A Comparison of Regional Isentropic-Sigma and Sigma Model Simulations of the January 1979 Chicago Blizzard

Abstract

In an initial effort in regional numerical weather prediction, results from the University of Wisconsin isentropic-sigma (UW ??σ) hybrid model and an ?identical? sigma model are compared. The two main objectives are to demonstrate the capability of the UW ??σ model for regional numerical weather prediction and to identify advantages of the hybrid model in simulating atmospheric water vapor transport and precipitation relative to the sigma model. The 72-h simulations produced by the two models extend over a region covering the western Pacific Ocean, North America, and the western Atlantic Ocean. The simulations begin at 0000 UTC 13 January 1979, a period during which an intense Chicago blizzard (sometimes called the Mayor Jane Byrne storm) develops over the central United States. This period also includes the rapid development of a cyclone in the western Pacific Ocean. Results using the Global Weather Experiment (GWE) ECMWF level IIIB data as initial and verification data indicate that both models produce reasonable and similar 72-b simulations, with the UW ??σ model mass and momentum distributions being slightly more accurate than the sigma model. Of particular importance for the Chicago blizzard is that the UW ??σ model more accurately simulates water vapor transport northward from the Gulf of Mexico and westward from the Atlantic Ocean. As a result, the hybrid model more accurately simulates observed precipitation, especially over the northeastern United States and southeastern Canada.