Department of Earth System Science

A data set of Intertropical Convergence Zone (ITCZ) extent and location in the eastern Pacific, 90°W–180°W, 0°N–25°N, is used to examine the diurnal cycle of cloudiness over the ocean. The data set was generated using a statistical model which utilizes 30 year, 3-hourly infrared (IR) satellite data from 1980 to 2009. The ITCZ envelope of convection has a significant diurnal cycle across the whole domain, and the area of cloudiness “pulses” in extent, peaking in the afternoon. The diurnal cycle of brightness temperatures within the ITCZ changes from east to west, but generally there are two minima in mean IR, implying cold cloud is at a peak: one in the morning, 0600–0900 LST, and another in the afternoon, 1300–1600 LST. Decomposition of cloud top temperatures show that high cloud is at a maximum in the early morning and midlevel cloud peaks in the afternoon. Low cloud is at a maximum in the late evening (2200–0000 LST) and is immediately followed by increases in high cloud, suggesting that the deepest convective systems grow rapidly, within a few hours overnight. There are seasonal changes in the diurnal cycle of cloud top temperature, and it is suggested that colder mean temperatures are more likely in the afternoon when the ITCZ is more extensive. This is shown via seasonal changes and also by comparing El Niño years, when ITCZ is more present, to La Niña years.