ISDAC cloud droplet number closure study: aerosol size distributions and case selection
Poster PDF
Authors
Steven J. Ghan — Pacific Northwest National Laboratory
Mikhail Ovchinnikov — Pacific Northwest National Laboratory
Charles Andrew Brock — NOAA - Earth System Research Laboratory
Don R. Collins — University of California, Riverside
J. Walter Strapp — Environment Canada
Peter Liu — Environment Canada
Alla Zelenyuk-Imre — Pacific Northwest National Laboratory
Ann Marie Macdonald — Environment Canada
Warren Richard Leaitch — Imre Consulting - Climate Chem. Meas. and Research Section
Michael Earle — Environment Canada
Nicole Shantz — Environment Canada
Category
Aerosol-Cloud-Radiation Interactions
Description
A cloud droplet number closure study is initiated using observational data from the U.S. Department of Energy (DOE) Indirect and Semi-Direct Aerosol Campaign (ISDAC). Aerosol, cloud droplet, and atmospheric state measurements were obtained using the National Research Council of Canada (NRC) Convair-580 aircraft. Aerosol composition was determined using a Single Particle Laser Ablation Time-of-flight mass spectrometer (SPLAT), and aerosol concentration and size measurements were obtained using an Ultra-High Sensitivity Aerosol Spectrometer (UHSAS; size range 0.055–1.0 µm), Passive Cavity Aerosol Spectrometer Probe (PCASP-100X; size range 0.12–3.0 µm), and Forward-Scattering Spectrometer Probe (FSSP-300; size range 0.3–20 µm). The performance of the aerosol probes was assessed by comparison with ground-based measurements from the North Slope of Alaska (NSA) site during missed approaches at Barrow and/or measurements from the National Oceanic and Atmospheric Administration (NOAA) P-3 aircraft during periods of coordinated flying. The size distributions from the PCASP-100X compared well with independent measurements from the NSA ground site and NOAA P-3 and are used in the modeling component of the droplet closure study. Size distributions from the UHSAS and FSSP-300 are used in limited capacity; the former are subject to instrumental artifacts that depend on particle size, and the latter show significant variation in humid conditions. The present work details the determination of aerosol size distributions and identifies cases appropriate for droplet closure. A sample case is presented to demonstrate how the aerosol size and composition data are used to compute cloud droplet concentrations in an adiabatic parcel model. A preliminary representation of the vertical (updraft) velocity is used in the parcel model, which is guided by results from large eddy simulations (LES). In addition, since aerosol size distributions are of interest to the broader modeling community, selected distributions are provided for periods of interest throughout the ISDAC project.
