Boreal Winter Storm Tracks and Associated Precipitation in North America: A Potential Vorticity Perspective

Storm tracks can have a profound impact on the climate by influencing the variability in cyclonic activity in the mid-latitudes. This study uses six-hourly Climate Forecast System Reanalysis (CFSR) data to investigate the behavior of Northern Hemisphere winter storm tracks and their relation to surface precipitation in North America. A Lagrangian approach based on Potential Vorticity (PV) diagnostics is employed to track cyclones over the Pacific and the North American-Atlantic (NAA) storm tracks that impact the weather and climate over North America. The mean intensities, or average strengths, of the Pacific and NAA storm tracks in the DJF season are shown to be strongest over their respective ocean basins. Other statistics using isentropic potential vorticity reveal small scaled features including regions of cyclogenesis and cyclolysis. The representation of storm tracks and their properties is found to be sensitive to the minimum PV threshold used to identify individual storms. However, despite minor differences, all main features remain the same regardless of the threshold that was chosen.

Much of the cyclone-associated CFSR precipitation occurs in regions where the Pacific and NAA storm tracks are strongest, indicating that storm tracks identified from PV values leave a strong footprint in surface precipitation. Even the weaker portions of the storm tracks over land are also shown to leave a strong footprint in precipitation. The Pacific cyclone-associated precipitation pattern reveals precipitation rates in excess of 5 mm day^-1 from Alaska to northern California along the North American west coast. The NAA storm track associated precipitation is concentrated over Florida and the surrounding Caribbean islands. An analysis was also performed using daily precipitation accumulations from the Global Precipitation Climatology Project (GPCP). The results show that precipitation associated with storms identified via a minimum intensity threshold occurs in nearby regions as the cyclone-associated precipitation from the reanalysis. The observational cyclone-associated precipitation rate exceeds 3 mm day^-1 along the western coast of Canada and the eastern coast of the United States, and it exceeds 4 mm day^-1 over the southeastern United States.

Local precipitation maxima are more prevalent in the observational precipitation patterns than in the reanalysis patterns. The magnitudes of both observed and reanalysis patterns over the Pacific Ocean are similar; however, the reanalysis appears to overestimate precipitation over the western coast of North America. A similar situation develops in the east where the observed precipitation pattern over the western North Atlantic Ocean is similar to that of the reanalysis, with the latter overestimating the precipitation patterns elsewhere.