The tropical super cloud cluster (SCC) is now well known to have wide-range of hierarchy of convections within its eastward-moving envelope spanning 2,000--5,000 km. The observations revealed that the hierarchy includes westward-moving cloud cluster of O (100km) in scale (Nakazawa,1988), which in turn consists of meso-scale convection of O (10km) (Chen et al.,1996), which would ultimately be an ensemble of cumulus convection. The origin of the hierarchical convections, however, remains unknown, particularly in terms of less-than cloud-cluster scales. Besides, we have not yet understood how the hierarchically organized SCC relates to Madden-Julian oscillation (MJO), and other tropical waves. This study aims to understand this issue by using the two-dimensional cloud-resolving model (Yamasaki,1984). Its horizontal domain, consisting of a central fine-mesh (1km-grid) area covering 10,000 km and a coaser-mesh area outside, spans 40,000 km in total, equivalent to the full circumference of the equator. It enables us to pursue behaviors of organized cumulus convections over wide-range of scales explicitly, and their interactions with a MJO-scale circulation as well. Also, we assume relatively high sea surface temperature (SST, 302K) approximately in the fine-mesh area, which mimics the warm pool where these phenomena are frequently observed.
The time integration revealed that a large-scale (4,000--6,000km) quasi-stationary convection develops over the high SST area. As observed, it includes synoptic-scale SSCs, each of which involves hierarchy of convections; cloud cluster (CC), meso-scale convection (MC), and cumulus convection. The key agent of the hierarchical organization and the eastward movement of SCC is gravity waves of meso-scale (O(10--100km)) induced by MC and CC. Following the low-level moistening by WISHE (Emanuel,1987), updrafts of the eastward-propagating waves initiate low-level clouds and subsequently help them develop into taller, vigorous convections through dynamical uplift as well as adiabatic cooling over the cloud (de-stabilization). Once developed, the convection induces gravity waves, which then repeat the above process to yield clustering of MC into CC, and CC into SCC (a similar mechanism proposed by Chao and Lin (1994)). The intensification of MC is an additional requirement for making the gravity waves intensive enough to trigger new convections, where the interaction between cold pool and low-level warm flows plays the role. It turned out that the SCC thus created propagates eastward at the speed of 3--6m/s, comparable to the observations.
Additionally, the SCC induces and cooperates with planetary-scale, wavenumber-1 gravity wave propagating eastward over 40,000km in 30 days (30-day wave). Due to the difference in phase velocity, SCC and 30-day wave exhibit cooperative and anti-cooperatve regimes. The cooperative regime is governed by wave-CISK, where dynamical and thermodynamical fields induced by SCC tends to destabilize 30-day wave, which in turn maintains SCC. The anti-cooperative regime without such interaction results in the decay of SCC. Furthermore, it is interesting to note that 30-day wave coexists with gravity waves of meso-to-synoptic scale which propagate at the speed of 40--45m/s as recent observational studies reported (Milliff and Madden,1996).