The impact of additional rawinsonde observations on a mesoscale model simulation of a lake-effect snow event

One of the primary objectives of the Lake-Induced Convection Experiment (Lake-ICE) was to investigate the interaction between the convective boundary layer over Lake Michigan and the larger synoptic-scale flow. As part of the field study, six supplementary Cross-Chain Loran Atmospheric Sounding System (CLASS) units and three Integrated Sounding System (ISS) units were used in addition to those from the standard synoptic upper-air network. The three ISS units were in the vicinity of Lake Michigan, and the six CLASS units were in the data sparse region of Central and Northeastern Ontario and Western Quebec. The impact of initializing a mesoscale forecast model with these additional observations is investigated using a series of simulations.

The Pennsylvania State University – National Center for Atmospheric Research Mesoscale Model (MM5V2) running on a doubly nested grid is used to simulate the lake-effect snow event from 4-6 December 1997. Model output from a 30 km horizontal resolution grid shows that the inclusion of the six CLASS soundings appreciably alters the initialized sea level pressure field over the region. This leads to improved simulation of the surface wind and pressure fields over the course of the 48-hour simulation. A nested 10 km horizontal resolution grid shows that the initialization data from the CLASS sites seemed to have a bigger influence on the development of the boundary layer in the vicinity of Lake Michigan than that from the ISS sites. The 10 km grid over Lake Michigan captures a lake-effect mesoscale vortex that propagates on-shore during the 21-24 hour period of the simulation. The inclusion of the additional rawinsonde data changes the track of the vortex and axis of heaviest snowfall by approximately 20 km. Most of this shift appears to be the result of a subtle change in the gradient flow caused by the additional CLASS soundings. The WSR-88D reflectivity and radial velocity data are used to validate the model output and indicate that the additional soundings at initialization lead to a more accurate simulation of the boundary layer flow and the track of the mesoscale vortex.