AEROSOL–CLOUD–METEOROLOGY INTERACTION AIRBORNE FIELD INVESTIGATIONS Using Lessons Learned from the U.S. West Coast in the Design of ACTIVATE off the U.S. East Coast ARMIN SOROOSHIAN, BRUCE ANDERSON, SUsaNNE E. BAUER, RACHEL A. BRAUN, BRIAN CAIRNS, EWAN CROsbIE, HOssEIN DADasHAZAR, GLENN DISKIN, RICHARD FERRARE, RICHARD C. FLagaN, JOHNATHAN HAIR, CHRIS HOSTETLER, HafLIDI H. JONssON, MARY M. KLEB, HONGYU LIU, ALEXANDER B. MACDONALD, ALLISON MCCOMISKEY, RICHARD MOORE, DAVID PAINEMAL, LYNN M. RUssELL, JOHN H. SEINFELD, MICHAEL SHOOK, WILLIAM L. SMITH JR., KENNETH THORNHILL, GEORGE TSELIOUDIS, HAILONG WANG, XUBIN ZENG, BO ZHANG, LUKE ZIEMba, AND PAQUITA ZUIDEMA Insights and limitations during 500+ flight hours with a single aircraft are used to motivate the dual-aircraft approach in ACTIVATE to study aerosol–cloud–meteorology interactions. he latest Intergovernmental Panel on Climate labor-intensive, expensive, and challenging efforts Change (IPCC 2013) report stated that the largest have resulted in several datasets that have not been T uncertainty in estimating global anthropogenic fully exploited because of inconsistencies in measure- radiative forcing is associated with the interactions ments and flight strategies between campaigns, and of aerosol particles with clouds. Furthermore, the the extensive time and resources needed for quality latest Decadal Survey for Earth Science (National assurance and in-depth analysis of the vast amount Academies of Sciences, Engineering, and Medicine of data collected (Sorooshian et al. 2018). The im- 2017) recommended a designated mission to study portance of this research field has been motivated in aerosols and clouds as one of the “most important” countless reports and review papers that examine the priorities for the Earth observing system. Warm state of the field and suggest future research needed marine boundary layer (MBL) clouds cover more to help answer some of the most pressing problems than 45% of the ocean surface (Warren et al. 1998) (Fan et al. 2016; Seinfeld et al. 2016; Wood et al. 2016; and consequently exert a large net cooling effect Mülmenstädt and Feingold 2018). (Hartmann et al. 1992). They are of special interest We begin by reflecting on a multiyear effort that owing to their pivotal role in unresolved climate aimed to address these limitations by keeping several change questions associated with climate sensitivity features in common: i) core group of instruments, ii) a and cloud feedbacks (Mülmenstädt and Feingold single aircraft, iii) geographic region, iv) time of year, 2018). Recent decades have experienced a prolifera- and v) quality control and assurance strategy. The tion of field experiments targeting aerosol–cloud– region off the coast of California is one of the most meteorology interactions for MBL clouds. These extensively studied for aerosol–cloud–meteorology AMERICAN METEOROLOGICAL SOCIETY AUGUST 2019 | 1511 interactions owing to proximity to aircraft bases and Extensive ship traffic in this study region served as a a wide range in aerosol concentrations coupled to a focal point of many experiments (e.g., Durkee et al. persistent marine cloud deck, especially in the sum- 2000; Russell et al. 2013) since the formation of ship mertime when experiments are usually conducted. tracks represents one of the clearest visual demonstra- tions of how aerosol perturbations impact clouds when viewed from space. Diversity of other pollutant sources, with varying characteristic physical and chemical properties, provides an additional benefit for studying this region. The lessons learned from the California studies sponsored by the Office of Naval Research (ONR) pro- vide motivation for a five-year NASA Earth Venture Suborbital (EVS-3) investigation off the opposite coast of the United States. A dual-aircraft approach with combined in situ and remote sensing instrumentation will be coupled to an unprecedented number of flights to maximize statistics in a region with diverse aerosol and meteorological condi- tions, including the continuum of warm cloud types spanning strati- form to cumulus. The Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment FIG. 1. Summary of flights conducted with the CIRPAS Twin Otter (ACTIVATE) is described in detail, in six summertime campaigns between 2005 and 2016. The coordi- nates for the expanded white box are 35.3°–37.5°N, 121.5°–123.5°W. with a description of data analysis (MASE = Marine Stratus/Stratocumulus Experiment; E-PEACE = East- and multiscale modeling that will ern Pacific Emitted Aerosol Cloud Experiment; NiCE = Nucleation address the complexity of the pro- in Cloud Experiment; BOAS = Biological and Oceanic Atmospheric cesses being examined ranging in Study; FASE = Fog and Stratocumulus Evolution Experiment.) spatial scale from ~10–7 to 106 m (i.e., AFFILIATIONS: SOROOSHIAN—Department of Chemical and La Jolla, California; WANG—Atmospheric Sciences and Global Environmental Engineering, and Department of Hydrology and Change Division, Pacific Northwest National Laboratory, Richland, Atmospheric Sciences, The University of Arizona, Tucson, Arizona; Washington; ZENG—Department of Hydrology and Atmospheric ANDERSON, DISKIN, FERRARE, HAIR, HOSTETLER, KLEB, MOORE, SHOOK, Sciences, University of Arizona, Tucson, Arizona; ZUIDEMA—Rosen- SMITH, AND ZIEMba—NASA Langley Research Center, Hampton, stiel School of Marine and Atmospheric Science, University of Virginia; BAUER, CAIRNS, AND TSELIOUDIS—NASA Goddard Institute Miami, Miami, Florida for Space Studies, New York, New York; BRAUN, DADasHAZAR, CORRESPONDING AUTHOR: Armin Sorooshian, AND MACDONALD—Department of Chemical and Environmental [email protected] Engineering, University of Arizona, Tucson, Arizona; CROsbIE, The abstract for this article can be found in this issue, following the table PAINEMAL, AND THORNHILL—NASA Langley Research Center, and of contents. Science Systems and Applications, Inc., Hampton, Virginia; FLagaN DOI:10.1175/BAMS-D-18-0100.1 AND SEINFELD—Department of Chemical Engineering, California Institute of Technology, Pasadena, California; JONssON—Naval A supplement to this article is available online (10.1175/BAMS-D-18-0100.2) Postgraduate School, Monterey, California; LIU AND ZHANG— In final form 1 April 2019 National Institute of Aerospace, Hampton, Virginia; MCCOMISKE— ©2019 American Meteorological Society Brookhaven National Laboratory, Upton, New York; RUssELL— For information regarding reuse of this content and general copyright Scripps Institution of Oceanography, University of California, information, consult the AMS Copyright Policy. 1512 | AUGUST 2019 single particle to synoptic scale). We conclude with a water paths (LWPs) ranged between 40 and 760 m preview of how the generated results can be used by and 10 and 310 g m–2, respectively. Clouds were typi- the research community. cally subadiabatic (average adiabaticity = 0.766 ± 0.134; Braun et al. 2018). On average, the observed COASTAL CALIFORNIA FLIGHTS. Data LWC lapse rate tended to be a fairly constant fraction were collected using the Center for Interdisciplinary of the adiabatic LWC lapse rate through the bottom Remotely-Piloted Aircraft Studies (CIRPAS) Twin 90% by height of the cloud; however, in the top 10% Otter on 113 flight days for a total of 514 flight hours of the cloud, a sharp decrease in LWC was observed in the areas shown in Fig. 1. The payload is sum- [Fig. 4 of Braun et al. (2018)], most likely due to marized in Table ES1 (https://doi.org/10.1175/BAMS processes such as cloud-top entrainment and pre- -D-18-0100.2) with datasets provided by Sorooshian cipitation. Cloud droplet number concentrations Nd et al. (2017, 2018). The general flight pattern included reached as low as ~20 cm–3 and as high as ~400 cm–3. level legs at various altitudes (below cloud, in cloud at This broad range is driven by variability in out-of- different levels, above cloud) with occasional vertical cloud aerosol levels, with both sub- and above-cloud soundings as either slants or spirals (Fig. 2). The level Passive Cavity Aerosol Spectrometer Probe (PCASP) legs (~10–15 min) were meant to generate statistics at a concentrations Dp (~0.1–2.6 µm) spanning three fixed altitude, in addition to providing sufficient time orders of magnitude. Aside from sea spray and for measurements with longer time resolutions (e.g., marine biogenic sources of aerosols (Modini et al. cloud water collection, scanning mobility particle 2015), ship exhaust was a major subcloud source sizers) and enhancing accuracy of measurements (Coggon et al. 2012; Wang et al. 2014). The major requiring the aircraft to remain level (e.g., wind mea- sources impacting the above-cloud aerosol budget surements). The soundings (~10–15 min at an incline were transported continental emissions of wildfire rate of 90 m min–1) were useful for characterizing plumes (Maudlin et al. 2015; Mardi et al. 2018), dust, the vertical environmental profile. For the full set of biogenic secondary organic aerosol (SOA), and urban calculations desired for aerosol–cloud interactions in pollution (Hegg et al. 2010; Prabhakar et al. 2014; these campaigns, it was necessary to have data below, Wang et al. 2014). The aerosols above cloud in the in, and above clouds, which amounted to a total of 297 cases, hereinafter referred to as “cloud events.” This number was reduced from the 439 events sampled owing to lack of data or poor data quality for at least one of the requisite parameters. Histograms of relevant parameters for aerosol– cloud interactions