Tropical cyclones often encounter and interact with potential vorticity (PV) anomalies associated with features like upper level troughs. While it is recognized that these interactions can bring about changes in the intensity of tropical cyclones, the nature of the interactions is poorly understood. Molinari et al. (1995) have argued that such outflow layer interactions have a significant impact on hurricane intensity. The aim of the current work is to use an idealized model to study the interaction of tropical cyclones and upper-level troughs at the outflow layer level with the aim of reproducing some of the features observed in nature.
The outflow layer is represented by a divergent barotropic shallow water model in a channel with periodic east-west boundaries. Following Ooyama (1987), the tropical cyclone is represented by a mass source with an associated momentum source. Two types of experiments are carried out: interactions with idealized troughs; and case studies of specific storms initialized using gridded outflow layer analyses from ECMWF. It was shown (Aiyyer 1997) that the outflow produced by a mass source alone is able to considerably deform the trough while promoting a net cyclonic spin up at inner radii.
The current work extends the same by including the effect of the momentum source. Trough interactions will be evaluated in terms of eddy angular momentum fluxes, PV evolution, and cross-storm flow, which gives a measure of vertical shear. It is shown that, while in the absence of any background flow, the outflow produced by a mass and momentum source combination appears to be fairly realistic, the interaction with idealized troughs has a complicated evolution due to the momentum parametrization. Such complications can be expected in simulations with multi-layered shallow water model also and warrant detailed study in order to successfully model trough interactions.
REFERENCES:
Aiyyer, Anantha, 1997: interaction of tropical cyclone outflow layer potential vorticity anomalies. Preprint, 22nd Conf. on Hurricanes and Tropical Meteorology, Ft. Collins, Colorado, Amer. Meteor. Soc., 197-198.
Molinari, J., Skubis, S. and Vollaro, D., 1995: External influences on hurricane intensity. Part III: Potential vorticity structure. J. Atmos. Sci., 52, 3593-3606.
Ooyama, K. V., 1987: Numerical experiments of steady and transient jets with simple model of the hurricane outflow layer. Preprint, 17th Conf. on Hurricanes and Tropical Meteorology, Miami,
Florida, Amer. Meteor. Soc., 318-320.