A Trajectory study of storm tracks and the midwinter precipitation characteristics over the west coast of the United States

Storm tracks and the mid-winter precipitation characteristics over the west coast of United States are investigated for the storm events during the winters (December-February, 2000-2010) using trajectory model calculations based on GMAO MERRA reanalysis data and observed hydrologic data. The calculated air parcel trajectories are classified based on a k-mean clustering analysis, which are used to identify trajectory pathways. This classification is in turn adopted to detect the storm events corresponding to the northern and southern pathways. It shows that intense precipitation events in the Sierra Nevada region are mainly related to strong low-level water vapor transports into the region from the tropics and strong storm tracks from the mid-latitude. In contrast, dry events resulting in weak precipitation are closely related to the intrusion of a cold air trough from the high latitude upper troposphere into the low latitude lower troposphere. The PV (Potential Vorticity), low-level air temperatures, water vapor flux, and large-scale circulation patterns corresponding to individual trajectory groups are well distinguished from each other. The climatological precipitation patterns corresponding to each group are also analyzed using the observations from AIRS and rain gauge-based NCEP daily precipitation analysis. The controls on the midwinter extreme precipitation over the west coast of United States will be also discussed.