The role of equatorial Rossby waves in tropical cyclogenesis .

摘要

A set of unique methodologies utilizing both idealized and real-data numerical simulations was employed to examine the links between the equatorial Rossby (ER) wave and tropical cyclogenesis. The anomalous circulations associated with the ER wave relevant to tropical cyclone (TC) genesis (e.g. low-level vorticity, low-level convergence, and vertical shear) were quantified for each of the ER waves. While portions of the ER wave were favorable for genesis, it was shown that when the ER wave anomalies were combined into a single genesis parameter (GP), the values of the parameter were near, but below, a threshold value commonly associated with genesis.;Based on this result it was hypothesized that the magnitude of the anomalous circulations of the ER wave alone is usually insufficient to cause TC genesis; rather, only when an intense ER wave combines with a favorable background flow does the likelihood of genesis increase significantly. A series of idealized monsoon trough (MT) structures were superimposed on the ER wave, and it was demonstrated that within certain portions of the wave, the GP exceeded the threshold value. Not surprisingly, the most favorable conditions for genesis were observed for the most intense (largest amplitude) ER wave in combination with the most intense (large background cyclonic vorticity) MT.;Two different idealized MT flow configurations were simulated for a nine day period. Both MT flows were shown to be relatively stable. That is, while they did not breakdown, their relative vorticity structure remained quasi-steady over the course of the simulations. The idealized MT flow configurations were then added to the ER wave. For both MT cases, the ER wave structure quickly deformed within the background horizontal shear region of the MT. A smaller-scale cyclonic circulation formed as a result of the wave-breaking process and was shown to have a horizontal length scale comparable to a TC. The wave-breaking process was accompanied with a shift of the location of enhanced convection from east of the ER wave cyclonic gyre center to a region co-located with the TC-scale circulation. The TC-scale circulation formed near the initial critical latitude in both cases.;The initial value idealized simulations were repeated, but with moisture and diabatic effects turned off. The resulting ER wave propagated at a phase speed about 1 m s-1 faster when compared to the moist ER wave. While no TCs formed when the idealized MT environments were added to the dry ER wave, the ER wave was observed to break regardless of moisture and diabatic effects in the vicinity of the critical latitude. As was the case in the moist simulations, the TC-scale circulation was stronger for the case of the stronger MT flow configuration.;In the final part of the study, real-data simulations were performed in which an ER wave was inserted into a more realistic background environment. Two TC genesis cases from the real-data simulations were examined in much more detail; one case (Case 1) in which an ER wave promoted TC genesis and the other (Case 2) in which the ER wave suppressed TC formation relative to the control simulation. For both cases, the local conditions prior to and at the time of genesis were documented. The large-scale environment in Case 1 was associated with anomalous cyclonic low-level vorticity, anomalous low-level convergence, and weaker vertical shear relative to the CON simulation. In Case 2, the large-scale environment featured much larger vertical shear and anticyclonic relative vorticity, owing to the circulations of the ER wave.;Results from the various numerical experiments demonstrated that ER waves have the ability to both enhance and suppress TC formation. In the case of ER wave-enhanced genesis, the anomalous low-level vorticity and convergence were the dominant factors. For the case of ER wave-suppressed activity, vertical shear was much more important. It appears that an anomalously intense ER wave is usually not sufficient on its own to initiate genesis. However, an anomalously intense ER wave placed within a favorable background environment (e.g. monsoon trough) makes genesis much more likely. The wave-breaking of the ER wave is a viable mechanism for the initial formation of the TC scale circulation. The timescale of the formation of this circulation occurred over a much faster timescale for the stronger MT flow configuration when compared with the weaker MT. (Abstract shortened by UMI.)