Thermodynamic and Kinematic Structure of a Snowband and Freezing Rain Event during STORM-FEST

Abstract

A unique wintertime storm occurred on 12 February 1992 during the Stormscale Operational and Research Meteorology-Fronts Experiment Systems Test (STORM-FEST) field project. This storm consisted of a narrow east–west-oriented snow swath (max 20-cm snow depth) with an extensive area of freezing rain to the south. The event was well observed by various networks and systems. Included in these systems were about twice the normal sounding sites, which released rawinsondes every 3 h, single- and dual-Doppler capabilities, and NCAR King Air and Wyoming King Air aircraft. Four aspects of the storm were investigated: the temporal evolution of the low-level jet, the vertical stability of various layers, the development of isothermal layers, and the dynamical effects resulting from melting. The freezing rain related to this storm was a result of an overrunning situation, and the snow swath was produced from conditional symmetric instability with respect to the ice process in the overrunning cloud layer. The warm frontal layer was dynamically unstable in terms of the Richardson number and contained some shear-induced gravity waves. A convective boundary layer was present near the surface. A low-level jet was present at the top of the convective boundary layer and along the snow–freezing rain interface. Isothermal layers developed just below the bright band as a result of diabatic cooling because of melting. The diabatic process of melting appears to have enhanced the speed of the low-level jet and triggered and focused the release of the thermodynamic instability so that an enhanced precipitation rate occurred over the snow–freezing rain interface.