Received: 30 Dec 2020 – Accepted for review: 20 Jan 2021 – Discussion started: 21 Jan 2021
Abstract. In southern Africa, widespread agricultural fires produce substantial biomass burning (BB) emissions over the region. The seasonal smoke plumes associated with these emissions are then advected westward over the persistent stratocumulus cloud deck in the Southeast Atlantic (SEA) Ocean, resulting in aerosol effects which vary with time and location. Much work has focused on the effects of these aerosol plumes, but previous studies have also described an elevated free-tropospheric water vapor signal over the SEA. Water vapor influences climate in its own right, and it is especially important to consider atmospheric water vapor when quantifying aerosol-cloud interactions and aerosol radiative effects. Here we present airborne observations made during the NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign over the SEA Ocean. In observations collected from multiple independent instruments on the NASA P-3 aircraft (from near-surface to 6–7 km), we observe a strongly linear correlation between pollution indicators (carbon monoxide (CO) and aerosol loading) and atmospheric water vapor content, seen at all altitudes above the boundary layer. The focus of the current study is on the especially strong correlation observed during the ORACLES-2016 deployment (out of Walvis Bay, Namibia), but a similar relationship is also observed in the August 2017 and October 2018 ORACLES deployments.
Using ECMWF and MERRA-2 reanalyses and specialized WRF-Chem simulations, we trace the plume-vapor relationship to an initial humid, smoky continental source region, where it is subjected to conditions of strong westward advection, namely the South African Easterly Jet (AEJ-S). Our analysis indicates that airmasses likely left the continent with the same relationship between water vapor and carbon monoxide as was observed by aircraft. This linear relationship developed over the continent due to daytime convection within a deep continental boundary layer (up to 5–6 km) which produced fairly consistent vertical gradients in CO and water vapor, decreasing with altitude and varying in time, but does not originate as a product of the BB combustion itself. Due to a combination of conditions and mixing between the smoky, moist continental boundary layer and the dry and fairly clean upper-troposphere air above (~6 km), the smoky, humid air is transported by strong zonal winds and then advected over the SEA (to the ORACLES flight region) following largely isentropic trajectories. HYSPLIT back trajectories support this interpretation. Better understanding of the conditions and processes which cause the water vapor to covary with plume strength is important to accurately quantify direct, semi-direct, and indirect aerosol effects in this region.
The manuscript titled “Exploring the elevated water vapor signal associated with the free-tropospheric biomass burning plume over the southeast Atlantic Ocean” by Kristina Pistone and co-authors investigated the association of CO-q with ORACLES aircraft data over SEA ocean. They have also analyzed the reanalysis and model simulations to understand the meteorological and dynamical dependence of BB plume-water vapour relationship. This manuscript is well written and scientifically sound. I recommend the publication of this manuscript in ACP.
The explanation for the source of water vapour in the continental plume and its close association with BB CO and aerosols is not adequate. Since reanalysis and model results also showed similar variabilities, this could be of a meteorological coincidence rather than direct emissions. But why such a strong association exist between CO and q is not yet clear and needs to be explained in detail. Is there any study on source tagging or isotopic measurements of water vapour and aerosols over SEA?
Authors discussed the influence of boundary layer evolution over land on the vertical transport of CO and water vapour over the continents. I could not find further quantitative supporting information on the boundary layer parameters (boundary layer height, fluxes: SHF and LHF etc) to supplement the arguments. Small write-up on the general boundary layer features and its diurnal structure during the BB events could be useful.
The year-to-year variability of CO-q relationship is worth noting. Authors mentioned the airmass history over the BB regions, but more information is required on this point. Whether airmass pattern shows significant difference between 2016, 2017 and 2018? Notwithstanding the variability in time and meteorological conditions, what about the CO-q association for co-located measurements made during 2016, 2017 and 2018? Whether re-analysis and model simulations also depict weak association during 2017 and 2018?
Section 4.1 analyze the isentropic and kinematic airmass back trajectories using HYSPLIT. Though authors made broad comments on the usefulness and issues of back trajectory analysis, this section did not add more to the association of CO-q. Page 32, Line 1-4: This point is interesting, but needs more clarity and supporting evidence.
There are several studies discussed the radiative implications of aerosols on water vapor and diurnal evolution of boundary layer over the continental locations. How does BB aerosols influence the CO-q association? I wonder whether diabatic heating due to absorbing aerosols has any effect on the elevated layers of water vapour. Moreover, photochemical oxidations and chemical reactions involving CH4 and OH can also affect the concentration of CO and water vapour. Though the strength of these mechanisms may not be adequate enough to explain the observed CO-q association, it is better to mention these possibilities in the discussion for the sake of completion.
Figure 2 is interesting. Authors mentioned that the humidity datasets (aircraft, COMA, WISPER) differ for measurements within the PBL and rapidly changing aircraft conditions. I still not able to understand why only PBL humidity measured from the three instruments differ? What is the problem with PBL humidity and why aircraft is more stable (fewer ascents/descents) in the free troposphere compared to PBL. What is the rationale for omitting the PBL data is not clear? Is it possible to screen the data close to clouds?
Page 11, Line 7-8: What is the rationale for selecting PBL height as 2 km? How do authors measure the PBL height (Page 12, Line 5)?
Page 13, Line 8-10: To assess the possibility of the hygroscopic growth of aerosols on the AOD versus q relationship, authors have to provide the ranges of relative humidity.
Page 25, line 29: replace “continental boundary layer over land” with “continental boundary layer” (or boundary layer over land).
Using aircraft-based measurements off the Atlantic coast of Africa, we found that the springtime smoke plume there was strongly linearly correlated with the amount of water vapor in the atmosphere (where there is more smoke, there is more humidity). We see the same general feature in satellite-assimilated (reanalyses) and free-running models, which suggests that the humidity-smoke relationship is not a product of burning but originates over the continent and is transported without mixing.
Using aircraft-based measurements off the Atlantic coast of Africa, we found that the springtime...
