The Diurnal Variation of Deep Convection and Inferred Precipitation in the Central Tropical Pacific During January–February 1979

Abstract

Infrared satellite data are used to determine the diurnal variation of deep convection and inferred rainfall in the central tropical Pacific during January-February 1979. The parameter employed to characterize the convection is the percent coverage of 1.5° latitude-longitude squares by clouds with tops colder than various specified equivalent blackbody temperatures. Rainfall estimates are based on an empirical relationship between precipitation rate and fractional coverage by cold clouds derived from measurements taken during the GARP Atlantic Tropical Experiment (GATE). In addition, the diurnal variation of vertical motion, determined kinematically from level III-b gridded wind analyses of the European Centre for Medium Range Weather Forecasts, is examined. Principal conclusions are: A pronounced diurnal variation of deep convection occurred throughout the region of study. At certain hours fractional coverage by clouds colder than ?36°C deviated by as much as 40% from the daily mean. The variation was especially large in the South Pacific Convergence Zone (SPCZ). The character of the diurnal cycle varied considerably within the region of study, distinctly different convective regimes being found in five subregions that were examined. The diurnal variation of inferred precipitation also differed from one subregion to another. The SPCZ exhibited a cycle similar to that observed in the GATE B-scale area where a prominent afternoon maximum occurred. Morning maxima, however, prevailed in the Intertropical Convergence Zone (ITCZ) and in tropical cloud intrusions that occurred frequently south of Hawaii. Vertical motions were upward relative to the mean at 2000 LST in the belt between 10 and 20°N and downward in the SPCZ and ITCZ. The opposite behavior occurred at 0800 LST. No obvious relationship existed between the cloud and vertical motion variations.