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Droplet closure analysis of Arctic stratocumulus clouds during ISDAC

 
Poster PDF

Authors

Michael Earle — Environment Canada
Peter Liu — Environment Canada
J. Walter Strapp — Environment Canada
Greg McFarquhar — University of Oklahoma
Alla Zelenyuk-Imre — Pacific Northwest National Laboratory
Dan Imre — Imre Consulting
Mikhail Ovchinnikov — Pacific Northwest National Laboratory
Nicole Shantz — Environment Canada
Steven J. Ghan — Pacific Northwest National Laboratory
Warren Richard Leaitch — Imre Consulting - Climate Chem. Meas. and Research Section

Category

Aerosol-Cloud-Radiation Interactions

Description

Figure 1: Comparison of cloud droplet number concentrations (CDNC), calculated using an adiabatic parcel model with two different representations of the updraft velocity, with CDNC values observed in-cloud. Six cases during ISDAC are considered: three from flights in clean aerosol conditions (F08, F30, F31) and three from flights in polluted aerosol conditions associated with biomass burning plumes (F23, F26, F27).

Motivated by the climate implications of aerosol-cloud relationships in the Arctic, the Indirect and Semi-Direct Aerosol Campaign (ISDAC) was conducted in Alaska in April 2008. An instrumented research aircraft provided the platform for process-based analysis of these relationships through an aerosol-cloud droplet closure study. Measurements of aerosol particles below the base of stratocumulus clouds, representing potential cloud condensation nuclei, were obtained using an optical probe (PCASP-100X) for number concentration and size information and a single particle mass spectrometer (SPLAT II) for composition information. Measurements of droplet properties within cloud were obtained using two optical probes (CDP and FSSP-100). The updraft velocity (w) was measured in-cloud using a gust probe and also simulated in a large-eddy simulation cloud model (LES). Using a separate adiabatic parcel model, representations of w based on both the measurements and LES output were used to simulate cloud droplet number concentrations (CDNC), with the objective of increasing knowledge of how updrafts influence CDNC.

Cases considered in the analysis were separated into two regimes with respect to aerosol particle number concentration (Na): three clean cases (Na < 250/cc), characteristic of background and boundary-layer concentrations in Arctic spring; and three polluted cases (Na > 250/cc) influenced by biomass burning plumes. The updraft velocity for each case was considered in terms of the normally distributed probability distribution function (PDF). Separate sets of parcel model simulations, in which CDNC was computed by integrating over the in-cloud PDF or by using a single, characteristic updraft velocity w*, gave modelled CDNC values within the estimated measurement uncertainty (Figure 1). However, the specific values of w* that produced the best agreement between measured and modelled CDNC differed for clean and polluted cases, owing to differences in activation between the regimes. Updraft velocity PDFs from the LES compared favourably with observations, pointing to their potential use in model parameterizations when measurements are not available. The results from this analysis improve our understanding of updraft-CDNC relationships in Arctic clouds, helping us move toward a better representation of aerosol activation processes in global models.