A Climatology and Synoptic-Dynamic Basis for Distinguishing Cool-Season Precipitation Events at St. John's, Newfoundland

• Main Author: Milrad, Shawn M.
• Format: Text
• Language: unknown
• Published: SelectedWorks 2010
• Subjects: Meteorology; Newfoundland
• Online Access: https://works.bepress.com/shawn_m_milrad/6

For St. John’s, Newfoundland, at the confluence of several North American storm tracks, we identify synoptic-scale characteristics and precursors of various classes of cool-season precipitation events.Such events, based upon a climatology for 1979-2005, are separated into three categories based on precipitation amount. We find that the storm systems responsible for extreme precipitation events originate farther south and east than corresponding features in moderate and light events. A wind climatology showsthat extreme precipitation events at St. John’s are characterized almost exclusively by easterly surface and 925 hPa geostrophic winds, and that both the surface and 925 hPa geostrophic winds rotate clockwise with decreasing precipitation amount.Focusing on extreme events, we utilize two methods of manual synoptic typing to further partition events. The first method uses backward air parcel trajectories to separate events by air parcel source region. One subset of events (“west”) is characterized by strong upper-level dynamics and high precipitable water values in the central United States, which helps to produce a strong cyclone upon reaching the Atlantic Ocean; this is not seen for “west” events outside the extreme category. The second method of synoptic typing utilizes time series ofthree ascent-forcing quasi-geostrophic (QG) variables. While most events are characterized by a strong upstream sea-level cyclone originating from the Gulf of Mexico (“cyclone”), a subset are dominated by strong low-level frontogenesis, in the absence of a substantial upstream cyclone (“frontal”).Finally, a dynamic and thermodynamic analysis, and forecast model evaluation is completed for consecutive extreme events in December 2008, which produced over 125 mm of precipitation over six days. The first event is a “cyclone”, and is marked by strong QG forcing for ascent in the presence of low static stability and high values of subtropical moisture. The second event is a “frontal” event, and is associated with a persistent ...