Preprints
https://doi.org/10.5194/acp-2021-153
https://doi.org/10.5194/acp-2021-153

  10 Mar 2021

10 Mar 2021

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

Cloud Drop Number Concentrations over the Western North Atlantic Ocean: Seasonal Cycle, Aerosol Interrelationships, and Other Influential Factors

Hossein Dadashazar1, David Painemal2,3, Majid Alipanah4, Michael Brunke5, Seethala Chellappan6, Andrea F. Corral1, Ewan Crosbie2,3, Simon Kirschler7, Hongyu Liu8, Richard Moore2, Claire Robinson2,3, Amy Jo Scarino2,3, Michael Shook2, Kenneth Sinclair9,10, K. Lee Thornhill2, Christiane Voigt7, Hailong Wang11, Edward Winstead2,3, Xubin Zeng5, Luke Ziemba2, Paquita Zuidema6, and Armin Sorooshian1,5 Hossein Dadashazar et al.
  • 1Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, USA
  • 2NASA Langley Research Center, Hampton, VA, USA
  • 3Science Systems and Applications, Inc., Hampton, VA, USA
  • 4Department of Systems and Industrial Engineering, University of Arizona, Tucson, AZ, USA
  • 5Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ, USA
  • 6Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA
  • 7Institute of Atmospheric Physics, German Aerospace Center
  • 8National Institute of Aerospace, Hampton, VA, USA
  • 9NASA Goddard Institute for Space Studies, New York, NY, USA
  • 10Universities Space Research Association, Columbia, MD, USA
  • 11Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA, USA

Abstract. Cloud drop number concentrations (Nd) over the western North Atlantic Ocean (WNAO) are generally highest during the winter (DJF) and lowest in summer (JJA), in contrast to aerosol proxy variables (aerosol optical depth, aerosol index, surface aerosol mass concentrations, surface cloud condensation nuclei [CCN] concentrations) that generally peak in spring (MAM) and JJA with minima in DJF. Using aircraft, satellite remote sensing, ground-based in situ measurements data as well as reanalysis data, we characterize factors explaining the divergent seasonal cycles and furthermore probe into factors influencing Nd on seasonal time scales. The results can be summarized well by features most pronounced in DJF, including features associated with cold air outbreak (CAO) conditions such as enhanced values of CAO index, planetary boundary layer height (PBLH), low-level liquid cloud fraction, and cloud-top height, in addition to winds aligned with continental outflow. Data sorted into high and low Nd days in each season, especially in DJF, revealed that all of these conditions were enhanced on the high Nd days, including reduced sea level pressure and stronger wind speeds. Although aerosols may be more abundant in MAM and JJA, the conditions needed to activate those particles into cloud droplets are weaker than in colder months, which is demonstrated by calculations of strongest (weakest) aerosol indirect effects in DJF (JJA) based on comparing Nd to perturbations in four different aerosol proxy variables (total and sulfate aerosol optical depth, aerosol index, surface mass concentration of sulfate). We used three machine learning models and up to 12 input variables to infer about most influential factors related to Nd for DJF and JJA, with the best performance obtained with gradient boosted regression tree (GBRT) analysis. The model results indicated that cloud fraction was the most important input variable, followed by some combination (depending on season) of CAO index and surface mass concentrations of sulfate and organic carbon. Future work is recommended to further understand aspects uncovered here such as impacts of free tropospheric aerosol entrainment on clouds, wet scavenging and giant CCN effects on aerosol–Nd relationships, updraft velocity, and vertical structure of cloud properties such as adiabaticity that impact the satellite estimation of Nd.

Hossein Dadashazar et al.

Status: open (until 05 May 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Hossein Dadashazar et al.

Hossein Dadashazar et al.

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Short summary
This study investigates the seasonal cycle of cloud drop number concentration (Nd) over the western North Atlantic Ocean (WNAO) using multiple datasets. Reasons for the puzzling discrepancy between the seasonal cycles of Nd and aerosol concentration were identified. Results indicate that Nd is highest in winter (when aerosol proxy values are often lowest) due to conditions both linked to cold air outbreaks and that promote greater droplet activation.
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