The Effects of Snow Cover on Midlatitude Cyclones in the Great Plains

Abstract

Abstract The impacts of snow cover on the structure and intensity of midlatitude cyclones are examined. The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (Penn State–NCAR) Mesoscale Model (MM5) was used to simulate eight synoptic events in which a well-developed cyclone moved across the central and northern Great Plains region of the United States. Two simulations were performed for each event: a control run with the actual snow cover and a perturbed run with an extensive snow cover. In all of the cases, increasing the snow cover, and thereby reducing the available potential energy, weakened the cyclones. Among the eight cases, the averaged minimum central low pressure of the cyclones in the perturbed runs was approximately 4 mb greater than the control cyclones. The reduction in temperature and moisture in the lower atmosphere was most pronounced in the warm sector, which significantly reduced the thermal and moisture gradients near the surface. This resulted in a weakening of the fronts, less convergence near the surface, and decreased precipitation. Averaged among the cases, the upward vertical velocity near the center of the low was about 3.5 cm s−1 less in the perturbed simulations. Accumulated vertically integrated rainwater was reduced by 0.64 × 109 m3 when averaged for all of the cases in the perturbed simulations. In addition, the weaker gradients across the surface fronts in the increased snow-covered simulations decreased thermal and moisture advection near the surface and may have contributed to limiting the cyclones’ intensification in some of the cases by dampening positive feedback processes between the surface and midtroposphere.