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Riko Oki, Akimasa Sumi, and David A. Short


It is known that spatially averaged rainfall rate 〈R〉 is highly correlated with the fractional area (F) of rain rate exceeding a preset threshold (τ), when the area is large enough to include numerous convective systems in various stages of their life cycles. Using this fact, a method to estimate area-averaged rain rate from F(τ), which is obtained from satellite observations, is proposed for Tropical Rainfall Measuring Mission (TRMM). There have been numerous studies investigating F–〈R〉 relationships and optimal thresholds at several radar observation sites around the world but no studies to confirm the performance of the method within Japan. In this study an analysis of radar–AMeDAS (Automatic Meteorological Data Acquisition System) precipitation data is presented. The F–〈R〉 relationships of radar–AMeDAS rain data have been examined systematically, with the result that the optimum threshold that maximizes the correlation between 〈R〉 and F(τ) is near 3.5 mm h−1 in every year and season of available data.

Using the threshold method with the average coefficients obtained when the threshold is set to 3.5 mm h−1, TRMM sampling of radar–AMeDAS rainfall is simulated. Fixing 5° × 5° areas, monthly mean area-averaged rain rate is estimated from the observational coverage that would be obtained by TRMM during a month. The errors from the threshold method are only 3%–4% larger than the sampling errors (14%–19% on average) obtained by using the full dynamic range of observed rain rates. Considering the dynamic range of TRMM sensors, the threshold method would be an effective method to estimate area-average rain rate.

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S. P. Hayes, L.J. Mangum, J. Picaut, A. Sumi, and K. Takeuchi

The importance of the El Niño-Southern Oscillation phenomenon in year-to-year fluctuations of the global climate has led to efforts to improve the real-time ocean observing system in the tropical Pacific. One element of this improved system is the TOGA-TAO (Tropical Atmosphere-Ocean) Array of wind and upper ocean thermistor chain moorings. This array, the result of an international effort, has already provided the rudiments of a basin-wide, real-time observing system and plans call for a major enhancement during the second half of the TOGA decade. The development of the TAO array is discussed, recent results from the pilot measurements are described, and plans for the expanded array are presented.

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I.-S. Kang, K. Jin, K.-M. Lau, J. Shukla, V. Krishnamurthy, S. D. Schubert, D. E. Waliser, W. F. Stern, V. Satyan, A. Kitoh, G. A. Meehl, M. Kanamitsu, V. Ya. Galin, Akimasa Sumi, G. Wu, Y. Liu, and J.-K. Kim


The atmospheric anomalies for the 1997/98 El Niño–Southern Oscillation (ENSO) period have been analyzed and intercompared using the data simulated by the atmospheric general circulation models (GCMs) of 11 groups participating in the Monsoon GCM Intercomparison Project initiated by the Climate Variability and Prediction Program (CLIVAR)/Asian–Australian Monsoon Panel. Each participating GCM group performed a set of 10 ensemble simulations for 1 September 1996–31 August 1998 using the same sea surface temperature (SST) conditions but with different initial conditions. The present study presents an overview of the intercomparison project and the results of an intercomparison of the global atmospheric anomalies during the 1997/98 El Niño period. Particularly, the focus is on the tropical precipitation anomalies over the monsoon–ENSO region and the upper-tropospheric circulation anomalies in the Pacific–North American (PNA) region.

The simulated precipitation anomalies show that all of the models simulate the spatial pattern of the observed anomalies reasonably well in the tropical central Pacific, although there are large differences in the amplitudes. However, most of the models have difficulty in simulating the negative anomalies over the Maritime Continent during El Niño. The 200-hPa geopotential anomalies over the PNA region are reasonably well reproduced by most of the models. But, the models generally underestimate the amplitude of the PNA pattern. These weak amplitudes are related to the weak precipitation anomalies in the tropical Pacific. The tropical precipitation anomalies are found to be closely related to the SST anomalies not only during the El Niño seasons but also during the normal seasons that are typified by weak SST anomalies in the tropical Pacific. In particular, the pattern correlation values of the 11-model composite of the precipitation anomalies with the observed counterparts for the normal seasons are near 0.5 for the tropical region between 30°S and 30°N.

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