above cloud, i.e., 3 % and 15 % for 2017 and 2018, respectively. The September 2016 profiles in this region on the other hand show less than 0.25 M m-1 Sr-1 aerosol backscatter coefficient, and hence an aerosol-free gap, almost 50 % of the time. Regardless of the exact threshold chosen, September 2016 showed by far the highest frequency of aerosol-free gaps above clouds when compared to the 2017 and 2018 deployments. The representativeness of this September maximum in aerosol-free gaps above clouds is unknown and will need to be explored with future satellite lidar observations of sufficient accuracy. Overall, we found significant spatial and temporal variability in the degree of contact between BB aerosol and Sc clouds in the SEA region and that the frequency of occurrence of aerosol-free gaps is likely much lower than previously assumed (Costantino and Bréon, 2013). These findings need to be considered when interpreting previous results on aerosol–cloud interactions in the region.

ORACLES data provide a useful test bed for algorithm development in support of future satellite missions, for example NASA's ACCP (Aerosols, Clouds, Convection and Precipitation) mission. For instance, ORACLES observations are currently being used to develop joint polarimeter–lidar retrievals of aerosol and cloud properties. Whether such observations will successfully detect features such as the clear-air layers in the SEA will depend on specific instrument characteristics, but the ORACLES measurements should provide useful benchmarks for the testing of candidate observing concepts.

The lidar-derived increase in extinction with height for September (2016) in Fig. 13 is accompanied by a similar increase with height of the mean in situ SSA (derived from the in situ PSAP absorption paired with nephelometer scattering at 530 nm) from 0.84 to 0.87 (Fig. 14, middle of top row). The SSA values are in reasonable agreement with values of ∼0.83 for dry conditions in Davies et al. (2019) derived using state-of-the-art photoacoustic and cavity ring-down instrumentation. Pistone et al. (2019) compare the ORACLES absorption–scattering measurements with SSA derived by several different airborne remote-sensing methods at wavelengths between 400 and 995 nm and found reasonable agreement both for specific case studies and for the range of measured spectral SSA over the full ORACLES-2016 deployment.

Black carbon is the primary absorber of sunlight within BB aerosol (e.g., Bond et al., 2013). Although a corresponding decrease with height of the refractory black carbon (BC) mass concentration relative to the mean organic aerosol (OA) mass concentration is not clearly apparent in Fig. 14, (middle of middle row), an example from an individual profile from the 24 September 2016 flight indicates more nitrate and organic aerosol above 3.5 km than below it, relative to the black carbon mass concentration (Fig. 14, middle of bottom row). This is consistent with an increase in SSA with height. Examples of individual profiles are shown from each year, broken down by aerosol species – black carbon, organic aerosol, nitrate (NO3), ammonium (NH4) and sulfates (SO4), indicate distinct vertical structures (Fig. 14 bottom row).

Profiles from August 2017 also indicate some vertical structure to the SSA without a clear mapping to the BC/OA ratio, while the mean SSA from the October 2018 deployment increases even more sharply with altitude than does the mean SSA from September (2016). For October 2018, the increase of SSA with height is clearly consistent with the proportional increase in organic aerosol relative to black carbon. Work is ongoing to attribute changes in SSA to both thermodynamically driven changes in gas–particle phase partitioning (e.g., Wu et al., 2020) and more irreversibly driven changes related to forms of photodegradation in this near-equatorial, sun-exposed environment. Overall the SSA values are less than has been previously assumed, with that implication further explored within the modeling study of Mallet et al. (2020).

Some of the differences between the individual profiles in Fig. 14, bottom row, can be related to differences in the prevailing meteorology shown in Fig. 8. The distinctive two-layer aerosol structure profiled on 24 September 2016 reflects the ability of strong winds at 4 km (see also Fig. 6) to disperse aerosol westward, with the aerosol lower down, at 2–3 km, resulting from an anticyclonic circulation (Fig. 8). The strong 4 km zonal winds are much less apparent in August, consistent with a lower-lying, less layered aerosol vertical structure. The free-tropospheric winds remain strong into October, but by then the fire emissions have reduced considerably and less aerosol appears to reach the altitude at which the zonal winds are strongest.

Ongoing analysis of the ORACLES dataset is relating the observed vertical structure in aerosol composition and optical properties to aerosol aging. Rather than using an aircraft to track the evolution of a smoke plume as it advects, or using an aerosol chemical component as a “chemical clock” to determine aerosol age, an alternative approach links a model-derived mean aerosol age to in situ aerosol characteristics. Particles, including at fire emission sources, are marked at the time of the model initialization and tracked in time thereafter (Saide et al., 2016), with the particle ages subsequently extracted along the flight legs and these modeled ages combined with the in situ datasets. An example is shown in Fig. 15 for 24 September 2016 (a profile from this flight is also shown in Fig. 14), using aerosol age derived from one of the ORACLES in-field aerosol forecasting models, WRF-AAM (see Sect. 3.3.5). The model-derived mean aerosol age height–latitude cross section is overlaid with measurements of f44, the mass-to-charge ratio m/z 44 relative to total organics. Higher fractions of f44 reflect the formation of carboxylic acids, which coincide with older aerosol ages up to approximately 10 d. The ORACLES f44 values shown here are on par with those reported for Siberian biomass burning aerosol after trans-Pacific transport to Alaska. Notable is that younger aerosol overlies older aerosol on this day, coinciding with the altitude range of strong zonal winds (not shown, but consistent with Fig. 6). This figure also highlights the wide range of aerosol ages beyond 4 d that ORACLES sampled during its flights, marking another unique contribution by the ORACLES campaign. Ongoing work is relating the model-derived aerosol aging to a continuous depletion of non-black-carbon aerosols.

ORACLES scientific objective 3 (Table 1) seeks to understand the extent to which smoke aerosol from southern Africa mixes into the MBL and impacts marine low cloud microphysical processes. Smoke may enhance the population of cloud condensation nuclei (CCN), which can lead to increases in cloud droplet concentration and smaller droplet sizes. It can also potentially induce changes in cloud condensate through suppressing precipitation rates, enhancing cloud top entrainment, and otherwise influencing cloud macrophysical properties. These indirect effects lead to significant increases in albedo in global models, but little in situ data are available over the SE Atlantic to test these models.

According to HSRL-2 observations (Fig. 13), a denser, more frequent smoke layer in the FT occurred in 2017 than in 2016 or 2018. CCN measurements made on all three ORACLES deployments (Kacarab et al., 2020) provide evidence that FT CCN concentrations were also higher in 2017 (Fig. 16a). Despite the frequent contact of the smoke with the sub-cloud planetary boundary layer (PBL) top, concentrations of CCN in the PBL are much lower than those in the FT (compare panels a and b in Fig. 16). Also, despite very similar FT CCN concentrations in 2016 and 2018, PBL CCN levels were actually significantly higher in 2018 than in 2016 (Fig. 16b), suggesting that the FT CCN concentration alone is not a unique determinant of microphysical properties in the PBL. As noted in Diamond et al. (2018), it is the history of subsidence and entrainment of smoke into the PBL, rather than the instantaneous presence of aerosol immediately above clouds, that sets the MBL CCN level and therefore cloud droplet concentration (Nc). Seasonally, satellite evidence (Fig. 10) indicates that the highest Nc values offshore occurred in 2017, and this is consistent with the highest measured PBL CCN concentrations (at supersaturations >0.25 %) occurring during 2017. October 2018 also showed high Nc values offshore consistent with the higher measured CCN concentrations.

In addition to entrainment of smoke from the FT, the concentration of CCN and resulting droplet concentrations in the marine PBL are also modulated strongly by coalescence scavenging by light precipitation (Wood et al., 2012). In addition, evidence suggests that precipitation can be suppressed in clouds with high CCN and Nc (e.g., Sorooshian et al., 2009). The APR-3 radar on the P-3 is sufficiently sensitive to detect this light precipitation. Measurements of precipitation from the APR-3 indicated significant differences in precipitation during the three campaigns (Fig. 17). Comparing the two campaigns flown out of São Tomé (2017 and 2018), precipitation was significantly lighter in 2017 (Fig. 17), which may indicate suppression due to higher droplet concentrations, but additional analysis of cloud thickness and liquid water path differences between the campaigns is underway to quantify these impacts (see also Dzambo et al., 2020).

Observations obtained by in situ probes installed on the NASA P-3 are being used to investigate cloud–aerosol interactions. Over the 3-year period of ORACLES, 397 vertical profiles were flown through clouds during either sawtooth profiles (see Fig. 11) or in individual cloud profiles isolated in time–space from other profiles. The vertical dependence of cloud properties in a common reference frame is examined using a normalized altitude ZN, defined as (Z-Zb)/(Zt-Zb), where Z is altitude, Zt cloud top altitude and Zb cloud base altitude. This allows us to composite the vertical microphysical structure of cloud decks from many different clouds.

We select 397 vertical profiles through the cloud layer when accumulation-mode (0.1<D<3 µm) aerosol concentration (Na) was measured at least 100 m above and below cloud using the Passive Cavity Aerosol Spectrometer Probe (PCASP). Each profile was classified as “Contact” or “Separated” based on whether the layer of enhanced above-cloud aerosol concentration (defined as PCASP Na>500 cm-3) was in contact with cloud top or separated from it by at least 100 m. Each of the “Contact” and “Separated” profiles was further classified based on whether the average PCASP Na within the 100 m layer below cloud base was greater than or less than 250 cm-3. Four different regimes were thus defined based on the below-cloud boundary layer Na and by whether or not the cloud layer was in contact with an overlying biomass burning aerosol (BBA) plume or separated from it.

Figure 18 shows the mean cloud droplet concentration Nc as a function of ZN for the four regimes. The average boundary layer Na values for the regimes are as follows: Contact/Na<250, 184 cm-3; Contact/Na>250, 507 cm-3; Separated/Na<250, 113 cm-3; Separated/Na>250, 318 cm-3. The average boundary layer Na is a good predictor of the vertical mean Nc across the four regimes, consistent with the primary source for droplets being aerosols ingested into cloud base (Diamond et al., 2018). Although Nc is approximately constant with height for the two Separated regimes and the Contact/Na<250 cm-3 regime, consistent with many previous studies (Nicholls and Leighton, 1986; Martin et al., 1994; Miles et al., 2000; Painemal and Zuidema, 2011; Wood, 2012), it experiences a significant and unexpected increase with ZN for the Contact/Na>250 cm-3 case. The highest Nc values occur where the MBL has both high Na and is in contact with overlying BB aerosol layers. The lowest boundary layer Na is found where there is separation from the overlying aerosol. It is perhaps surprising that clouds within boundary layers with higher Na but not overlain by aerosol, or clouds within boundary layers with lower Na but overlain by aerosol, possess very similar Nc. This further suggests that clouds' history of interacting with aerosol within the prior days (Mauger and Norris, 2007; Diamond et al., 2018) can have an impact on Nc but that aerosols entrained from above cloud top also have an impact on Nc. These findings are further described by Gupta et al. (2020).

Approximately one-half of all of the flights were devoted to a routine path, motivated by a desire to facilitate model improvement through an unbiased sampling performed frequently enough to adequately capture the monthly mean (see Sect. 3.3.6). An a priori evaluation based on the random sampling of clear-sky aerosol optical depths rationalized the decision to allocate six to eight flights to a routinely sampled flight line, on random days during the deployment. The initial model–observation comparison (Shinozuka et al., 2020), which is based on the 2016 measurements only, includes model versions similar to those used for the aerosol forecasts in the field (WRF-CAM5 (Weather Research and Forecasting – Community Atmosphere Model 5) and GEOS-5 (Goddard Earth Observing System, Version 5)), as well as the UK Meteorological Office Unified Model (UM), the French ALADIN-Climate (Aire Limitée Adaptation dynamique Développement InterNational – Climate, Mallet et al., 2019), the global GEOS-Chem (Goddard Earth Observing System – Chemistry) model, and the E3SM (Energy Exascale Earth System Model) Atmosphere Model (EAM). Measured variables which are compared include the aerosol layer top/bottom boundaries (as determined from lidar and in situ), aerosol extinction (lidar and in situ), black carbon and organic aerosol mass concentrations (SP2 and AMS), carbon monoxide, scattering and absorption Ångström exponents, and the single-scattering albedo (in situ). The new datasets allow an extension beyond previous assessments emphasizing the aerosol layer boundaries only (Das et al., 2017; Koffi et al., 2012). Not all of the models include all of these variables, and no effort is made to standardize model features such as the emissions databases and frequency of initialization. The project confronted the issue of how best to compare infrequently sampled but detailed measurements to frequently sampled but coarsely resolved model output, through aggregating both measured and model data into approximately 2 by 2∘ grid boxes centered on the routine flight track (see Fig. 19 for an example). This approach is similar to that applied within the AeroCom community (e.g., Katich et al., 2018; Myhre et al., 2013). The smaller grid spacings applied within a larger domain also clarify the ability of models to transport aerosol further offshore. A further study will apply a similar approach to model–observation comparisons based on the 2017 and 2018 deployments.

Shinozuka et al. (2020) assessed modeled aerosol properties through examining three separate layers and concluded that the upper 3–6 km layer generally contains less aerosol in the models than is observed, primarily because the aerosol layer tops are placed too low. Another approach, applied within Doherty et al. (2021) is examining the vertical distribution of aerosol (as in Fig. 19) and those cloud properties (cloud fraction and optical depth) important for quantifying the direct aerosol radiative effect. For example, they find significant differences in both the absolute value and vertical structure of SSA in the observations versus in the models, as well as significant differences between models. Consistent with Shinozuka et al. (2020), the model bias has an altitude dependence that results from the models generally placing the plume at too low an altitude. The covariance in the model bias in extinction and SSA with altitude affects the column SSA – the parameter of interest for determining plume aerosol direct radiative effect. Therefore Doherty et al. (2021) also compare observed to modeled extinction-weighted (or column-aerosol) SSA for the smoke plume. This and related analyses in the paper lay the groundwork for determining which modeled parameters are contributing most to biases in modeled aerosol direct radiative effect of the smoke.

As alluded to in Sect. 2.1, a subordinate, yet important objective for ORACLES was the acquisition of data that can be used for the refinement and testing of retrieval capabilities for instrument concepts that have a potential for deployment to space. Among the remote-sensing instruments participating in ORACLES, the following have a link to space-based instrument concepts: HSRL-2 (future HSRL in space), APR-3 (CloudSat's CPR, GPM's DPR, RainCube's radar, EarthCARE's CPR, and candidate radars for future missions targeting clouds, convection, and precipitation), RSP (APS on Glory), AirMSPI (MAIA), and eMAS (MODIS). Examples of algorithm developments using ORACLES data include Xu et al. (2018), Segal-Rozenhaimer et al. (2018), and Miller et al. (2020), who created new cloud retrieval algorithms for the AirMSPI and RSP instruments. Additionally, the NASA Ames 4STAR instrument was used to provide AERONET-like retrievals of aerosol microphysics on the basis of sky radiance measurements from an airborne platform (Pistone et al., 2019), the SSFR instrument provided within-atmosphere spectral radiative flux observations for direct measurements of scene and cloud albedos both from the P-3 and from the ER-2 in 2016 (Cochrane et al., 2019), and HSRL-2 successfully deployed a very stable and accurate density-tuned interferometer as a prototype for spaceborne instrumentation and leveraged the resulting high accuracy in the measurements to infer aerosol microphysical properties (Burton et al., 2018). In combination, these instruments provide a powerful and unprecedented toolset for retrieving atmospheric trace gases, aerosol, and cloud properties, with simultaneous closure opportunities against the spectral radiative flux observations (see Fig. 20). The resulting dataset will be used in the testing of instrument and algorithm concepts for future satellite missions, such as the NASA Aerosol, Clouds-Convection and Precipitation (ACCP) mission, recommended by the 2018 Decadal Survey for Earth Observations from Space (National Academies of Sciences, Engineering, and Medicine, 2018) and the NASA Plankton, Aerosol, Cloud, Ocean Ecosystem (PACE) mission (Werdell et al., 2019), due for launch in 2023.