Model projections of greenhouse-gas-induced climate warming indicate enhanced sensitivity at high latitudes owing largely to positive feedbacks involving sea-ice and snow (e.g. Mitchell et al., 1990; Kattenberg et al., 1996; Rind et al., 1995). A worry is that this sensitivity may be an artifact of the crude treatment of sea-ice and snow processes in such models. For example, some coupled models treat sea-ice as a motionless thermodynamic slab, ignoring ice deformation and advection, while others use a simple "ice drift" approach. Some coupled models do include more realistic treatments of ice dynamics and the snowpack on land, and comparison of the models participating in CMIP will allow some evaluation of whether improvements in the models' representation of cryospheric processes indeed improves simulation of the cryosphere, and the climate as a whole.
Our goal is to measure the models' ability to simulate snow and ice and to relate errors in these quantities to model parameterizations of cryospheric processes or biases in other aspects of the climate system.
We will assemble data sets of observed snow and ice extent, and available estimates of snow and ice thickness. We will compute from these data, and from the model output, various large-scale quantities like snow-covered area in North America and Eurasia, ice-covered area and maximum thickness in the NH and SH etc. Where possible, the difference, model minus observed, will be used as a measure of model performance for each of these quantities, and from descriptions of the model parameterizations, we will seek commonalities between those models which perform well and those which do not (e.g. do models which include ice dynamics have a smaller error in ice extent than those which do not? Do models which include flux adjustment have a smaller error? Does model error in snow extent decrease with increasing sophistication of the land- surface scheme?). An initial examination of some of these relationships was conducted by G.J. Boer and S. Lambert and presented in Gates et al. (1996). We will also explore relationships between errors in other available quantities (e.g. overturning streamfunction, surface pressure patterns) and cryospheric variables. Since time-series of cryospheric quantities are not available from CMIP, we will examine the correlation between surface air temperature and ice/snow line in our model to see if it can be used as a proxy for cryospheric variability in the other participating models (for example, the location of the 0-degree isotherm should be a fairly good indicator of ice edge location, at least in winter).
We will use the basic CMIP model output: atmospheric surface fields (especially snow and ice depth); ocean annual means and zonal cross-sections; and surface air temperature time-series. In addition we will make use of climatological data available at our institution (e.g. ice and snow extent, surface air temperature and pressure, ocean temperature and salinity).
Gates, W.L. plus 9 others, Climate Models - Evaluation. Chapter 5 in Climate Change 1995, Cambridge University Press, 572pp., 1996.
Kattenberg, A. plus 8 others, Climate Models - Projections of Future Climate, Chapter 6 in Climate Change 1995, Cambridge University Press, 572pp., 1996.
Mitchell, J.F.B., S. Manabe, V. Meleshko, and T. Tokioka, Equilibrium Climate Change - and its Implications for the Future. Chapter 5 in Climate Change: The IPCC Scientific Assessment, Cambridge University Press, 365pp., 1990.
Rind, D., R. Healy, C. Parkinson, D. Martinson, The role of sea ice in 2xCO2 climate model sensitivity. Part I: The total influence of sea ice thickness and extent. J. Climate, 8:449-463, 1995.
Other relevant publications by subproject leader:
Flato, G.M. and W.D. Hibler III, Modeling pack ice as a cavitating fluid. J. Phys. Oceanogr., 22:626-651, 1992.
Flato, G.M. and R.D. Brown, Variability and climate sensitivity of landfast Arctic sea ice. J. Geophys. Res., 101:25,767-25,777, 1996.
Flato, G.M. and D. Ramsden, Sensitivity of an atmospheric general circulation model to the parameterization of leads in sea-ice. Annals of Glaciol., 25, in press, 1997.
Lemke, P. W.D. Hibler III, G. Flato, M. Harder, and M. Kreyscher, On the improvement of sea ice models for climate simulations: the sea ice model intercomparison project. Annals of Glaciol., 25, in press, 1997.
Randall, D., J. Curry, D. Battisti, G. Flato, plus 6 others, Status and outlook for large-scale modeling of atmosphere-ice-ocean interactions in the Arctic. Bull. Am. Meteorol. Soc., submitted, 1997.