Generating regional precipitation from observed and GCM synoptic-scale pressure fields, southern Alberta, Canada

Observed (1960 to 1989) synoptic-scale fields of sea level pressure (SLP) and 50 kPa geopotentials were objectively typed using the Kirchhofer technique to determine the frequency of occurrence of weather patterns in western Canada. Twenty different synoptic types were generated for both variables (SLP and 50 kPa) and compared to those simulated in the CCC GCM (Canadian Climate Centre's general circulation model). The SLP fields were poorly simulated by the GCM, but the 50 kPa fields were reliable. Calculation of the precipitation efficiencies for 1960-69 allowed a quantitative assessment of the mean amount of precipitation which each 50 kPa synoptic type brings to southern Alberta in each month. This information was used to generate synthetic precipitation for southern Alberta from (1) observed synoptic climatology for 1970-89, and (2) 1xCO(2) and 2xCO(2) runs of the CCC GCM. For (1), total amounts of synthetic precipitation during 1970-89 approximate the observed values, but the variability is much less. For (2), using the 1xCO(2) run of the CCC GCM to generate synthetic precipitation scenarios gives more realistic amounts than the precipitation modelled by the GCM itself, although the variability is poorly replicated. The GCM suggests little change in the synoptic control of precipitation arising from a doubling of CO2.