An analysis of the genesis of Hurricane Guillermo (1991).

Abstract

A case study of tropical cyclogenesis in the eastern Pacific Ocean is investigated. The tropical cyclone developed in 1991 during the Tropical Experiment in Mexico (TEXMEX) project. In this case study, the initial circulation originated west of Central America and after a period of intensification this circulation became Hurricane Guillermo. The purpose of this research is to identify the physical mechanisms that are active in the formation of the initial circulation and the role that the topography plays in this formation. A documentation of the characteristics of the large scale-flow present prior to the detection of the initial circulation is performed. The observations used include data derived from upper-air soundings and satellite imagery. These observations show that a synoptic-scale easterly wave moved over the Caribbean Sea and while the wave approached the topography of Central America, a low-level, mesoscale circulation developed over the eastern Pacific. It is observed that the modification of the easterly flow by the mountains is an important element in organizing the initial circulation. In order to investigate the dynamics involved in the formation of the circulation model simulations are performed. The objective of these simulations is to reproduce the formation of the circulation and analyze the contribution from the topographic modification of the flow in the formation of the vortex. This objective is accomplished by performing numerical simulations with a mesoscale model. The model fields indicate that the initiation of the circulation occurred while the easterly wave axis moved close to the mountains of Central America. The changes in the direction of the upstream easterly flow, induced by the wave, and the deflection of the winds by the mountains generated a pair of easterly jets in the eastern Pacific. These two elements, along with the Intertropical Convergence Zone (ITCZ), provided a mesoscale area of enhanced vorticity that defined the formation of a closed circulation. A further intensification of the circulation occurred and the storm evolved into a stronger system.