Synoptic sea-level pressure patterns generated by a general circulation model: Comparison with types derived from NCEP/NCAR re-analysis and implications for downscaling

摘要

A principal component analysis (PCA)-based synoptic typing scheme is used to assess the ability of the Canadian Centre for Climate Modelling and Analysis (CCCma) Coupled Global Climate Model (CGCM2) to reproduce daily mean-sea-level (MSL) synoptic patterns and their frequencies for the Pacific Northwest region of North America. Model output for the 'control' period 1961-1989 is compared against the climatology based on National Center for Environmental Prediction (NCEP) re-analysis data. Although CGCM2 is able to reproduce the full range and seasonality of 13 synoptic types it significantly under-represents three cold types and over-represents (by about 50%) three warm/wet winter types. This effect is most pronounced in winter months. Differences in frequencies between the CGCM2 runs (1961-1989 climatology and 1990-2100 IPCC SIZES 'A2' greenhouse gases (GHG) and aerosol forcing scenario) are smaller than the differences between NCEP and CGCM2 synoptic type frequencies for the 1961 - 1989 control. Unresolved orographic influences and atmosphere-ocean coupling are cited as possible explanations for model deficiencies. Results suggest that application of CGCM2 output in downscaling studies examining regional impacts should take account of these potential biases. The approach adopted provides a methodology for not only assessing progress in emerging generations of more sophisticated higher resolution General Circulation Models (GCMs) (e.g. CGCM4 is under development), but also choosing the most appropriate model for regional downscaling studies. Copyright (C) 2006 Royal Meteorological Society.