A comparison of large-scale influences on tropical cyclogenesis in the Eastern Pacific

In a given hurricane season, several tropical disturbances propagate across environments favorable for development; however, only a few disturbances actually strengthen into tropical cyclones. The lack of a consolidated theory on tropical cyclogenesis makes it difficult for forecasters to predict a storm’s development. Previous studies have approached this problem by comparing large-scale influences on storms that developed into tropical cyclones and on those that did not. This study used a similar approach to characterize the environmental influences on cyclogenesis in the 2005 Eastern Pacific Hurricane season. Data for each storm were taken from the NCEP/NCAR Final Analysis model and analyzed over a 48-hour period during the development stage. The non-developing storms were selected based on certain atmospheric parameters to resemble the developing storms prior to cyclogenesis. Composites and spatial averaging were used to compare 12 developing storms and 11 non-developing storms during this season. The results showed that the environments of the developing storms had large regions of increased moisture above the boundary layer and greater temperatures in the upper troposphere. Regions of increased potential vorticity penetrated deeper into the troposphere for the developing storms. Lastly, the storms that developed were in environments with relatively strong wind shear to the south of the vortex. The results suggest that the moisture, temperature, and wind shear fields preceded development, while the vorticity fields were more of an indicator of development. Identifying these large-scale characteristics as possible determining influences can lead to a better understanding of tropical cyclogenesis.