Observational Analysis of Tropical Cyclone Formation. Part I: Basic Description of Data Sets

Abstract

Twelve composite data sets are constructed from rawinsonde data in the tropical northwest Pacific and tropical northwest Atlantic Oceans. Each data set is a composite average of approximately 80 individual disturbances. Four different types of non-developing oceanic tropical disturbance are composited. For comparison, disturbances in each ocean are composited at four different stages of intensification from pre-hurricane to hurricane and pre-typhoon to typhoon. In total, 912 different tropical weather systems go into the composites and approximately 40 000 rawinsonde observations are used. Details are presented on data density, number of individual weather systems averaged and mean position for each composite system. The basic thermodynamic and dynamic properties of the systems are discussed as well as regional differences between the Pacific and the Atlantic. The analyses presented here form a framework for Part II and subsequent papers which use these composite data sets to investigate the genesis and intensification of tropical cyclones. The composited systems in both oceans exist in easterly winds at low and middle tropospheric levels. The Pacific systems have westerlies close to the south, whereas the Atlantic systems are completely embedded in the easterly trade winds. At the 200 mb level, systems in both oceans are in the vicinity of the subtropical ridge with easterlies to the south and westerlies to the north. The cloud clusters and tropical cyclones in both oceans are characterized by an upper level warm core and a local positive moisture anomaly. They have a two-layer wind structure with convergence and cyclonic vorticity between the surface and 350 mb and divergence and anticyclonic vorticity above that level. Most of the inflow occurs above the frictional boundary layer and is subsequently believed to be flow down the pressure gradient. Regional differences between systems in the two oceans are discussed in terms of Gray's seasonal genesis parameter and it is shown that the background surface evaporation rate is much greater in the Pacific than in the Atlantic.