[1] Changes in the Baiu rainband owing to global warming are assessed by the pseudo global warming downscaling method (PGW-DS). The PGW-DS is similar to the conventional dynamical downscaling method using a regional climate model (RCM), but the boundary conditions of the RCM are obtained by adding the difference between the future and present climates simulated by coupled general circulation models (CGCMs) into the 6-hourly reanalysis data in a control period. We conducted the multiple PGW-DS runs using the selected Coupled Model Intercomparison Project Phase 3 (CMIP3) multimodel data set, giving better performance around East Asia in June, and the PGW-DS run using the multiselected CGCM model ensemble mean (PGW-MME run). The PGW-MME and PGW-DS runs show an increase in precipitation over the Baiu rainband and the southward shift of the Baiu rainband. The PGW-MME run has good similarity to the average of all PGW-DS runs. This fact indicates that an average of the multiple PGW-DS runs can be replaced by a single PGW-DS run using the multiselected CGCM ensemble mean, reducing the significant computational expense. In comparison with the GCM projections, the PGW-DS runs reduce the intermodel variability in the Baiu rainband caused by the CGCMs themselves.

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Sensitivity of precipitation diurnal cycle to the horizontal grid spacing was investigated using a mesoscale model without cumulus parameterization. Four numerical experiments with changing horizontal resolution are performed over one month with NCEP reanalysis boundary forcing. The studied range of grid spacing is from 3.5 km to 28 km which has been known as the intermediate scale to simulate mesoscale systems with/without cumulus parameterization. The target area is the Tibetan Plateau where pronounced diurnal cycle of convective systems is observed during the spring season.Lower resolution runs (14 and 28 km grid) show delayed formation and delayed mature stage of the cumulus convection in comparison to satellite observations. On the other hand, higher resolution runs (3.5 and 7 km grid) reproduce the proper development of the clouds after local noon which is consistent with observations. The total hydrometeor content and rainfall rate increase with grid size. Such systematic relationship of resolution dependencies are confirmed even in the monthly mean diurnal cycle, although most of previous studies examined only short periods. These results suggest that finer resolution at less than 7 kilometer is necessary to simulate realistic phase of the precipitation diurnal cycle over the Tibetan Plateau.The mechanism that is responsible for the resolution dependency is discussed. We suggest that the Journal of the Meteorological Society of Japan Vol. 86A 18

Early summer climate in the western North Pacific is largely represented by the baiu phenomenon. The meridional fluctuations of the baiu front on interannual time scales and the associated large-scale circulations are examined using the empirical orthogonal function (EOF) analysis and composite or correlation analyses based on the EOF time coefficients. The first EOF mode indicates a 5- or 6-yr low-frequency fluctuation (LF mode) appearing south of 35°N. The development is concurrent with horseshoe sea surface temperature anomalies (SSTAs) in the entire tropical Pacific that are associated with the El Niño–Southern Oscillation (ENSO). SSTAs in the western North Pacific control the anomalous southward expansion of the baiu front through a modification of the convection at around 20°–35°N. The LF mode is negatively correlated with the south-southeast Asian summer monsoon. The second EOF mode is characterized by a meridional seesawlike fluctuation with a node at around 28°N and a time scale of biennial oscillation (BO mode). The horseshoe SSTAs again control the anomalous meridional circulations, but with a different spatial phase through a convection off the Philippines. The spatial phase difference between the two horseshoe patterns is about 90° in both the zonal and meridional directions. The BO mode is negatively correlated with the tropical western North Pacific monsoon. SSTAs associated with the BO mode tend to be confined to the tropical western Pacific, while the signals of the LF mode extend rather broadly in the tropical Pacific–Indian Ocean sector, suggesting that the tropical BO is an aborted ENSO in the tropical central–western Pacific. The spatial phase of horseshoe SSTAs adjusts the interannual variability of the meridional fluctuation of the baiu front in the western North Pacific.

This study investigates precipitation over the Maritime Continent, comparing the precipitation simulated by a 20 km-grid Meteorological Research Institute General Circulation Model (MRI-GCM) and the nearsurface rain data of TRMM 2A25. The focus is particularly the diurnal cycle and its phase distribution of precipitation. The features of the simulated precipitation mostly agree well with observations made over islands and straits having horizontal scales smaller than 200 km. However, these are quite different around larger islands, such as Sumatra and Borneo, particularly in the phase of the diurnal cycle. The MRI-GCM indicates maximum pre cipitation in the early afternoon on these islands, while the observed precipitation has its maximum at night. In particular, over the inland areas of the larger islands, the simulated diurnal cycle has almost a reversed phase. The simulated precipitation is remarkably weaker than the observation around the western coast of Sumatra Island, where a large discrepancy is also found in the phase distribution along a line perpendicular to the coast. A higher-resolution simulation using a nonhydrostatic model without convective parameterizations substantially improves the phase distribution over Borneo Island. The non-hydrostatic model simulates well the migration of the precipitation zone and the daily maximum at night in the inland areas. In contrast, the GCM fails to simulate the diurnal cycle over islands whose horizontal scale is larger than 200 km, although the 20 km grid spacing is small enough to reproduce the major aspects of the local circulations. The cause for this seems to be the cumulus convective parameterization, which may not adequately represent the coupling of convection and local circulations.