A Radar Analysis of Urban Snowfall Modification in Minneapolis-St. Paul

A better understanding of urban snowfall climatology will help to mitigate winter weather hazards in highly populated cities, such as Minneapolis-St. Paul, Minnesota. Winter road maintenance accounts for roughly 25% of Minnesota Department of Transportation maintenance budgets, and state and local agencies spend more than $2.3 billion on snow and ice control operations annually. Urban snowfall has also been shown to enhance health problems and increase mortality rates. Further research on urban snowfall climatology is needed to more accurately diagnose urban regions susceptible to such winter health risks. The winter urban heat island effect has been suggested to reduce snowfall downwind of city centers because of localized energy and moisture flux variations, but previous research lacks spatial detail since it is primarily based on sparse surface observations. This project utilizes radar data for two studies-a 25-event snowfall composite and an individual-event analysis of 13 snow-only events-occurring from 1995 to 2012 and passing over the Minneapolis-St. Paul urban area to quantify the change in radar reflectivity values as a proxy for snowfall intensity downwind of the city. Results show that for the summed maximum composite event snowfall was significantly decreased downwind of the urban region; however, the highest maximum composite event, as well as 4 of the 13 individual snowfall events evaluated, did not have significantly decreased snowfall downwind of the city center, with the highest maximum composite and two of the three individual events having increased reflectivity values downwind. Analysis of related atmospheric variables for events with increased downwind reflectivity suggests that atmospheric instability and convergence may play a critical role in urban snowfall modification.