Methodology:

The UK Met Office Unified Model (known as MetUM) uses the GA6 (Global Atmosphere 6.0; Walters et al., 2017) configuration of version 11.1, which is suitable for grid scales of 10 km or coarser. The HIRHAM5 model combines the physics of the ECHAM5 general circulation model and the hydrostatic dynamical core of the HIRLAM7 numerical weather prediction model (Christensen et al., 2007). The MetUM (HIRHAM5) uses 70 (31) vertical levels in the atmosphere. The ice mask used by both models is derived from Advanced Very High-Resolution Radiometer (AVHRR) data from the period 1992 to 1993 at a resolution of 1 km.

Both models are run over Antarctic CORDEX domains from December 1979 to February 2019 at grid spacing of around 12 km, with the MetUM (HIRHAM5) grid consisting of 392 x 504 (406 x 542) grid points. Initial and boundary conditions for both models are supplied by ERA-Interim reanalysis data. The MetUM hindcast uses a frequent re-initialization approach, consisting of twice-daily 24 hr forecasts. Output at T+12, T+15, T+18, and T+21 hr from each of the forecasts is concatenated together to form a seamless series of 3-hourly model outputs, with the earlier output discarded as spin-up. The HIRHAM5 hindcast uses a long-term continuous integration approach, with no nudging.

Christensen, O. B., M. Drews, J. H. Christensen, K. Dethloff, K. Ketelsen, I. Hebestadt, and A. Rinke, 2007: The HIRHAM regional climate model, Version 5 (beta), Danish Climate Centre, Danish Meteorological Institute, Danish Meteorological Institute, Technical Report. ISSN 1399-1388

Walters, D., I. Boutle, M. Brooks, T. Melvin, R. Stratton, S. Vosper, H. Wells, K. Williams, N. Wood, T. Allen, A. Bushell, D. Copsey, P. Earnshaw, J. Edwards, M. Gross, S. Hardiman, C. Harris, J. Heming, N. Klingaman, R. Levine, J. Manners, G. Martin, S. Milton, M. Mittermaier, C. Morcrette, T. Riddick, M. Roberts, C. Sanchez, P. Selwood, A. Stirling, C. Smith, D. Suri, W. Tennant, P. L. Vidale, J. Wilkinson, M. Willett, S. Woolnough, and P. Xavier, 2017: The Met Office Unified Model Global Atmosphere 6.0/6.1 and JULES Global Land 6.0/6.1 configurations. Geosci. Model. Dev., 10, 1487-1520, https://doi.org/10.5194/gmd-10-1487-2017.

Data quality:

Summer daily maximum temperatures derived from both the MetUM and HIRHAM5 hindcasts were compared in a statistical analysis with observations from twenty weather stations situated either on or close to Antarctic ice shelves. This comparison showed that the MetUM simulated temperatures were characterised by a) a cold bias for fifteen out of the twenty stations, which for ten of the stations was less than 1 K, b) a root-mean-square-error of 2-3 K, and c) a correlation from 0.62-0.91. The HIRHAM5 summer daily maximum temperatures typically have a larger cold bias, larger root-mean-square-error, and lower correlation than the MetUM results. The relatively good performance of the MetUM (and to a lesser extent HIRHAM5) suggests that daily maximum temperatures over Antarctic ice shelves derived from these model outputs are broadly representative of actual conditions.