A Statistical and Synoptic Investigation of Tropical Cyclone Intensity Changes Over the Gulf Stream Along the Southeast Coast of the United States

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Title: A Statistical and Synoptic Investigation of Tropical Cyclone Intensity Changes Over the Gulf Stream Along the Southeast Coast of the United States
Author: Bright, Robert James Jr.
Advisors: Dr. Lian Xie, Committee Chair
Dr. Len Pietrafesa, Committee Member
Dr. Al Riordan, Committee Member
Abstract: Cases of unexpected rapid intensity changes of tropical cyclones (TCs) close to the coast of the United States (U.S.) such as Opal (1995), Bertha (1996), and Lili (2002) have prompted more research into the factors that affect TC strength. It is well known that along with large-scale environmental and storm-scale internal influences, TC intensity is also influenced by interactions with the underlying ocean. In particular, warm ocean currents such as the Loop Current (LC) in the Gulf of Mexico and the Gulf Stream (GS) along the U.S. East Coast can provide these storms with extra energy through enhanced heat and moisture fluxes. However, the extent to which each of these factors is important is not as well understood. Although previous studies have investigated the potential impact of the LC and its associated warm-core eddy in helping to strengthen TCs, none have focused exclusively on what impact the GS may have. Thus, this research attempts to document the importance of the GS in influencing the strength of TCs approaching the Southeast Coast of the U.S. This was accomplished by performing a climatological study of 48 TCs that crossed the GS during the period 1944-2000 as well as case studies of three recent landfalling hurricanes in North Carolina. In addition, an empirical-statistical prediction scheme was developed to forecast the net intensity changes of TCs over the GS. The statistical analysis based on historical "best track" data suggested that the GS helped to either enhance or at least maintain the strength of most storms. In fact, 75% and 72% of TCs either intensified or maintained their strength over the GS according to maximum wind speed (MWS) and minimum central pressure (MCP), respectively. Moreover, most of these storms experienced greater intensification rates than they had prior to reaching the GS. Composite analysis revealed that strengthening of TCs over the GS was typically associated with a trough northwest of the TC similar to the results of previous studies. It is believed that the trough could contribute positively to the storm by steering it along the GS, which would allow for increased TC-GS interaction, and/or through direct TC-trough interaction. On the other hand, weakening was associated with two different synoptic scenarios. The first is a situation in which no trough is present northwest of the TC, thus allowing the storm to move across the GS rather than along it. This would essentially limit the TC-GS interaction. In fact, the climatological study revealed that the weakening cases spent less than half the time over the GS on average than the storms that intensified. The second scenario is one in which the trough northwest of the TC is stronger than for the intensifying cases. Such a setup would likely lead to increased wind shear over the storm. Thus, the negative effect of shear would likely dominate any positive contributions from the trough (e.g., increased upper-level outflow, enhanced momentum fluxes, etc.). Based on the results of the climatological study, a regional TC intensity prediction scheme was developed to forecast the net intensity changes of TCs over the GS. Stepwise multiple linear regressions were performed to fit climatological, persistence, and synoptic predictors to the observed net MWS and MCP changes, respectively. Results indicate that the storm's duration over the GS, initial intensity, and previous 12-h MWS change were the statistically significant parameters in the MWS model. The important variables in the MCP model include the storm's duration over the GS, 12-h MCP change, and the magnitude of the 850-200-hPa shear at the time the storm left the GS. The coefficients of the parameters in each model are physically consistent and agree with the results of the climatological study. For example, the longer a TC spends over the GS the more likely it is to intensify. Also, intensification is more likely if the storm is already intensifying prior to the GS. Forecasts based on the MWS model were then compared to average forecast errors from various operational models as well as the National Hurricane Center official forecasts. Overall, the MWS model performed well despite it being a simple persistence-type model. Case studies of three recent landfalling hurricanes in North Carolina were also performed to highlight the different types of intensity changes experienced by TCs over the GS. Hurricane Bertha (1996) rapidly intensified prior to landfall as it crossed the GS and interacted with a short-wave trough. It appeared this event was due to a combination of enhanced TC-GS interaction while in a favorable upper-level environment. Hurricane Bonnie (1998) basically maintained its strength over the GS. However, the storm did experience a slight MCP decrease after the GS, but its proximity to the coast likely limited any chance for intensification. On the other hand, Floyd weakened while traversing the GS, albeit more slowly than it had been prior to reaching the GS. Dry air entrainment and increased vertical wind shear seemed to be the likely causes for Floyd's general weakening trend during that period. However, the storm then maintained its MWS after crossing the GS prior to making landfall. Overall, the results of the three case studies support the findings of the climatological study and other previous studies that indicate the positive influence warm ocean features can have on TC intensity.
Date: 2003-10-06
Degree: MS
Discipline: Marine, Earth and Atmospheric Sciences
URI: http://www.lib.ncsu.edu/resolver/1840.16/971

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