Spatial and Annual Variations in the Diurnal Cycle of Large-Scale Tropical Convective Cloudiness and Precipitation

Abstract

Three years of three-hourly infrared satellite data from the American geostationary satellites were used to determine the large-scale spatial and temporal variations in the diurnal cycle of tropical convective precipitation. The region examined extended from 50°N to 50°S, 175°E to 25°W. The satellite data were related to convection through the fractional coverage of 2.5° subareas by clouds colder than several threshold temperatures. Seasonal maps showing mean fractional coverage, total diurnal variance in cold clouds, as well as variance associated with the first and second harmonic, respectively, present the results. Seasonal maps showing vectors of the amplitude and phase of the first harmonic are also shown. In general, our results agreed with previous studies. The mean positions and annual variations of the maxima and minima in tropical convection were accurately depicted. The diurnal cycle over the tropical continents and the other continents during summer was much larger than that over the oceans. In virtually all areas where the diurnal variation was large, the first harmonic explained most of this variance. The interior of South America during summer had an 1800 LST maximum, with coastal and mountain regions showing somewhat earlier maxima. Over the Central American mountains in summer, late evening or early nighttime maxima were apparent, with near noontime maxima over the adjacent waters. The diurnal cycle observed over the United States in summer was also consistent with previous results. Early morning maxima along the Gulf Coast, the Florida peninsula and the Ohio River Valley, afternoon maxima over the western plains and mountains, evening maxima in the upper Mississippi River Valley, and an area of small diurnal variation and ambiguous phase extending southwestward from the Great Lakes were all present in the data. Substantial diurnal cycles over the oceans were apparent only in the convergence zones. These regions were generally characterized by near-noontime maxima. Although the principle contrasts of continent/ocean, convective/nonconvective, and high/low relief were apparent in each year, substantial interannual fluctuations in the variances of the diurnal cycle were also noted. Some of these fluctuations such as the one that occurred during the 1982?83 ENSO event, could be attributed to shifts in convection. Other interannual variations have no clear explanation and may represent sampling fluctuations.