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N. Phillips

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Phillip A. Arkin and Bernard N. Meisner

Abstract

Estimates of areal- and time-averaged convective precipitation derived from geostationary satellite imagery using a simple thresholding technique are presented. The estimates are based on measurements of the monthly mean fraction of 2.5° × 2.5° areas covered by clouds whose equivalent blackbody temperature in infrared imagery is below 235 K. The transformation between fractional coverage and rainfall amount is based upon comparisons of fractional coverages using a variety of temperature thresholds and spatial and temporal averaging scales with areal averaged rainfall from the GARP Atlantic Tropical Experiment.

Three-year means of the estimated precipitation for the period December 1981-November 1984 are shown for each of the (3-month) calendar seasons and compared with published descriptions of the long-term seasonal mean rainfall fields. Over the tropical oceans agreement is quite good with no evidence of any systematic errors. Over the Americas, long-term means derived from station observations of rainfall show less extensive areas of heavy rainfall than those derived here, and a slight tendency for lower peak values during the rainy season.

The interannual variability during the 3-yr period is described and compared with station observations of rainfall. The relationship between cloud cover and rainfall in the tropics (30d°N-30°S) is found to be similar to that found in previous studies, with a threshold of 235 K giving highest correlations, while observations between 30° and 50° were best correlated with a threshold of 220 K. The large changes in rainfall distribution over South America associated with the 1982-83 ENSO episode and the breaking of the drought in Northeast Brazil during 1984 are clear in the estimates presented here, but the amplitude of the changes is somewhat over-estimated. Warm season rainfall observed over the United States is less than the estimates, except near the Gulf of Mexico and southeast United States coast where the degree of overestimation increases away from the coast.

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Bernard N. Meisner and Phillip A. Arkin

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.

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