A Climatology of Surface Baroclinic Zones

Frederick Sanders Marblehead, Massachusetts

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Eric G. Hoffman Natural Science Department, Plymouth State College, Plymouth, New Hampshire

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Abstract

Analyses of surface potential temperature for one year over the contiguous United States, southern Canada, northern Mexico, and adjacent oceanic areas showed three regions of relatively high frequency of strong gradients. These were the Atlantic and Gulf coasts, the Pacific coast, and the eastern slopes of the North American Cordillera. The Atlantic and Gulf zone was most pronounced at 1200 UTC during winter. The Pacific coast zone was most frequent at 0000 UTC in summer. The zone on the eastern slopes was present at both times and during all seasons but was most pronounced at 1200 UTC in winter. The locations of the maxima and the pronounced diurnal changes provide circumstantial evidence that the horizontal variation of diabatic heating and cooling due to surface heat flux is a more important physical mechanism than confluence is in the creation of surface baroclinic zones. An example is shown of the importance of the East Coast zone for weather forecasting during the cold season. In January of 2000 a low pressure center propagated along the surface baroclinic zone, separating cold air over land from warm air over the water, rather than following the deep baroclinic zone in the troposphere. During the warm season, the “backdoor cold front,” separating cool air over the water from hot air over land, presents a challenging forecast problem. Many analyzed fronts are not associated with significant horizontal density gradients. Because of these and other surface analysis issues, short-range forecasting might benefit from routine analysis of the surface temperature field.

Corresponding author address: Dr. Frederick Sanders, 9 Flint St., Marblehead, MA 01945-3716. Email: fnmisander@aol.com

Abstract

Analyses of surface potential temperature for one year over the contiguous United States, southern Canada, northern Mexico, and adjacent oceanic areas showed three regions of relatively high frequency of strong gradients. These were the Atlantic and Gulf coasts, the Pacific coast, and the eastern slopes of the North American Cordillera. The Atlantic and Gulf zone was most pronounced at 1200 UTC during winter. The Pacific coast zone was most frequent at 0000 UTC in summer. The zone on the eastern slopes was present at both times and during all seasons but was most pronounced at 1200 UTC in winter. The locations of the maxima and the pronounced diurnal changes provide circumstantial evidence that the horizontal variation of diabatic heating and cooling due to surface heat flux is a more important physical mechanism than confluence is in the creation of surface baroclinic zones. An example is shown of the importance of the East Coast zone for weather forecasting during the cold season. In January of 2000 a low pressure center propagated along the surface baroclinic zone, separating cold air over land from warm air over the water, rather than following the deep baroclinic zone in the troposphere. During the warm season, the “backdoor cold front,” separating cool air over the water from hot air over land, presents a challenging forecast problem. Many analyzed fronts are not associated with significant horizontal density gradients. Because of these and other surface analysis issues, short-range forecasting might benefit from routine analysis of the surface temperature field.

Corresponding author address: Dr. Frederick Sanders, 9 Flint St., Marblehead, MA 01945-3716. Email: fnmisander@aol.com

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