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Phillip A. Arkin and Philip E. Ardanuy


Measurement of climatic-scale precipitation (defined here as averages over areas of >104 km2 and periods of five days or longer) is impractical for many areas of the earth without the use of space-based observations. We briefly discuss the history of satellite rainfall estimation schemes and their application to climate studies. Two approaches—direct and indirect—have dominated work until very recently, when attempts to use more integrated techniques began. Indirect schemes, primarily based on visible and infrared (IR) observations of the characteristics of clouds, have been used in the majority of such studies. Direct schemes, such as those that use microwave observations of raindrop-sized hydrometeors, have been limited by a relative lack of the required measurements. A large number of studies have used datasets not originally intended as precipitation estimates at all, such as the NOAA outgoing longwave radiation data, to produce estimates of very large scale rainfall. Current and prospective attempts to overcome some of the difficulties affecting climatic-scale precipitation estimation are described. The Global Precipitation Climatology Project will integrate data from surface obserations, geostationary IR sensors, and polar-orbiting microwave and IR sensors to produce near-global analyses of monthly rainfall. The proposed Tropical Rainfall Measuring Mission will use a single satellite with an instrument package that will make visible, IR, and microwave radiometric observations. The package will also include a precipitation radar. We discuss certain other proposed satellite missions and international programs and their contributions to the production of climatic-scale precipitation estimates. Finally, we propose the development of a global rainfall analysis system.

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John E. Janowiak, Philip A. Arkin, Pingping Xie, Mark L. Morrissey, and David R. Legates


Very few (if any) in situ measurements of rainfall are available in the Pacific ITCZ east of the Line Islands (157°W). Hence, climatological datasets, which are assembled from various in situ sources, and satellite-derived analyses of precipitation are frequently relied upon to provide information on the distribution of rainfall in this important region. A substantial amount of disagreement exists among these information sources as demonstrated in this paper. In particular, the east–west gradient of estimated rainfall intensity in the eastern Pacific ITCZ is quite different during the Northern Hemisphere warm season among six different satellite algorithms (one infrared and five microwave) and two climatologies that are examined. Some of these data suggest that a local minimum in rainfall intensity is located near 140°W in the Pacific ITCZ during northern summer, with increasing intensity toward the east and west, while the others depict steadily decreasing rainfall intensity from west of the Americas to the date line. Conversely, all of the precipitation estimates that are examined depict a rainfall maximum in the Pacific ITCZ near 140°W during the Northern Hemisphere cool season, although the magnitudes vary substantially among them.

The authors examine estimates of seasonal mean rainfall over the eastern Pacific ITCZ (cast of the date line) from two rainfall climatologies and six satellite precipitation estimation techniques during July 1987 through June 1990. Inconsistencies among the precipitation analyses are investigated by examining several independent datasets that include atmospheric circulation data, sea surface temperature data, and ship reports of weather type.

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