**Author details**

formations with a single MCS, some of them associated with easterly waves usually take an average of 13 h to form a TC. In contrast, for monsoon trough-related formations, there are usually more than two MCSs, and the average time to form a TC is about 1 day. Very often, more than one MCS coexist at the same time within the monsoon trough. Whereas recent studies on TC formations, such as Refs. [10, 21, 22], focused on contributions from convectivescale systems such as vortical hot towers (VHTs) and convectively induced vorticity anomalies (CVAs), their interactions with mesoscale circulations, and mechanisms to generate the core surface vortex, less discussion was on the relationship between convection patterns and development times of TCs. The numerical simulations of typhoons Dan and Ketsana here illustrate that the pace of early development of a TC depends much on the convection

It might seem to be counterintuitive that the multiple MCS convection pattern is not speeding up the TC development process due to spinning up of larger relative vorticity within more areas in the TC, but rather slowing the process down compared with the single MCS pattern. Nevertheless, this phenomenon may be explained from the point of view of competing for convective resources. Whenever deep convection occurs, the environmental CAPE is consumed. Descent of cold and dry air left by heavy precipitation is unfavorable for new convection development, which has to wait until the environmental CAPE increases again by heat and moisture fluxes from the ocean surface. When Ref. [21] analyzed the simulated formation of Hurricane Dolly (2008), similar processes were found under the TC system scale, namely, there were episodes of deep convection during the development of Hurricane Dolly, but in between there was a period when the CAPE was minimum. This interpretation is consistent with the simulation for Typhoon Ketsana, especially during the development of MCS3 and MCS4. These two MCSs developed at about the same time with a large area of convection, but did not lead to TC formation right after. The reason may be that MCS4 to the east was consuming part of the CAPE within the TC area, making the MCS3 to the west less conducive to developing more deep convection and subsequent low-level vorticity generation. In fact, MCS3 dissipated first, and the system intensification slowed down before another MCS developed

It is interesting that this kind of CAPE argument is also applicable to the convective scales, as has been briefly discussed in Ref. [21]. Various convection processes, such as waxes and wanes, merging and splitting within each MCS are common during the early development of TC. That is, these processes represent the variability of the convective-scale VHT and CVA activities of the MCS, which is likely also due to the consumption and recovery cycles of CAPE. One point to note is that since the single-MCS convection pattern has been identified as the most common one at the end of early development, that is, for the TC to enter the intensification phase, the convection within that single MCS has to be long-lasting enough for the system to intensify to a sustainable level. In other words, there may be a hypothesis that the CAPE consumption and recovery cycles to sustain convection are more efficient under convective scales compared with mesoscales. This may be due to the fact that the areas for the surface fluxes to keep the lowlevel atmosphere warm and moist are simply much smaller, and/or that merging of the VTHs/ CVAs is effective to sustain deep convection, especially when these systems are moving toward

configuration, namely, whether it is a single MCS or interacting multiple MCSs.

130 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

in the core region.

Kevin K. W. Cheung\* and Guoping Zhang

\*Address all correspondence to: kevin.cheung@mq.edu.au

Department of Environmental Sciences, Macquarie University, Sydney, New South Wales, Australia
