IASC Cryosphere/Atmosphere/Marine Cross-Cutting WorkshopIce at the Interface: Atmosphere-Ice-Ocean Boundary Layer Processes

and Their Role in Polar ChangeJune 25-27, 2012

NOAA David Skaggs Research CenterBoulder, Colorado USA

The atmosphere-ocean boundary layer in which sea ice resides includes many complexprocesses that require a more realistic treatment in GCMs, particularly as models movetoward full earth system descriptions. The primary purpose of the workshop was to de-fine and discuss such coupled processes from observational and modeling points of view,including insight from both the Arctic and Antarctic systems. The workshop met each ofits overarching goals, including fostering collaboration among experimentalists, theoristsand modelers, proposing modeling strategies, and ascertaining data availability and needs.Several scientific themes emerged from the workshop, such as the importance of episodicor extreme events, precipitation, stratification above and below the ice, and the marginalice zone, whose seasonal Arctic migrations now traverse more territory than in the past.

OVERVIEW

Organizers: Elizabeth Hunke Los Alamos National LaboratoryWalt Meier National Snow and Ice Data CenterJim Overland NOAA Pacific Marine Environmental LaboratoryJeremy Wilkinson Scottish Marine InstituteOla Persson CIRES/NOAA Earth System Research Laboratory

Participants: A total of 47 scientists attended the workshop, including 15 early-careerscientists, 5 of whom are currently working toward the Ph.D. The workshop attractedsenior experts (“heroes”) and younger scientists representing a diverse mix of physical sci-ence, biology, chemisry and modeling. A list of participants is included at the end of thisreport. Several other scientists joined the webcast/teleconference set up by NOAA.

Agenda: The agenda was organized each day beginning with a biogeochemistry moti-vational talk, followed by in-depth discussions of specific topics. The first day focussedprimarily on ice-ocean interactions, the second day on ice-atmosphere interactions, withbiogeochemistry and future directions (fieldwork and modeling) the focus of the third (half)day. Discussion leaders gave overview talks, leaving roughly half of their allotted time fordiscussion. All participants were invited to bring slides to present informally during thediscussion sessions, and at final count we had 12 topical overviews, 3 talks describing cur-rent practices in GCMs, and 14 informal presentations in support of the discussion. Theagenda appears at the end of this report, including the names of everyone presenting dis-cussion slides in addition to the discussion leaders.

MOSAiC Workshop Synergy: The Boundary Layer (BL) workshop immediately pre-ceded a second IASC-sponsored workshop entitled “Multi-disciplinary drifting Observatoryfor the Study of Arctic Climate” (MOSAiC), and together the two workshops attracted

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more participants than either would have, alone. In addition, ideas arising from modeler-experimentalist interactions during the BL workshop strongly contributed to later fieldworkplanning discussions during the MOSAiC workshop.

Since both workshops were largely funded through IASC, we shared expenses when con-venient. For instance, MOSAiC covered expenses for some of their participants who wishedto arrive early for our workshop, and we covered expenses for some of our participants whowished to stay longer to attend the MOSAiC workshop. Also, MOSAiC covered all of therefreshments during both workshops, while BL workshop funds covered the joint banquet,held at the Gold Hill Inn on Wednesday evening, June 27.

Direct Funding Sources:

IASC €20,500SCAR $4000 for early-career travel onlyCliC 5000 CHF primarily for early-career travel

IASC paid for the workshop dinner and travel expenses for 5 expert (senior) scientistsand 5 early-career scientists. SCAR covered travel expenses for 2 early-career scientistsineligible for IASC funds. CliC covered travel expenses for three others, including 2 early-career scientists also ineligible for IASC funds. Although the US Department of Energy didnot provide a specified amount of funding, it did provide travel reimbursements for DOEscientists (Barton, Elliott, Hunke, Jeffery, Turner) and E. Hunke’s salary for her time spentorganizing the workshop.

SCIENTIFIC OUTCOMES

The atmosphere-ocean boundary layer in which sea ice resides includes many complexprocesses that require a more realistic treatment in GCMs, particularly as models movetoward full earth system descriptions. The primary purpose of the proposed workshopwas to define and discuss such coupled processes from observational and modeling pointsof view, including insight from both the Arctic and Antarctic systems. There were threeoverarching goals.

Goal 1: To foster collaboration among diverse research groups toward better understandingof boundary processes in the atmosphere-ice-ocean system and their contribution to biogeo-chemical cycling within the climate system.

Three collaborative discussion groups formed as a result of the workshop, whose discus-sions are ongoing:- ice-ocean dynamics/stress coupling- carbonate chemistry/dissolved inorganic carbon pump- dimethylsulfide (DMS) and methane.Each of these groups has entrained other scientists who did not attend the workshop, andeach promises fruitful collaborations stemming from close interactions among field experts,theorists, and modelers.

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Although the workshop touched on many different aspects of the atmosphere-ice-oceanboundary layer, a few themes emerged. First, averages do not often apply in polar regions—episodic or extreme events (pulses) are critical for describing the evolution of the system.Examples include fracturing and slippage along leads (which causes the inertial motion ofsea ice to be nonstationery, for instance), plumes due to biomass burning, DMS emissionsfrom the marginal ice zone, the bimodal distribution of clouds that controls the surfaceradiative budget, and freezing fronts passing vertically through sea ice that control seawaterentrainment and thus nutrient availability within the ecosystem.

A second theme could be described in terms of stratification, both above and belowthe ice. For instance, the changing age (thickness) of sea ice is significantly altering dragcharacteristics and mobility. Increased stratification in the western Arctic is modifyingexchanges of heat, salt and nutrients between the ice-ocean boundary layer system and thedeeper ocean, and ocean currents are rapidly intensifying in the western Arctic. Climatemodels include three boundary layers in their atmospheric components: over ice, overocean, and under clouds. However, these models include only one boundary layer in theocean, and it generally is under-resolved. Considerable discussion during the workshopcentered on stratification under the ice, including freshwater lenses and their effects onmomentum coupling and biogeochemistry.

A rather fundamental question arose during the workshop, still needing to be addressed:why is there more snow in the Southern Hemisphere than in the Northern Hemisphere?There are many processes that make the Arctic and Antarctic sea ice pack different, buttwo of these stand out. In the Arctic during the melt season, ponds are ubiquitous on topof sea ice, often sparkling clear blue; in the Antarctic, such ponds are uncommon; instead,liquid water lies at the base of a deep snow pack, often flooded by sea water and refrozen atthe interface. Snow—how much of it falls and how it drifts to define the surface topographyof the pack—is a central component in this difference between the hemispheric ice packs.Furthermore, snow is critical for biogeochemical considerations because, for example, ittraps and concentrates chemicals and other inclusions such as black carbon.

For each of these themes and others, a motivating idea underlying the discussion wasthat fact that the Arctic ice pack is changing. As older ice disappears and the pack becomesyounger, scientists are not sure how the balance of physical processes will change, nor howthese changes will affect the ecosystem. The Antarctic system, which is already largelycomposed of first-year ice, may provide some clue, but it has other dominant processesthat may obscure such a comparison with the Arctic. The ongoing evolution of the Arcticsystem and its fundamental differences with the Antarctic system motivate both modelers,to design more robust and flexible parameterizations, and observationalists, to gather thedata needed to put these changes in context and fully understand them. In addition tostratification effects, marginal ice zone (MIZ) processes, where sea ice is largely brokenup, are likely to become more important in the Arctic. Workshop discussions includedthe effect of wave action in forming the MIZ as well as specific MIZ processes such as theformation of pancake ice, biological and chemical interactions (including nucleation of bothclouds and frazil ice), and the need for floe size distributions in models.

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Goal 2: To develop modeling strategies that capture the important physics in ways appro-priate for earth system modeling.

The modelers in attendance highlighted known weaknesses in existing GCM implemen-tations. Most obvious among these are snow physics over sea ice, fluid flow within sea ice(and related biogeochemical processes), ice-ocean dynamical interactions, and treatmentof clouds and radiation in regions containing sea ice, all of which are being worked on byvarious research groups. The field experts pointed out other weaknesses that the modelershad not fully appreciated. Some of these can be investigated in current models; such “low-hanging fruit” will lead quickly to scientific publications. In particular, roughness lengthover and under sea ice is currently parameterized as a constant value, but could easily bevaried. Questions that can be studied include how the effective surface roughness (andtherefore drag) depends on stratification and on resolution in both time and space. Inaddition, models are ripe for inclusion of new processes, such as under-ice ponds, superim-posed ice (i.e., melt water that infiltrates sea ice and refreezes), improved representationsof sea-water flooding and snow-ice formation, and floe size distribution, all of which willrequire observational data for detailed development.

Goal 3: To determine what observations and measurements are presently available andwhat additional data could be collected for use in both model development and validation.

In addition to those mentioned under Goal 1, some themes were built into the agenda.For instance, biogeochemical connections are a driving force for both observations andmodeling. Throughout the workshop, participants recognized the need to improve ourunderstanding of the basic boundary layer physics in order to properly evaluate the role ofbiogeochemical processes in the system. Feedbacks require complete observations of boththe physical system and its biological and chemical aspects. Such data can inspire newmodeling approaches and will be indispensable for model development and validation.

A working document being compiled by the NCAR CESM Polar Climate Working Groupcollects and describes the kinds of data desired by modelers for calibration and validationof GCMs and the parameterizations used therein. This document was discussed and latercirculated among the workshop participants. Many discussions focussed on what currentmeasurements indicate in terms of the boundary layer physics, and contributed ideas re-garding needed observations and measurements for further discussion during the MOSAiCworkshop. Among the many specific needs discussed during the BL workshop was a generaldesire for more automated in situ measurements.

As a final note, this workshop had a stronger focus on CICE and CESM than other seaice models and GCMs because of the balance of participants. This was due in part becausethe workshop was held in Boulder and attracted a number of CESM modelers from NCAR,and partly because it was organized by the primary CICE model developer (E. Hunke).

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Acronyms:

BL Boundary LayerCESM NCAR’s Community Earth System ModelCICE Los Alamos Sea ice Model (not an acronym)CliC WCRP’s Climate and Cryosphere projectDOE US Department of EnergyDMS DimethylsulfideGCM Global Climate ModelIASC International Arctic Science CommitteeMIZ Marginal Ice ZoneMOSAiC Multi-disciplinary drifting Observatory for the Study of Arctic ClimateNCAR National Center for Atmospheric ResearchNOAA National Oceanic and Atmospheric AdministrationSCAR Scientific Committee for Antarctic ResearchWCRP World Climate Research Program

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Participants † early-career WorkshopName Institution Country fundingSteve Ackley University of Texas San Antonio USA IASCRichard Allard Naval Research Laboratory USAEd Andreas NorthWest Research Associates USA IASCDave Bailey National Center for Atmospheric Research USA† Neil Barton Lawrence Livermore National Laboratory USA† Luke Bennetts University of Adelaide Australia SCAR† Jeff Bowman University of Washington USA IASC† Lauren Candlish University of Manitoba Canada IASC† Sylvia Cole Woods Hole Oceanographic Institution USA CliCClara Deal IARC UAF/ University of Purdue USAGijs de Boer CIRES/NOAA Earth System Research Lab USAKlaus Dethloff Alfred Wegener Institute GermanyScott Elliott Los Alamos National Laboratory USABaylor Fox-Kemper University of Colorado Boulder USASebastian Gerland Norwegian Polar Institute NorwayAndrey Grachev CIRES USADavid Hebert Naval Research Laboratory USAMarika Holland National Center for Atmospheric Research USAElizabeth Hunke Los Alamos National Laboratory USANicole Jeffery Los Alamos National Laboratory USA† Jen Kay National Center for Atmospheric Research USA† Thomas Keitzl Max Planck Institute Germany IASC† Shelly Knuth University of Colorado Boulder USA† Olivier Lecomte Universite catholique de Louvain Belgium CliC† Brice Loose Woods Hole Oceanographic Institution USA IASC† Torge Martin Massachusetts Institute of Technology USAWieslaw Maslowski Naval Postgraduate School USAJulie McClean Scripps Institution of Oceanography USAMiles McPhee McPhee Research Company USA IASCWalt Meier National Snow and Ice Data Center USAMarcel Nicolaus Alfred Wegener Institute GermanyJim Overland NOAA Pacific Marine Environmental Lab USA IASCDon Perovich Cold Regions Research and Engineering Lab USAOla Persson NOAA Earth System Research Laboratory USA† Chris Polashenski Dartmouth College USAMarilyn Raphael University of California Los Angeles USAJeff Ridley UK Met Office UKAndrew Roberts Naval Postgraduate School USA IASC† Ben Saenz University of California Santa Cruz USAMatthew Shupe NOAA Earth System Research Laboratory USAJacqueline Stefels University of Groningen NetherlandsGunilla Svensson Stockholm University SwedenMichael Tjernstrom Stockholm University SwedenAdrian Turner Los Alamos National Laboratory USA CliC† Teresa Valkonen University of Helsinki Finland IASC† Gaelle Verge-Depre Universite catholique de Louvain Belgium SCARShouping Wang Naval Research Laboratory USA