Abstract
As the climate changes, the ability to predict changes in the frequency of tropical cyclogenesis is becoming of increasing interest. A unique approach is proposed that utilizes threshold values in potential intensity, wind shear, vorticity, and normalized saturation deficit. Prior statistical methods generally involve creating an index or equation based on averages of important meteorological parameters for a given region. The new method assumes that threshold values exist for each important parameter for which cyclogenesis is unlikely to develop. This technique is distinct from previous approaches that seek to determine how each of these parameters interdependently favors cyclogenesis. To determine three of the individual threshold values (shear, potential intensity, and vorticity), an idealized climate is first established that represents the most advantageous but realistic (MABR) environment. An initial numerical simulation of tropical cyclone genesis in the MABR environment confirms that it is highly favorable for cyclogenesis. Subsequent numerical simulations vary each parameter individually until no tropical cyclone develops, thereby determining the three threshold values. The new method of point downscaling, whereby background meteorological features are represented by a single vertical profile, is used in the numerical simulations to greatly simplify the approach. The remaining threshold parameter (normalized saturation deficit) is determined by analyzing the climatological record and choosing a value that is statistically observed to prevent cyclogenesis. Once each threshold value is determined, the fraction of time each is exceeded in the location of interest is computed from the reanalysis dataset. The product of each fraction for each of the relevant parameters then gives a statistical probability as to the likelihood of cyclogenesis. For predicting regional and monthly variations in frequency of genesis, this approach is shown to generally meet or exceed the predictive skills of earlier statistical attempts with some failure only during several off-season months. This method also provides a more intuitive rationale of the results. Since the genesis frequency index (GFI) method has shown to meet or exceed the predictive skill of other indices, analysis of the impact of El Niño and La Niña on cyclogenesis is performed for each of the six ocean basins. Although the GFI method generally corroborates other studies that have shown El Niño years are conducive for storms in the eastern Pacific and detrimental to storms in the Atlantic, the GFI method reveals important differences within each basin that other studies have not found. In particular the GFI method shows a region north of South America, known for its paucity of storms regardless of global weather phenomenon, is not greatly affected by El Niño events reinforcing the claim the GFI method is a more robust predictor of cyclogenesis. The first calculation of GFI used NCEP reanalysis to determine each of the four fractions (or frequencies). Since the NCEP reanalysis data has shown to underestimate mid and upper-level humidity, two other data sources are used to determine if a more accurate humidity profile can improve the GFI method. Although some of the basins did show an improved skill, there was not a global improvement from a particular data set and therefore the reanalysis data set chosen to calculate GFI was not deemed an important consideration. In an attempt to explain the threshold nature of cyclogenesis, analyses of momentum and moisture budgets on a nascent vortex were performed. Although momentum budgets revealed that friction did not contribute significantly to whether the storm would develop or not, no other processes revealed a threshold-like behavior in cyclogenesis. The moisture budgets, however, revealed an inverse relationship of advected water vapor to latitude. The cause of this was found to be that radial winds are significantly affected by the Coriolis parameter and at higher latitudes these winds (and therefore advected water vapor) were reduced. Since a developing storm requires the latent heat release from moisture and with less moisture available at higher latitudes, the storm must be more efficient at converting this heat to kinetic energy. Investigation into inertial stability found that although the Coriolis parameter was responsible for reduced radial winds at higher latitudes, it was also responsible for the increased stability. With increased stability, the heat produced within the inner vortex was confined to the same region. At low latitudes, even though there was abundant moisture, the inertial stability was too low to allow development, therefore revealing part of the threshold-like behavior of cyclogenesis.