Articles | Volume 15, issue 4
Atmos. Chem. Phys., 15, 1745–1768, 2015
Atmos. Chem. Phys., 15, 1745–1768, 2015

Research article 19 Feb 2015

Research article | 19 Feb 2015

Observations of the temporal variability in aerosol properties and their relationships to meteorology in the summer monsoonal South China Sea/East Sea: the scale-dependent role of monsoonal flows, the Madden–Julian Oscillation, tropical cyclones, squall lines and cold pools

J. S. Reid1, N. D. Lagrosas2, H. H. Jonsson3, E. A. Reid1, W. R. Sessions4, J. B. Simpas2, S. N. Uy2, T. J. Boyd5, S. A. Atwood6, D. R. Blake7, J. R. Campbell1, S. S. Cliff8, B. N. Holben9, R. E. Holz10, E. J. Hyer1, P. Lynch11, S. Meinardi7, D. J. Posselt12, K. A. Richardson1, S. V. Salinas13, A. Smirnov14, Q. Wang3, L. Yu15, and J. Zhang16 J. S. Reid et al.
  • 1Marine Meteorology Division, Naval Research Laboratory, Monterey, CA, USA
  • 2Manila Observatory, Ateneo de Manila University campus, Quezon City, Philippines
  • 3Department of Meteorology, Naval Postgraduate School, Monterey, CA, USA
  • 4CSC, Naval Research Laboratory, Monterey, CA, USA
  • 5Biogeochemistry Section, Naval Research Laboratory, Washington, DC, USA
  • 6Dept. of Atmospheric Science, Colorado State University, Ft. Collins, CO, USA
  • 7Department of Chemistry, University of California, Irvine, CA, USA
  • 8Department of Applied Science, University of California, Davis, CA, USA
  • 9NASA Goddard Space Flight Center, Biospheric Sciences Laboratory, Code 618, MD, USA
  • 10Space Sciences Engineering Center, University of Wisconsin, Madison, WI, USA
  • 11CSC Inc. at Naval Research Laboratory, Monterey, CA, USA
  • 12Dept. of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, MI, USA
  • 13Centre for Remote Imaging Sensing and Processing, National University of Singapore, Singapore
  • 14Science Systems and Applications, Inc., Lanham, MD, USA
  • 15Dept. of Civil and & Environmental Engineering, National University of Singapore, Singapore
  • 16Dept. of Meteorology, University of North Dakota, Grand Forks, ND, USA

Abstract. In a joint NRL/Manila Observatory mission, as part of the Seven SouthEast Asian Studies program (7-SEAS), a 2-week, late September 2011 research cruise in the northern Palawan archipelago was undertaken to observe the nature of southwest monsoonal aerosol particles in the South China Sea/East Sea (SCS/ES) and Sulu Sea region. Previous analyses suggested this region as a receptor for biomass burning from Borneo and Sumatra for boundary layer air entering the monsoonal trough. Anthropogenic pollution and biofuel emissions are also ubiquitous, as is heavy shipping traffic. Here, we provide an overview of the regional environment during the cruise, a time series of key aerosol and meteorological parameters, and their interrelationships. Overall, this cruise provides a narrative of the processes that control regional aerosol loadings and their possible feedbacks with clouds and precipitation. While 2011 was a moderate El Niño–Southern Oscillation (ENSO) La Niña year, higher burning activity and lower precipitation was more typical of neutral conditions. The large-scale aerosol environment was modulated by the Madden–Julian Oscillation (MJO) and its associated tropical cyclone (TC) activity in a manner consistent with the conceptual analysis performed by Reid et al. (2012). Advancement of the MJO from phase 3 to 6 with accompanying cyclogenesis during the cruise period strengthened flow patterns in the SCS/ES that modulated aerosol life cycle. TC inflow arms of significant convection sometimes span from Sumatra to Luzon, resulting in very low particle concentrations (minimum condensation nuclei CN < 150 cm−3, non-sea-salt PM2.5 < 1 μg m−3). However, elevated carbon monoxide levels were occasionally observed suggesting passage of polluted air masses whose aerosol particles had been rained out. Conversely, two drier periods occurred with higher aerosol particle concentrations originating from Borneo and Southern Sumatra (CN > 3000 cm−3 and non-sea-salt PM2.5 10–25 μg m−3). These cases corresponded with two different mechanisms of convection suppression: lower free-tropospheric dry-air intrusion from the Indian Ocean, and large-scale TC-induced subsidence. Veering vertical wind shear also resulted in aerosol transport into this region being mainly in the marine boundary layer (MBL), although lower free troposphere transport was possible on the western sides of Sumatra and Borneo. At the hourly time scale, particle concentrations were observed to be modulated by integer factors through convection and associated cold pools. Geostationary satellite observations suggest that convection often takes the form of squall lines, which are bowed up to 500 km across the monsoonal flow and 50 km wide. These squall lines, initiated by cold pools from large thunderstorms and likely sustained by a veering vertical wind shear and aforementioned mid-troposphere dry layers, propagated over 1500 km across the entirety of the SCS/ES, effectively cutting large swaths of MBL aerosol particles out of the region. Our conclusion is that while large-scale flow patterns are very important in modulating convection, and hence in allowing long-range transport of smoke and pollution, more short-lived phenomena can modulate cloud condensation nuclei (CCN) concentrations in the region, resulting in pockets of clean and polluted MBL air. This will no doubt complicate large scale comparisons of aerosol–cloud interaction.

Short summary
This paper reports on the first measurements of aerosol particles embedded in the convectively active southwest monsoonal flow of the South China Sea. The paper describes the research cruise and discusses how variability in aerosol characteristics relates to regional meteorological phenomena such as and the Madden Julian Oscillation, tropical cyclones, squall lines and the monsoonal flow itself. Of special interest is how aerosol transport relates to meteorological drivers of convective activity.
Final-revised paper