Fires in Africa affect clouds and climate over the Atlantic Ocean

Key Points & Overview

  • ORACLES aircraft campaign studying smoke-cloud-climate interactions over western Africa/southeast Atlantic Ocean
  • Strong relationship found between below-cloud smoke and cloud properties, but weak relationship between above-cloud smoke and cloud properties
  • Accounting for the time it takes to mix smoke from above-cloud into the cloudy layer will be important for future work
Rob Wood
Michael Diamond in front of the P-3 Orion research aircraft used in all three NASA ORACLES campaigns before a flight out of São Tomé on October 10th, 2018. Photo credit: Rob Wood.

In order to better understand this region and the effects these smoke-cloud interactions have for climate, NASA has been deploying aircraft outfitted with a suite of scientific instruments to study the clouds and smoke from the air. This field campaign, given the lovably clunky acronym ORACLES (for ObseRvations of Aerosols above CLouds and their intEractionS), was based out of Namibia in September 2016 and São Tomé in August 2017 and October 2018. The NASA P-3 Orion aircraft, which can fly through and sample the smoke and clouds directly, was used in all three years and is pictured (with the author) in Image 1.

My new paper, written with Professor Rob Wood (University of Washington Department of Atmospheric Sciences) and several colleagues from other universities and NASA centers, was recently published in the open-access peer-reviewed journal Atmospheric Physics and Chemistry about results from the September 2016 ORACLES deployment. By coincidence, we started writing the paper during the 2017 deployment, and it was published in the middle of the 2018 deployment!

Michael Diamond
Image 2.  The relationship between below-cloud CCN (left) and cloud properties (Nd) is much stronger than above-cloud (right). CCN is measured at 0.3% supersaturation (SS) in units of number of particles per cubic centimeter (cc). Nd is measured by the Phase Doppler Interferometer (PDI) instrument in units of number of droplets per cubic centimeter. The markers refer to different maneuvers being undertaken by the P-3. CLD refers to level in-cloud legs, SAW to sawtooth legs (porpoising above, through, and below the clouds), SQS to square spirals (spiral ascents or descents around a fixed location), and RMP to ramps (ascents or descents where the plane also travels horizontally). Purple lines show the mean slope (solid) and its 99% confidence interval (dashed).

Our main result, summarized in Image 2, is that the relationship between cloud properties (here given as the number of droplets in a given volume of air, or Nd) and the number of smoke and other airborne particles that can form cloud droplets (cloud condensation nuclei, or CCN) is as strong as expected when measuring smoke beneath the clouds, but the relationship is surprisingly weak when measuring the smoke above the clouds (which is most similar to what satellites can do). Through a combination of observations, modeling, and theory, we argue that this is because it takes a long time — several days — for clouds to mix enough smoke above them into the cloudy layer below, where they can form new cloud droplets. Previous studies in this region using satellites assumed this mixing occurred instantaneously and looked at snapshots of smoke-cloud contact to evaluate their interactions. Our results suggest that these studies are likely to underestimate the true effects.  Future studies will need to account for the prior history of smoke-cloud mixing instead of using instantaneous snapshots in order to accurately quantify the effects of smoke on these stratocumulus clouds and assess their ultimate effect on climate.

 

About the Article

Co-Authors: Amie Dobracki, Steffen Freitag, Jennifer Griswold, Ashley Heikkila, Steve Howell, Mary Kacarab, Jim Podolske, Pablo Saide, Rob Wood
Published: |
Atmospheric Chemistry and Physics