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Abstract. We present a method to extract the atmospheric signal of trace gas mixing ratios from firn and bubbly ice measurements. This method, validated using data from Antarctic sites (Vostok and DE08), includes a numerical model that simulates air transport in the firn, and inverse theory. We focus here on atmospheric CH4 reconstruction, but the method can be used to reconstruct recent changes in any trace gas elemental or isotopic composition measured in firn and ice, that has no chemical interaction with solid H20. This study also provides useful information on the effective diffusity-porosity relationship and quantifies the smoothing effect due to gas diffusion in firn and at pore closure, showing the typical time periods of events that are filtered by the firn and therefore not observed in the gas record of ice cores.

ABSTRACT. A sta nd a rd num eri cal ex perim ent fea turing th e R oss Ice Shelf, Anta rcti ca, is prese nted as a tes t pac kage for th e d evelopm ent a nd interco mpa rison of ice-shelf mod els. Th e em ph asis o f this package is solu tion of s tress-eq uili bri um equ a ti o ns for a n ice-shelf velocit y consistent with prese n t observa ti o ns. As a d emo nstra tion, we compa re fi ve ind epend entl y d e"eloped ice-shelf m od els based on finite-d ifference a nd finite-elem ent m eth od s. Our res ults sugges t th a t th ere is lit tle difference between finite-element a nd finite-difference method s in capturing th e basic, la rge-scale flo w fea tures of th e ice shelf. W e additi ona ll y show th a t the fit between mod el a nd observed velocity d epends strongly on th e ice-shelf tempera ture fi eld , fo r whi ch th ere is presently littl e observa tiona l control. Th e ma in differences betwee n mod el res ults a re du e to th e equ a ti ons being so lved , th e bo und a ry conditi ons a t th e ice fr ont a nd th e di sc reti za ti on method (finite elem ent vs finite difference) .

A standard numerical experiment featuring the Ross Ice Shelf, Antarctica, is presented as a test package for the development and intercomparison of ice-shelf models. The emphasis of this package is solution of stress-equilibrium equations for an ice-shelf velocity consistent with present observations. As a demonstration, we compare five independently developed ice-shelf models based on finite-difference and finite-element methods. Our results suggest that there is little difference between finite-element and finite-difference methods in capturing the basic, large-scale flow features of the ice shelf. We additionally show that the fit between model and observed velocity depends strongly on the ice-shelf temperature field, for which there is presently little observational control. The main differences between model results are due to the equations being solved, the boundary conditions at the ice from and the discretization method (finite element vs finite difference).

Measurements made during the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS, 1973–78) are used to determine the large-scale rheological conditions of the Ross Ice Shelf, Antarctica. Our method includes a numerical ice-shelf model based on the stress-equilibrium equations and control theory. We additionally perform a few tests on simplified geometries to investigate the precision of our method. Our results consist of a map of the depth-averaged viscosity of the central part of the Ross Ice Shelf to within an uncertainty of 20%. We find that the viscosity variations are consistent with Glen’s flow law. Application of a more realistic flow law in our study provides little enhancement of ice-shelf model accuracy until uncertainties associated with basal melting conditions and with temperature profiles at inflow boundaries are addressed. Finally, our results suggest a strong viscosity anomaly in the west-central part of the ice shelf, which is interpreted to be associated with changes in the dynamics of Ice Stream A or B at least 1000 years ago. This feature conforms to the prevailing notion that the West Antarctic ice streams are unsteady.