21 Jan 2021

21 Jan 2021

Review status: this preprint is currently under review for the journal ACP.

Exploring the elevated water vapor signal associated with the free-tropospheric biomass burning plume over the southeast Atlantic Ocean

Kristina Pistone1,2, Paquita Zuidema3, Robert Wood4, Michael Diamond4, Arlindo M. da Silva5, Gonzalo Ferrada6, Pablo Saide7, Rei Ueyama2, Ju-Mee Ryoo8,2, Leonhard Pfister2, James Podolske2, David Noone9,10, Ryan Bennett1, Eric Stith1,a, Gregory Carmichael6, Jens Redemann11, Connor Flynn11, Samuel LeBlanc1,2, Michal Segal-Rozenhaimer1,2,12, and Yohei Shinozuka1,2 Kristina Pistone et al.
  • 1Bay Area Environmental Research Institute, Moffett Field, CA, USA
  • 2NASA Ames Research Center, Moffett Field, CA, USA
  • 3University of Miami/Rosenstiel School of Marine and Atmospheric Science, Miami, FL, USA
  • 4University of Washington, Seattle, WA, USA
  • 5NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 6Center for Global and Regional Environmental Research, The University of Iowa, Iowa City, IA, USA
  • 7University of California, Los Angeles, Los Angeles, CA, USA
  • 8Science and Technology Corporation, Moffett Field, CA, USA
  • 9Department of Physics, University of Auckland, Auckland, NZ
  • 10College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, OR, USA
  • 11University of Oklahoma, Norman, OK, USA
  • 12Department of Geophysics, Porter School of the Environment and Earth Sciences, Tel-Aviv University, Tel-Aviv, Israel
  • anow at: JT4, LLC, Las Vegas, NV, USA

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.

Kristina Pistone et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2020-1322', Anonymous Referee #1, 15 Feb 2021
  • RC2: 'Review of Pistone et al.', Anonymous Referee #2, 29 Mar 2021

Kristina Pistone et al.

Data sets

ORACLES 2016 archival dataset for P-3 observations ORACLES Science Team

ORACLES 2017 archival dataset for P-3 observations ORACLES Science Team

ORACLES 2018 archival dataset for P-3 observations ORACLES Science Team

Kristina Pistone et al.


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Short summary
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.