A Family of Frontal Cyclones over the Western Atlantic Ocean. Part II: Parameter Studies

Da-Lin Zhang Department of Meteorology, University of Maryland, College Park, Maryland

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Ekaterina Radeva Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada

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John Gyakum Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada

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Abstract

In this study, a series of sensitivity experiments is performed to study the relative influence of latent heating, surface friction, and surface heat fluxes on the development of a family of frontal cyclones that occurred over the western Atlantic Ocean, using the simulation presented in Part I as a control run. It is shown that dry dynamics determines the initiation and track of all the frontal cyclones, and it accounts for about 59% of the deepening of a major frontal cyclone. Vorticity budget calculations reveal that in the absence of latent heating, preexisting upper-level cyclonic vorticity associated with a ring of potential vorticity provides the necessary forcing for the initiation and movement of the frontal cyclones, whereas the low-level thermal advection is responsible for a large portion of their amplifications as well as for their shallow circulations.

The impact of surface sensible and latent heat fluxes on the frontal cyclogenesis depends on the cyclones’ location with respect to the warm water surface. In the absence of latent heating, the surface fluxes have very weak impact, through modifying the low-level baroclinicity, on the evolution and final intensity of the frontal cyclones. When latent heating is allowed, however, the surface fluxes could result in more rapid cyclogenesis as a result of reduced static stability and increased moisture content in the maritime boundary layer; the impact is as pronounced as the latent heating. It is found that (dry) frontal cyclogenesis could still occur over a vast continental surface, although it is the slowest moving and deepening system among all the sensitivity tests being conducted.

The results reveal that (i) the frontal cyclones in the present case are baroclinically driven in nature, although they are markedly modulated by diabatic heating and surface fluxes; and (ii) the rapid frontal cyclogenesis phenomena tend to occur more frequently over a warm ocean surface due to its associated weak surface friction and its generated weak static stability in the maritime boundary layer.

Corresponding author address: Dr. Da-Lin Zhang, Department of Meteorology, University of Maryland, Room 2213, Space Science Building, College Park, MD 20742-2425.

Email: dalin@atmos.umd.edu

Abstract

In this study, a series of sensitivity experiments is performed to study the relative influence of latent heating, surface friction, and surface heat fluxes on the development of a family of frontal cyclones that occurred over the western Atlantic Ocean, using the simulation presented in Part I as a control run. It is shown that dry dynamics determines the initiation and track of all the frontal cyclones, and it accounts for about 59% of the deepening of a major frontal cyclone. Vorticity budget calculations reveal that in the absence of latent heating, preexisting upper-level cyclonic vorticity associated with a ring of potential vorticity provides the necessary forcing for the initiation and movement of the frontal cyclones, whereas the low-level thermal advection is responsible for a large portion of their amplifications as well as for their shallow circulations.

The impact of surface sensible and latent heat fluxes on the frontal cyclogenesis depends on the cyclones’ location with respect to the warm water surface. In the absence of latent heating, the surface fluxes have very weak impact, through modifying the low-level baroclinicity, on the evolution and final intensity of the frontal cyclones. When latent heating is allowed, however, the surface fluxes could result in more rapid cyclogenesis as a result of reduced static stability and increased moisture content in the maritime boundary layer; the impact is as pronounced as the latent heating. It is found that (dry) frontal cyclogenesis could still occur over a vast continental surface, although it is the slowest moving and deepening system among all the sensitivity tests being conducted.

The results reveal that (i) the frontal cyclones in the present case are baroclinically driven in nature, although they are markedly modulated by diabatic heating and surface fluxes; and (ii) the rapid frontal cyclogenesis phenomena tend to occur more frequently over a warm ocean surface due to its associated weak surface friction and its generated weak static stability in the maritime boundary layer.

Corresponding author address: Dr. Da-Lin Zhang, Department of Meteorology, University of Maryland, Room 2213, Space Science Building, College Park, MD 20742-2425.

Email: dalin@atmos.umd.edu

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