A COMPARISON OF REGIONAL ISENTROPIC-SIGMA AND SIGMA-MODEL SIMULATIONSOF THE JANUARY 1979 CHICAGO BLIZZARD

In an initial effort in regional numerical weather prediction, results from the University of Wisconsin isentropic-sigma (UW theta-sigma) hy brid model and an ''identical'' sigma model are compared. The two main objectives are to demonstrate the capability of the UW theta-sigma mo del for regional numerical weather prediction and to identify advantag es of the hybrid model in simulating atmospheric water vapor transport and precipitation relative to the sigma model. The 72-h simulations p roduced by the two models extend over a region covering the western Pa cific Ocean, North America, and the western Atlantic Ocean. The simula tions begin at 0000 UTC 13 January 1979, a period during which an inte nse Chicago blizzard (sometimes called the Mayor Jane Byrne storm) dev elops over the central United States. This period also includes the ra pid development of a cyclone in the western Pacific Ocean. Results usi ng the Global Weather Experiment (GWE) ECMWF level IIIB data as initia l and verification data indicate that both models produce reasonable a nd similar 72-h simulations, with the UW theta-sigma model mass and mo mentum distributions being slightly more accurate than the sigma model . Of particular importance for the Chicago blizzard is that the UW the ta-sigma model more accurately simulates water vapor transport northwa rd from the Gulf of Mexico and westward from the Atlantic Ocean. As a result, the hybrid model more accurately simulates observed precipitat ion, especially over the northeastern United States and southeastern C anada.