Sunday, 11 January 2009
Seasonal frequency of fronts and surface baroclinic zones in the Great Lakes region
Phoenix Convention Center
Frontal and trough passages over the Great Lakes are a critical factor in influencing weather in the region. For example, fronts often have a significant role in the initiation and evolution of severe thunderstorms, winter storms, and lake-effect snow storms. Frontal movement and the circulation of an associated cyclone can also have important implications on air mass distribution, air trajectories, and pollution transport in the Great Lakes region. A six year climatological study was undertaken to understand the spatial distribution and temporal frequency of fronts and troughs in the region. National Center for Environmental Prediction (NCEP) surface analyses and North American Regional Reanalysis data were used to determine the frequency of different frontal types in relation to each of the Great Lakes and the presence of baroclinic zones relative to analyzed fronts. Frontal locations and evolution were determined by examining the six-hour surface analyses (00, 06, 12, 18 UTC) for January 2000 through December 2005. A total of 2,173 fronts and 1,075 troughs were identified during the six-year period investigated. Cold fronts were the most frequent frontal type, followed by stationary, warm, and occluded fronts, respectively. Lake Michigan experienced the highest number of frontal passages while Lake Ontario experienced the least. The analyses also provide information regarding the intra-annual and interannual variability of fronts and troughs.
Surface baroclinic zones were identified using two-meter potential temperature analyses. Our investigation identified the presence of both strong and moderate surface baroclinic zones defined as having a potential temperature gradient between 3.5 and 7.0 °C (100 km)-1 and greater than 7.0 °C (100 km)-1, respectively. Our investigation of the correspondence of analyzed fronts with surface baroclinic zones found that fronts in the Great Lakes region were associated with moderate or strong baroclinic zones on 51% of the over 8,500 surface analyses examined. Warm and stationary fronts were found to have a corresponding baroclinic zone on ≥ 60% of the analyses; however, only 30% had moderate or strong surface baroclinicity in the vicinity of occluded fronts. Lastly, surface baroclinic zones were present on 26%, 48%, and 75% of analyses in association with synoptic-scale, lake-aggregate, and mesoscale thermal troughs, respectively.