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Hurricane Maximum Intensity: Past and Present

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Hurricane Maximum Intensity: Past and Present 1704 MONTHLY WEATHER REVIEW VOLUME 129 Hurricane Maximum Intensity: Past and Present J. PARKS CAMP National Weather Service, Sterling, Virginia MICHAEL T. M ONTGOMERY Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado (Manuscript received 6 March 2000, in ®nal form 15 September 2000) ABSTRACT Hurricane intensity forecasting has lagged far behind the forecasting of hurricane track. In an effort to improve the understanding of the hurricane intensity dilemma, several attempts have been made to compute an upper bound on the intensity of tropical cyclones. This paper investigates the strides made into determining the maximum intensity of hurricanes. Concentrating on the most recent attempts to understand the maximum intensity problem, the theories of Holland and Emanuel are reviewed with the objective of assessing their validity in real tropical cyclones. Each theory is then tested using both observations and the axisymmetric hurricane numerical models of Ooyama and Emanuel. It is found that ambient convective instability plays a minor role in the determination of the maximum intensity and that the Emanuel model is the closest to providing a useful calculation of maximum intensity. Several shortcomings are revealed in Emanuel's theory, however, showing the need for more basic research on the axisymmetric and asymmetric dynamics of hurricanes. As an illustration of the importance of asymmetric vorticity dynamics in the determination of a hurricane's maximum intensity it is shown, using Ooyama's hurricane model, that the maximum intensity of a tropical cyclone may be diminished by convectively generated vorticity anomolies excited outside the primary eyewall. The vorticity anomolies are parameterized by adding a concentric ring of vorticity outside the primary eyewall that acts to cut off its supply of angular momentum and moist enthalpy. It is suggested that the generation of vorticity rings (or bands) outside the primary eyewall is a major reason why tropical cyclones fail to attain their maximum intensity even in an otherwise favorable environment. The upshot of this work points to the need for obtaining a more complete understanding of asymmetric vorticity processes in hurricanes and their coupling to the boundary layer and convection. 1. Introduction intensity problem, numerous attempts have been made to compute an upper bound on tropical cyclone intensity Tropical cyclones plague tropical and subtropical for given atmospheric and oceanic conditions. For the oceans as well as neighboring land areas around the purposes of the present study, the maximum intensity world on an annual basis. Research into understanding of a hurricane [hereafter called maximum potential in- these deadly vortices has been ongoing for many years. tensity (MPI)] is de®ned as the maximum intensity (as Much progress has been made into understanding some determined by minimum surface pressure or maximum of the basic mechanisms that govern tropical cyclones, tangential winds) that a tropical cyclone may achieve allowing for signi®cant strides to be made in the fore- for a given atmospheric and oceanic thermal structure. casting of track. However, the associated forecasts of Generally, we assume that upper-level winds are fa- tropical cyclone intensity have not shown the same im- vorable for development. Investigations into MPI have provement (DeMaria and Kaplan 1999 and references). begun from different starting points leading to differing Understanding what controls hurricane intensity is vital viewpoints on the structure of tropical cyclones. The in order to properly warn those in the path of storms, primary goals of this paper are twofold. The ®rst goal as well as in predicting the impacts of global climate is to evaluate the current state of MPI theory and de- change on the character of tropical cyclones (Knutson termine which proposed theory best correlates with the and Tuleya 1999 and references). structure and behavior of observed tropical cyclones. As a step toward understanding the tropical cyclone The second is to suggest an answer to why most tropical cyclones fail to reach their MPI. Corresponding author address: J. Parks Camp, National Weather Kleinschmidt (1951), Miller (1958), and Malkus and Service, 44087 Weather Service Rd., Sterling, VA 20166. Reihl (1960) conducted the ®rst such studies aimed at E-mail: [email protected] determining the upper limit of tropical cyclone intensity. q 2001 American Meteorological Society JULY 2001 CAMP AND MONTGOMERY 1705 uel decreases with decreasing relative humidity such that a surface relative humidity of 100% at the eyewall does not support any circulation whatsoever. Converse- ly, the strongest storms in Holland's theory occur with 100% surface relative humidity under the eyewall, with only weak circulations (tropical depression strength) supported at 75% RH. This result is striking and sug- gests that the two models are fundamentally different. As we shall see, the hypothesized role of air±sea energy exchange and convective available potential energy (CAPE) lies at the heart of this difference. The outline of this paper is as follows. We ®rst review the basis of the MPI theories of Holland (1997) and Emanuel [1986, 1988a,b, 1989, 1991, 1995a,b, 1997, FIG. 1. Dependence of minimum sea level pressure (MSLP) on collectively referred to hereafter as Emanuel (1986± eyewall surface relative humidity (see text for details) predicted by 97)]. This is followed by a discussion of shortcomings Holland's (1997) (solid) and Emanuel's (1997) (dashed) MPI theories. present in each model, including a comparison of Eman- Initial conditions: T 5 300 K and p 5 1015 mb. s env uel's theory with the numerical models of Emanuel (1995b) and Rotunno and Emanuel (1987). The adverse Kleinschmidt (1951) modeled the tropical cyclone with effects of secondary eyewalls on tropical cyclone in- a frictional boundary layer beneath a conditionally neu- tensity are examined next using Ooyama's (1969) hur- tral out¯ow layer very similar, in many respects, to the ricane model. It is suggested that secondary eyewalls recent work of Emanuel (1986) [see Gray (1994) for a and the convectively generated vorticity anomalies that comparison of these two approaches]. Malkus and Reihl spawn them are the principle reason why most hurri- (1960) examined parcel trajectories in the in¯ow layer canes fail to reach their MPI even under favorable en- of tropical cyclones, while Miller (1958) developed an vironmental conditions based on SST, out¯ow temper- MPI theory controlled by sea surface temperature (SST) ature, vertical shear, etc. Finally, the main ®ndings are and the height of the convective equilibrium level (EL). summarized and paths for future research are discussed. Camp (1999) provides a detailed review of each of these approaches. 2. Holland (1997) In recent years, the most widely recognized investi- gations of MPI are those of Emanuel (1986, 1988a,b, Nearly 40 years after Miller introduced his MPI the- 1991, 1995a, 1997) and Holland (1997). While the ory, Holland (1997) introduced a similar, yet revised method used by Holland is similar to that of Miller, MPI theory. As with Miller (1958), Holland's theory Emanuel takes a very different approach leading to a relies heavily on the presence of ambient CAPE to de- revised view of tropical cyclone thermodynamic struc- termine the MPI. Both theories assume that surface air ture. The MPI in both models is governed by SST and rises moist adiabatically in the eyewall before sinking surface relative humidity. Additionally, each MPI for- dry adiabatically (with mixing from the eyewall) within mulation is also a function of the thermal structure of the eye. The surface pressure fall due to the warming the upper troposphere, with Holland's MPI regulated by from moist-adiabatic ascent is termed the ``one cell'' the EL, and Emanuel's MPI regulated by the average theory. The surface pressure fall due to the warming temperature of the out¯ow region. Figure 1 shows the from the adiabatic descent of parcels following moist MPI as a function of surface relative humidity at the adiabatic ascent is termed the ``two cell'' theory. eyewall calculated by Holland's and Emanuel's meth- Miller's (1958) and Holland's (1997) approaches are ods. The relative humidity in Holland's model is spec- modi®cations of the one and two-cell theories. The pri- i®ed underneath the eyewall, presumably at the radius mary change made by Holland (1997) to Miller's (1958) of maximum updraft, while the relative humidity in the approach is to utilize the pressure dependence of moist Emanuel model is strictly valid at the radius of maxi- entropy [proxied by equivalent potential temperature mum winds. The calculations performed were made us- (ue)], so that as convection warms the eyewall and low- ing unaltered code generously provided by K. Emanuel ers the surface pressure, the boundary layer ue increases and G. Holland, respectively. With similar initial con- as well, allowing for a further warming in the eyewall ditions, a sea surface temperature of 300 K, and a sur- and a further pressure drop at the surface. For most face relative humidity of 86%, the two theories predict initial conditions, the pressure drops under the eyewall approximately the same MPI. However, as is shown by become smaller with each iteration, leading to a con- Fig. 1, this agreement is purely coincidental. While both vergence of the surface pressure under the eyewall. theories are sensitive to the surface relative humidity Once the eyewall surface pressure converges, an eye near the eyewall radius, they are sensitive in opposite parameterization is used if the net pressure fall under ways. The minimum sea level pressure given by Eman- the eyewall is more than 20 mb. It is argued that a system 1706 MONTHLY WEATHER REVIEW VOLUME 129 FIG. 3. Minimum sea level pressure from Miller's (1958) model (solid), and Holland's (1997) model (dot), as a function of sea surface temperature with RHs 5 85%.
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