Genesis of Typhoon Chanchu (2006) from a Westerly Wind Burst Associated with the MJO. Part I: Evolution of a Vertically Tilted Precursor Vortex

Abstract

Although previous studies have shown the relationship between the Madden?Julian oscillation (MJO) and tropical cyclogenesis (TCG), many scale-interactive processes leading to TCG still remain mysterious. In this study, the larger-scale flow structures and evolution during the pregenesis, genesis, and intensification of Typhoon Chanchu (2006) near the equator are analyzed using NCEP?s final analysis, satellite observations, and 11-day nested numerical simulations with the Advanced Research Weather Research and Forecast model (ARW-WRF). Results show that the model could reproduce the structures and evolution of a synoptic westerly wind burst (WWB) associated with the MJO during the genesis of Chanchu, including the eastward progression of a WWB from the Indian Ocean into the Pacific Ocean, the modulation of the associated quasi-symmetric vortices, the initial slow spinup of a northern (pre-Chanchu) disturbance at the northeastern periphery of the WWB, and its general track and intensification. It is found that the MJO, likely together with a convectively coupled Kelvin wave, provides the necessary low-level convergence and rotation for the development of the pre-Chanchu disturbance, particularly through the eastward-propagating WWB. The incipient vortex evolves slowly westward, like a mixed Rossby?gravity wave, on the northern flank of the WWB, exhibits a vertically westward-tilted circulation structure, and eventually moves northward off of the equator. Results show that the interaction of the tilted vortex with moist easterly flows assists in the downtilt-right (i.e., to the right of the upward tilt) organization of deep convection, which in turn forces the tilted vortex to move toward the area of ongoing deep convection, thereby helping to partly decrease the vertical tilt with time. It is shown that despite several days of continuous convective overturning, sustained surface intensification does not commence until the vortex becomes upright in the vertical. A conceptual model is finally presented, relating the decreasing vortex tilt to convective development, storm movement, TCG, and surface intensification.