A family of frontal cyclones over the western Atlantic Ocean. Part II: Parameter studies

In this study, a series of sensitivity experiments is performed to study th e relative influence of latent heating, surface friction, and surface heat fluxes on the development of a family of frontal cyclones that occurred ove r 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 t rack of all the frontal cyclones, and it accounts for about 59% of the deep ening of a major frontal cyclone. Vorticity budget calculations reveal that in the absence of latent heating, preexisting upper-level cyclonic vortici ty associated with a ring of potential vorticity provides the necessary for cing for the initiation and movement of the frontal cyclones, whereas the l ow-level thermal advection is responsible for a large portion of their ampl ifications as well as for their shallow circulations. The impact of surface sensible and latent heat fluxes on the frontal cyclog enesis depends on the cyclones' location with respect to the warm water sur face. In the absence of latent heating, the surface fluxes have very weak i mpact, through modifying the low-level baroclinicity, on the evolution and final intensity of the frontal cyclones. When latent heating is allowed, ho wever, the surface fluxes could result in more rapid cyclogenesis as a resu lt of reduced static stability and increased moisture content in the mariti me boundary layer; the impact is as pronounced as the latent heating. It is found that (dry) frontal cyclogenesis could still occur over a vast contin ental 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 ba roclinically driven in nature, although they are markedly modulated by diab atic heating and surface fluxes; and (ii) the rapid frontal cyclogenesis ph enomena 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.