THE DIURNAL CYCLE OF TROPICAL CONVECTION

The diurnal cycle of tropical convection is investigated with global cloud imagery constructed from 11 mum radiance measurements taken aboard six satellites. Four harmonics of the diurnal cycle are resolved in the cloud imagery with about 50 km horizontal resolution. To isolate deep convective activity from other processes which cause diurnal fluctuations in longwave radiance, an index of deep convective activity is constructed by thresholding to brightness temperatures less than 230 K. Significant diurnal amplitude of deep convection is found only over tropical landmasses. Over the tropical oceans the diurnal cycle is weak and is barely discernible from the background red spectrum of convective variance. Nonetheless, oceanic convection exhibits a systematic diurnal fluctuation with maximum intensity in the early morning. Nocturnal subsidence along the cloud-free equator is postulated to play a role in forcing diurnal variation in the intertropical convergence zones. Other mechanisms are also implied to contribute as a similar early morning maximum in deep convection is seen even where no adjoining cloud-free regions occur. The diurnal cycle of deep convection is found to be organized on planetary scales predominantly in nonmigrating modes (i.e., modes with phase speeds not equal to that of the Sun). The nonmigrating modes are postulated to be produced by the nonuniform distribution of convective centers. The nonmigrating, zonally symmetric component is found to be especially large. While the role of this mode in forcing diurnal variations of the tropospheric circulation (e.g., the Hadley cell) is questionable, its role in modulating the ionospheric electric potential is well established. Discrepancies with the traditional notion that cloud-to-ground lightning is the dominant mechanism by which the Earth is negatively charged are discussed. In particular, the probable role of nonlightning-producing clouds (e.g., the stratiform portion of convective complexes) in diurnally modulating the negative charge on Earth is discussed.