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Medium-range numerical prediction of Antarctic sea ice

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Roberts, Andrew Frank 2005 , 'Medium-range numerical prediction of Antarctic sea ice', PhD thesis, University of Tasmania.

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Abstract

This research investigates the use of a multiple-thickness sea-ice model for opera-
tional Antarctic sea-ice forecasts. Deterministic predictions are generated with a finite
difference sea-ice model for the Southern Ocean. The forecast model includes a new
method for ridging sea ice on semi-diurnal timescales and includes a modified Coulombic
rheology. This approach differs from formalistic techniques used in existing thickness
distribution models, and is introduced to simulate oriented fractures with multiple thickness-
categories. Behaviour of the dynamic component of the model is assessed on a test
grid to illustrate its properties and numerical limitations. The dynamic component is
then combined with a constant salinity, enthalpy-conserving thermodynamic model on
a 50 km resolution polar stereographic grid using 11 thickness categories to simulate
circum-Antarctic sea ice.
Sea-ice assimilations are generated for observation intensive years (1992 and 1996) to
compare model output with drafts from Upward Looking Sonar, ice velocities from
drifting buoys and concentration from satellite measurements. The assimilations are
forced with atmospheric fields from the European Centre for Medium Range Weather
Forecasting (ECMWF) and National Oceanic and Atmospheric Administration Sea
Surface Temperatures. Climatological-mean ocean currents are used as a background
geostrophic field beneath the ice. The sea-ice analyses are then filtered with Special
Sensor Microwave/Imager (SSM/I) derived sea-ice concentrations, adjusting the inno-
vation sequence (sequence of observation minus forecast at each timestep) for the unique
thermodynamic lag of each ice-thickness category. This method provides an estimate
of oceanic heat flux and explicitly adjusts vertical ice temperature profiles. It improves
approximations of sea-ice extent and mean thickness, suggesting a significant source of
error in the unassimilated model results from incorrect heat flux specification between
ice, ocean and atmosphere. However the scheme was less successful at assimilating
individual thickness categories of the thickness distribution, and possible future im-
provements to the scheme are discussed.
A series of G-day forecast case studies designed to test short-term predictability of the
model are provided in the final part of this thesis. Pack forecasts are generated with
both ECMWF and National Centre for Environment Prediction (NCEP) atmospheric
reanalyses. They are verified against assimilations and SSM/I-derived sea-ice concen-
tration. The results demonstrate that as long as the initial sea-ice state used to initialise
forecasts is physically consistent, the medium-range forecast will itself be quite skilled.
Information obtained from two sets of forecasts, one forced with ECMWF and the
other with NCEP, shows both sets of forcings generate regionally different but simi-
larly skilled forecasts. This suggests that ensembles methods may improve operational
forecast guidance. Overall, the work paves the way for regular operational Antarc-
tic sea-ice forecasts and the potential of determining Antarctic sea-ice thickness from
remotely-sensed observations.

Item Type: Thesis - PhD
Authors/Creators:Roberts, Andrew Frank
Keywords: Sea ice
Copyright Holders: The Author
Copyright Information:

Copyright 2005 the author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Includes CD-ROM inserted in Appendix C on page 183. "Antarctic Climate and Ecosystems Cooperative Research Centre". Thesis (Ph.D.)--University of Tasmania, 2005. Includes bibliographical references

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