Type of Document Thesis Author Watson, Michael Author's Email Address firstname.lastname@example.org URN etd-07122005-133321 Title A Recalculation of Mpi Using Upper-Ocean Depth Averaged Temperatures: Climatology and Case Studies Degree Master of Science Department Meteorology, Department of Advisory Committee
Advisor Name Title Robert Hart Committee Chair Carol Anne Clayson Committee Member T.N. Krishnamurti Committee Member Keywords
- Maximum Potential Intensity
- Upper Ocean Heat Content
Date of Defense 2005-07-08 Availability unrestricted AbstractTropical cyclone track forecasts have improved greatly in recent years. However, intensity forecasts still pose a large problem in tropical meteorology. Several theories have been developed over the past fifty years which attempt to arrive at an upper bound or Maximum Potential Intensity (MPI) of tropical cyclones. Emanuel (1988, 1995), in particular, uses the SST and atmospheric sounding to arrive at a maximum intensity (EMPI) for the hurricane using a Carnot Engine cycle approach to the energetics of the storm. While this approach has captured the upper bound or greatest intensity of hurricanes reasonably well, the use of SST for EMPI calculations may be overestimating the maximum intensity. Hurricanes draw their energy from a significant depth of the upper ocean, the average temperature of which is almost always less than the SST. Consequently, there may be more cases of tropical cyclones approaching or exceeding their EMPI than an SST-based EMPI would dictate alone. Presumably, a purely SST-based EMPI calculation would only be valid for intense storms if the entire layer affected by hurricane mixing was isothermal. Furthermore, the Carnot approach is based on an axisymmetric tropical cyclone thus ignoring potential asymmetric influences on a tropical cyclone’s strength.
A recalculation of the MPI climatology acknowledging this oceanic variability (NMPI) is performed here in an attempt to better isolate those storms that have approached or exceeded a more realistic MPI. Using the NODC (Levitus) World Ocean Atlas Data (1994) oceanic temperatures over five layers to a depth of 50m have been weighted to capture the upper ocean heat content, producing a climatology of mean upper ocean temperature over the globe. Having captured the upper ocean heat content a new calculation of MPI is performed. The weighting of oceanic layers chosen is supported by observations taken during the passage of Felix over the Bermuda testbed mooring on 15 August 1995 (Dickey, et al. 1998).
A revised MPI climatology for the named tropical systems in the North Atlantic basin from the years 1982-2003 is produced. Results have shown that there is a dramatic impact upon the EMPI climatology when the mean upper oceanic temperature is used instead of just SST. With only the skin SST being used for EMPI, the only storms to actually exceed their MPI are recurving, poleward moving systems, a consequence of the storm accelerating rapidly and moving over cooler waters while weakening more slowly than the timescale required for the storm to come into thermodynamic balance with the decreasing SST. With the new NMPI calculation using a mean oceanic temperature, several storms in the Atlantic basin actually exceed their NMPI.
Two case studies are performed. The first case study looks at Isabel (2003) and indicates the upper ocean may be a limiting factor in preventing a tropical cyclone from reaching its EMPI. The second case study examines Alex (2004) indicating that asymmetric influences, in this case trough interactions, may allow for tropical cyclones to exceed their EMPI.
Further, it was discovered that hurricanes influence the atmospheric and oceanic environment such that stabilization processes from tropical cyclones may preclude development of another storm immediately after storm passage. The findings show the response time necessary for the atmosphere and the ocean to thermodynamically and synoptically readjust such that support exists for tropical cyclone formation. For the strongest storms in the Atlantic basin, it is shown that the most intense estimate of the EMPI was not found under the storm at analysis time, but rather 10-11 days before the storm actually reached the location. This shows that strong tropical cyclones begin to affect the ambient environment well before the TC center arrives at the location, and that the thermodynamic stabilization process of the environment and ocean begins a week or more prior to a strong TC's passage. It is unclear whether this stabilization is a result of the TC outflow itself, or of the modification of the intensity of the Hadley and Walker circulations in the process of TC generation and intensification.
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