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Riko Oki and Akimasa Sumi


Sampling errors of rainfall retrieved by a low-inclination, low-altitude satellite, such as the Tropical Rainfall Measuring Mission (TRMM) satellite, are estimated by using radar–AMeDAS (Automatic Meteorological Data Acquisition System) rainfall data around Japan. The data are composite radar-derived rainfall maps calibrated with a dense surface rain gauge network (AMeDAS).

In this study, sampling errors of monthly rainfall have been estimated over 5° × 5° and 2.5° × 2.5° (latitude × longitude) domains using a 43-month time series of radar–AMeDAS data. For a 5° × 5° area the sampling error around Japan would be approximately 16% with the swath width of the TRMM microwave imager (TMI) and approximately 20% with the swath width of the TRMM precipitation radar. Using the swath width of TMI, the sampling error for a 2.5° × 2.5° area was 24%. These errors are relatively larger than found in previous studies using Global Atmospheric Research Program (GARP) Tropical Atlantic Experiment (GATE) radar data, because this study is based on the long-term actual data around Japan, where 1) rain related to midlatitude cyclones and tropical convective rainfall are included, and 2) rainfall over the land as well as the ocean is included. A probable source of additional error for monthly averages is due to the fact that Japan is at the extreme northern edge of the TRMM orbit where it takes six weeks to cover the diurnal cycle.

A strong seasonal dependency of sampling errors was found. For the 5° × 5° area, errors in the summer rainy season (June–September) were 13% and 19% in the dry season (November–March) with the swath width of TMI. In terms of the autocorrelation coefficient of area-averaged rainfall, this difference of sampling errors between summer and winter is reasonable because the summer e-folding time is longer than that for winter.

The sampling errors could be reduced considerably, especially at higher latitudes, by two TRMM satellites flying one-half orbit apart, providing a greater number of independent samples and a complete sampling of the diurnal cycle within one month.

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Quanzhen Geng, Akimasa Sumi, and Atusi Numaguti


By using the National Centers for Environmental Prediction reanalysis data and a nonlinear barotropic model, the transient activities and the possible role of transient vorticity fluxes in the maintenance of east Asian summer seasonal mean circulation anomalies associated with Japan’s extremely cool and wet summer of 1993 and hot and dry summer of 1994 are examined. Data analysis shows that the summertime atmospheric circulation and the transient activity anomalies exhibit nearly opposite patterns over east Asia during these two years. Vorticity budget calculations indicate that the anomalies of transient activities in these two years produced strong transient vorticity flux forcing anomalies that are comparable in magnitude to the divergent forcing anomalies over east Asia.

The nonlinear barotropic model, when forced by the anomalous divergence and transient vorticity flux forcing together, produces simulations that bear resemblance to the observed summertime seasonal mean circulation anomalies of 1993 and 1994. The response to the anomalous transient vorticity flux forcing is found to be similar in magnitude with the response to the anomalous divergence. The solution deteriorates if the transient vorticity flux forcing anomaly is not included.

Both the vorticity budget analysis and model simulations in the present paper show that the transient vorticity flux forcing anomaly plays an important role in maintaining the observed circulation anomalies over east Asia during the 1993 and 1994 summers.

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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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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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