Observational analysis of tropical cyclone recurvature

May 1991.Also issued as author's thesis (M.S.) -- Colorado State University, 1991.Includes bibliographical references.Twenty-one years (1957-77) of North Pacific rawinsonde data are used to show how the large scale synoptic pattern interacts with the tropical cyclones' environment just prior to, and during the recurvature process. This study is believed to be the first to quantitatively examine how the environmental wind fields at all levels of the troposphere are related to tropical cyclone motion prior to, and during, recurvature. Significant changes in the upper tropospheric zonal wind fields were found to the north and northwest of tropical cyclones one to two days prior to beginning recurvature. These cyclones actually began to recurve when positive zonal winds penetrated into the mid and upper troposphere, 6° from the cyclones' center. Tropical cyclones which did not recurve showed negative zonal winds at this radius. Based on the results of this study, a recurvature forecasting scheme was developed using the environmental windfields measured in the northwest region of the cyclone. This recurvature scheme was then tested on 55 tropical cyclones which developed in the northwest Pacific in 1984-86. It was found that tropical cyclone direction related fairly well to the mid and upper tropospheric windfields to the north, northwest and west of the cyclone. This recurvature scheme was then used in real-time during the Tropical Cyclone Motion (TCM-90) experiment which was conducted during the summer of 1990 in the Northwest Pacific. This scheme was found to be generally successful.On cover page: ONR N00014-87-K-0203 P05

Cluster analysis of downscaled and explicitly simulated North Atlantic tropical cyclone tracks

A realistic representation of the North Atlantic tropical cyclone tracks is crucial as it allows, for example, explaining potential changes in US landfalling systems. Here we present a tentative study, which examines the ability of recent climate models to represent North Atlantic tropical cyclone tracks. Tracks from two types of climate models are evaluated: explicit tracks are obtained from tropical cyclones simulated in regional or global climate models with moderate to high horizontal resolution (1° to 0.25°), and downscaled tracks are obtained using a downscaling technique with large-scale environmental fields from a subset of these models. For both configurations, tracks are objectively separated into four groups using a cluster technique, leading to a zonal and a meridional separation of the tracks. The meridional separation largely captures the separation between deep tropical and sub-tropical, hybrid or baroclinic cyclones, while the zonal separation segregates Gulf of Mexico and Cape Verde storms. The properties of the tracks’ seasonality, intensity and power dissipation index in each cluster are documented for both configurations. Our results show that except for the seasonality, the downscaled tracks better capture the observed characteristics of the clusters. We also use three different idealized scenarios to examine the possible future changes of tropical cyclone tracks under 1) warming sea surface temperature, 2) increasing carbon dioxide, and 3) a combination of the two. The response to each scenario is highly variable depending on the simulation considered. Finally, we examine the role of each cluster in these future changes and find no preponderant contribution of any single cluster over the others