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Munehisa K. Yamamoto, Fumie A. Furuzawa, Atsushi Higuchi, and Kenji Nakamura

1. Introduction Diurnal variations in precipitation systems are of interest in meteorology. The global distribution of such diurnal variations has been studied with ground-based meteorological observations ( Dai 2001 ) and with observations from meteorological satellites carrying either infrared radiometers (e.g., Janowiak et al. 1994 ; Nitta and Sekine 1994 ; Yang and Slingo 2001 ) or microwave radiometers ( Chang et al. 1995 ). Geostationary infrared radiometers observe clouds hourly, and

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Song Yang and Eric A. Smith

Diurnal Variability of Precipitation through Observations and Models . Since the advent of high-quality multiyear and globally distributed observations of precipitation from spaceborne sensors, there has yet to be a focused study concerning the topic of the diurnal variability of precipitation. This special issue of the Journal of Climate contains 11 papers that seek to bring about this focus. For over a century observational and modeling studies have demonstrated that diurnal processes forced by

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Tianjun Zhou, Rucong Yu, Haoming Chen, Aiguo Dai, and Yang Pan

optimal combination of microwave rain estimates from TRMM, Special Sensor Microwave Imager (SSM/I), Advanced Microwave Scanning Radiometer (AMSR), and Advanced Microwave Sounding Unit (AMSU) to adjust IR estimates from geostationary IR observations. The 3B42 estimates are scaled to match the monthly rain gauge observations. To improve the comparability, both the PERSIANN and TRMM data were remapped onto the same 0.5° grid as the rain gauge data. At each grid box and for each hour, the JJA averages of

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J. Li, S. Sorooshian, W. Higgins, X. Gao, B. Imam, and K. Hsu

Prediction (NCEP) Eta Model (48-km horizontal resolution) and found that diurnal variations over the SMO were weaker than the satellite estimates. He argued that these differences were reasonable because the satellite rainfall estimates were based on the maximum instantaneous rainfall in the afternoon, while the model forecast was integrated over time. The arguments above motivate the need for high spatial and temporal resolution ground-based observations to validate both modeled and satellite

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R. Cifelli, S. W. Nesbitt, S. A. Rutledge, W. A. Petersen, and S. Yuter

determining the local phase of the diurnal cycle resulting from either the propagation of convective systems or external nonconvective forcing such as gravity waves. Over the open ocean, studies regarding the diurnal cycle of precipitation are often hampered by the paucity of observations ( Dai 2001 ). This is particularly true in the east Pacific ITCZ region, where atmospheric–oceanic processes are poorly represented in numerical models ( Mechoso et al. 1995 ; Raymond et al. 2003 ) and large

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Song Yang and Eric A. Smith

1. Introduction Understanding space–time variations of precipitation is an important topic in climate research, in which modern, high quality, global-scale precipitation observations are essential. Detailed analyses of such datasets reveal the evolution and lifetime of precipitating clouds, including the embedded convection and stratiform forms of precipitation, all of which help improve rainfall climate predictions. High quality, global-scale coverage of precipitation first became available

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Alex C. Ruane and John O. Roads

were initialized from reanalyzed observations 4 times each day (at 0000, 0600, 1200, and 1800 UTC), and successive 3- and 6-h forecasts link together to form a comprehensive time series. Each forecast time represents the 3-h period preceding it from the same initialization, so a 6-h forecast of precipitation represents the mean precipitation rate between 3 and 6 h of model time, for example. As precipitation and evaporation are not assimilated, these short model forecasts are required to simulate

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Song Yang, Kwo-Sen Kuo, and Eric A. Smith

new types of research concerning diurnal variability. This has been particularly true for precipitation since the advent of the Tropical Rainfall Measuring Mission (TRMM) and its associated TRMM Microwave Imager (TMI) radiometer and Precipitation Radar (PR) rain-rate retrievals dating back to November 1997. Many mechanisms have been proposed to explain diurnal precipitation behavior (see Table 1 for definitions of acronyms used in explaining dynamically, thermodynamically, and radiatively forced

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