Sea Ice Thermodynamics and ITD considerations Marika Holland NCAR

Sea ice thermodynamics Simulate vertical heat transfer (conduction, SW absorption) Balance of fluxes at ice surface (ice-atm exchange, conduction, ice melt) Balance of fluxes at ice base (ice-ocn exchange, conduction, ice melt/growth) F ocn F sw  F sw F LW F SH F LH hihi hshs T1T1 T2T2 T3T3 T4T4 -k dT/dz

Vertical heat transfer (from Light, Maykut, Grenfell, 2003) (Maykut and Untersteiner, 1971; Bitz and Lipscomb, 1999; others) Assume brine pockets are in thermal equilibrium with ice Heat capacity and conductivity are functions of T/S of ice Assume constant salinity profile Assume non-varying density Assume pockets/channels are brine filled

Albedo Parameterized albedo depends on surface state (snow, temp, h i ). Issues: Implicit ponds, optically thick snow, no snow aging, constant fraction of SW absorbed in surface layer, constant extinction coefficients (Perovich et al., 2002)

New Albedo Formulation High ice/snow albedo due to multiple scattering associated with individual snow grains, inclusions of gas, brine, etc. New multiple-scattering sea ice radiative transfer has been developed by Bruce Briegleb and Bonnie Light Dependent on snow/ice inherent optical (microscopic) properties Allows for inclusion of soot, algae, etc in a general and consistent manner (biological implications) Allows for improvements to numerous parameterizations (e.g. snow aging effects, melt ponds) (currently being tested within CCSM)

Melt Pond Albedo Parameterization Accumulate fraction of snow and surface ice melt into pond volume reservoir. Compute pond area/depth from simple empirically-based relationship. Pond volume advected as a tracer. Albedo depends on pond fraction and depth. July Pond Concentration (Based on Ebert and Curry, 1993)

Ice Thickness Distribution Previous studies with Single Column Models (Maykut, 1982) Basin-scale models (Hibler, 1980) Coupled models of intermediate complexity (simplified atmos) Fully coupled models (Holland et al) Have shown ITD influences mean climate state: Thicker ice Warmer SAT More saline Arctic Ocean Changes in atmosphere, ocean circulation Schramm et al., 1997

Ice Thickness Distribution (Thorndike et al., 1975) Evolution depends on: Ice growth, lateral melt, ice divergence, and mechanical redistribution (riding/rafting)

Calculation of ITD - Mechanical Redistribution Parameterized after Rothrock,1975; Thorndike et al., 1975; Hibler, 1980; Flato and Hibler, 1995 Convergence and shear produce ridges Thin ice replaced by smaller area of thicker, ridged ice Thinnest 15% of ice participates in ridging Distribution of ridged ice results Assumptions regarding ridge formation (participation function, ridged distribution, etc.) and its relationship to ice strength Sea ice simulations sensitive to these assumptions For example - What to do with snow on ridging ice?

Boundary layer exchange in presence of ITD Resolving an ITD improves ice-ocn-atm exchange But ocean and atmospheric boundary layers do not differentiate between lead and ice covered surfaces

Near-term improvements for thermo/ITD SNOW - metamorphosis (aging, etc - important for radiation), blowing snow, others? Soot, algae, other impurities in ice - important for coupling to biology Sea ice "hydrology”: including melt ponds, brine pockets and drainage, percolation and snow-ice formation Exchange with ocean/atm - new possibilities with ITD, improvements based on observations (e.g. exchange coefficients, double diffusion) Mechanical redistribution - observed studies to refine and improve parameterizations

Albedo Most climate models use - empirical formulae to calculate albedo (function of surface state) - optically thick snow - constant fraction of radiation absorbed in surface layer (1-i o ) - constant extinction coefficient within ice - tuned ice albedo to implicitly include effects of surface melt water Not consistently related to inherent optical properties of snow/ice Only loosely tied to physical properties of snow/ice system Difficult to generalize for improved treatments of snow, meltwater, and impurities

Albedo 1. Existing scheme emphasizes albedo, absorption within ice and transmission to ocean are secondary Absorption and transmittance are difficult to validate, yet important! Absorbed light immediately available for melting Transmitted light heats upper ocean, available for primary productivity 2. While a tuned albedo parameterization may produce reasonable results for a sea ice model, the strength of the ice-albedo feedback and the character of radiative interactions with the atmosphere may require a more complex treatment of shortwave radiation (Curry et al., 2001)

For climate studies Need to include processes that are important for: Representing climatological state Representing feedbacks –realistic variability and sensitivity Physics appropriate to the models spatial scale Parameterize important non-resolved processes Trade off between complexity and computational cost

Enhanced albedo feedback in ITD run Larger albedo change for thinner initial ice With ITD have larger a change for ice with same initial thickness Suggests surface albedo feedback enhanced in ITD run ITD (5 cat) 1 cat. 1cat tuned Holland et al., 2006

Fundamentals - Thermodynamics (Beer’s Law) where Fraction transmitted below surface layer Albedo Vertical heat transfer

Fundamentals - Thermodynamics Vertical heat transfer (from Light, Maykut, Grenfell, 2003) where and Non-varying density; assume brine filled pockets/channels (Maykut and Untersteiner, 1971)

Fundamentals - Thermodynamics Vertical heat transfer Boundary Conditions: Assume balance of fluxes at ice surface: Where q(S,T) is the amount of energy needed to melt ice And base:

Boundary layer exchange in presence of ITD Resolving an ITD improves ice-ocn-atm exchange But ocean and atmospheric boundary layers do not differentiate between lead and ice covered surfaces Observations indicate that this can be important

Ice Thickness Distribution (Thorndike et al., 1975) Evolution depends on: Ice growth, lateral melt, ice divergence, and mechanical redistribution (riding/rafting)