Diurnal Variation of Deep Cumulus Convection

This paper presents observational evidence in support of the existence of a large diurnal cycle (one daily maximum and one daily minimum) of oceanic, tropical, deep cumulus convection. The more intense the deep convection and the more associated it is with organized weather systems, the more evident is a diurnal cycle with a maximum in the morning. At many places heavy rainfall is 2–3 times greater in the morning than in the late afternoon-evening. Many land stations also show morning maxima of heavy rainfall. The GATE observations show a similar diurnal range in heavy rainfall, but the time of maximum occurrence is in the afternoon. This occurrence is 6–7 h later than in most other oceanic regions and is probably a result of downwind influences from Africa and the fact that the GATE heavy rainfall was often associated with squall lines. Diurnal variations in low-level, layered and total cloudiness show a much smaller range. The variability of deep convection and heavy rainfall is not readily observable from those satellite pictures which cannot well resolve individual convective cells nor is it easily obtained from surface observations of the percent of sky coverage which are heavily weighted to the presence of low-level and layered clouds. A comparison of previous observational studies is made. It is hypothesized that the diurnal cycle in deep convection with a morning maximum is associated with organized weather disturbances. This diurnal cycle likely results from day versus night variations in tropospheric radiational cooling between the weather system and its surrounding cloud-free region.