DIURNAL-VARIATION AND LIFE-CYCLE OF DEEP CONVECTIVE SYSTEMS OVER THE TROPICAL PACIFIC WARM POOL

Satellite infrared data and in situ surface measurements from the Trop ical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experim ent (TOGA COARE) are used to examine the diurnal variations of deep co nvection in two distinct large-scale now regimes over the western Paci fic warm pool. Large-scale atmospheric dynamic and radiative processes strongly affect the life-cycle of deep convective systems in the trop ics. The observed diurnal variation of tropical cloud systems suggests that diurnal heating of the tropical atmosphere and ocean surfaces pr ovides favoured conditions in the afternoon for the formation of cloud systems and, as the cloud systems grow and decay with time, the diurn al cycle of cloudiness reflects the life-cycle (initiation, growth, an d dissipation) of cloud systems. During the convectively suppressed ph ases of the intra-seasonal oscillation (ISO), the cloud systems are sp atially small and their lifetimes are generally short (<3 h). They for m, reach maximum size, and die preferentially in the afternoon, at the time of day when the ocean surface and overlying atmospheric surface layer are warmest from solar heating. During the convectively active p hases of the ISO, the cold cloud coverage is dominated by spatially la rge, long-lived cloud systems. They tend to form in the afternoon (140 0-1900 LST) and reach a maximum areal extent of very cold cloud tops ( <208 K) before dawn (0000-0600 LST). As part of their life-cycle, the subsequent decay of these large systems extends into the next day; the satellite-observed maximum cloud coverage is dominated by successivel y warmer cloud tops, from 208-235 K in the early afternoon (similar to 1400 LST) to 235-260 K in the early evening (similar to 1800 LST) Mea nwhile the frequency of small cloud systems exhibits two peaks-one in the afternoon and the other in the predawn hours. The latter is eviden tly triggered by outflows from the large convective systems. The life- cycle of the large, long-lasting convective systems introduces horizon tal variability into the pattern of observed cold cloud tops during th e active phases of the ISO. Because the life-cycle of large convective systems can take up to a day, they leave the boundary layer filled wi th air of lower moist-static energy and a cloud canopy that partially shades the ocean surface from the sunlight the following day. So the d ay after a major large convective system, the surface conditions do no t favour another round of convection; therefore, convection occurs in neighbouring regions unaffected by the previous convective systems. We call this spatially selective behaviour of the large systems diurnal dancing. The boundary-layer recovery phase leads to a tendency for the large systems to occur every other day at a given location. This 2-da y periodicity appears to phase-lock with westward-propagating equatori al inertio-gravity waves of similar frequency. The combination of the diurnal surface-cloud-radiation interaction and equatorial inertio-gra vity waves may explain the observed westward-propagating 2-day disturb ances in cold cloud tops over the warm pool.