The maximum potential intensity (MPI) of hurricanes is a topic that has beenexplored extensively over the past several years. Since the originaltheory of Miller (1958), several other hypotheses have been suggested which set an upper bound on hurricane intensity given a set of environmentalconditions. Holland (1997) offers an extension of Miller's original thermodynamic argument. In this theory, a combination of hydrostatic pressure falls from the vertical redistribution of surface moist entropy and eye warming leads to a minimum attainable central pressure. Emanuel (1986, 1988, 1997) argues that the radial circulation of a mature tropical cyclone acts as a carnot heat engine. The minimum central pressure and associated maximum tangential wind speed is found from a balance between the extraction of energy from the sea surface and dissipation in the frictional boundary layer and outflow. Gray (1998) argues that maximum potential intensity is attained when the updraft in the inner core generated through eyewall buoyancy and frictional convergence exactly balances the vertical motion needed to fulfill the momentum requirements of the circulation.
Oddly enough, all three authors presents results which appear to be in agreement with observations. Is this a coincidence? Can all three theories be correct? Each makes certain assumptions to which the results are sensitive. Two areas of concern involve the meteorological conditions in the boundary layer. For example, a change in surface eyewall RH from 80% to 100% in Holland's model results in an additional surface pressure drop of 96 mb. Likewise, a four degree increase in the surface temperature lowers the mininum attainable pressure by 131 mb. The temperature and the relative humidity in the boundary layer have large impacts on the surface moist entropy budget which is important to the development of tropical cyclones. There has been considerable debate as of late, as to the values of these parameters in the boundary layer of a tropical cyclone in nature.
In the present work, Ooyama's (1969) classical tropical cyclone model as well as a modified version will be used to assess the validity of the above MPI theories and the impact of boundary layer conditions on their results. In particular, variations of the surface moist entropy due to a range of surface temperatures and relative humidities will be used to test whether or not the theoretical values of maximum wind speed and minimum central pressure from the MPI theories agree with values generated by this model.
REFERENCES
Emanuel, K. A., 1986: An air-sea interaction theory for tropical cyclones. Part 1: Steady-state maintenance. J. Atmos. Sci., 43, 585-604.
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Gray, W. M., 1998: The role of the hurricane's inner-core upper-level cyclonic outflow as a proxy for cyclone intensity and intensity change. Preprints, Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, Amer. Meteor. Soc., 86-92.
Holland, G. J., 1997: The maximum potential intensity of tropical cyclones. J. Atmos. Sci., 54, 2519-2540.
Miller, B. I., 1958: On the maximum potential intensity of hurricanes. J. Meteor., 15, 184-195.
Ooyama, K., 1969: Numerical simulation of the life cycle of tropical cyclones. J. Atmos. Sci., 26, 3-30.