Seasonal variations of tropical oceanic rainfall characteristics

Abstract

Using infrared cloud top temperatures observed by the Geostationary Operational Environmental Satellite (GOES) - West, an attempt is made to quantify the area-averaged rainfall by using the GOES precipitation index (GPI). The diurnal and seasonal variations of tropical oceanic rainfall are investigated by using the GPI for the period 19 December 1987 to 31 December 1988 over the central and eastern tropical Pacific Ocean. The coefficient of variation is found as a simple function of the mean. Seasonal variations of the coefficient of variation are small except during Northern Hemispheric summer. The time scales of area-averaged rainfall over the South Pacific Convergence Zone (SPCZ) are longer than those over the Intertropical Convergence Zone (ITCZ) during Northern Hemispheric winter; however, they have almost the same values as those over the ITCZ region during Northern Hemispheric summer. The coefficient of variation over the Pacific ITCZ region is reasonably close to that in the GARP Atlantic Tropical Experiment (GATE) region. There are seasonal amplitude variations in the diurnal and semidiurnal cycles. During Northern Hemispheric winter, diurnal and semidiurnal cycle amplitudes are large over the SPCZ region. Power spectrum analysis shows that the SPCZ region has suppressed rainfall at night. Rainfall is enhanced from morning to midafternoon and reaches a maximum between 1400 to 1700 local standard time (LST). During Northern Hemispheric summer, strong diurnal and semidiurnal variations were observed over the ITCZ region. Amplitudes are also large to the north of the ITCZ. The composited diurnal and semidiurnal cycles show a rainfall minimum around midnight and a rainfall maximum at 1500 LST. There is also minor enhancement around 0700 LST. These results are consistent with results over the GATE region.