Diurnal variation and life‐cycle of deep convective systems over the tropical pacific warm pool

Abstract

AbstractSatellite infrared data and in situ surface measurements from the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA CORE) are used to examine the diurnal variations of deep convection in two distinct large‐scale flow regimes over the western Pacific warm pool. Large‐scale atmospheric dynamic and radiative processes strongly affect the life‐cycle of deep convective systems in the tropics. the observed diurnal variation of tropical cloud systems suggests that diurnal heating of the tropical atmosphere and ocean surfaces provides favoured conditions in the afternoon for the formation of cloud systems and, as the cloud systems grow and decay with time, the diurnal cycle of cloudiness reflects the life‐cycle (initiation, growth, and dissipation) of cloud systems.During the convectively suppressed phases of the intra‐seasonal oscillation (ISO), the cloud systems are spatially small and their lifetimes are generally short (< 3 h). They form, 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 phases of the ISO, the cold cloud coverage is dominated by spatially large, long‐lived cloud systems. They tend to form in the afternoon (1400‐1900 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 successively warmer cloud tops, from 208‐235 K in the early afternoon (∼ 1400 lst) to 235‐260 K in the early evening (∼ 1800 lst). Meanwhile the frequency of small cloud systems exhibits two peaks‐one in the afternoon and the other in the predawn hours. the latter is evidently triggered by outflows from the large convective systems.The life‐cycle of the large, long‐lasting convective systems introduces horizontal variability into the pattern of observed cold cloud tops during the 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 with air of lower moist‐static energy and a cloud canopy that partially shades the ocean surface from the sunlight the following day. So the day after a major large convective system, the surface conditions do not 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‐day periodicity appears to phase‐lock with westward‐propagating equatorial inertio‐gravity waves of similar frequency. the combination of the diurnal surface‐cloudradiation interaction and equatorial inertio‐gravity waves may explain the observed westward‐propagating 2‐day disturbances in cold cloud tops over the warm pool.