Tosiyuki Nakaegawa

A study of dynamical seasonal prediction of potential water resources based on an atmospheric GCM experiment with prescribed sea-surface temperature

Abstract The inter-annual variability and potential predictability of seasonal mean potentially available water resources (precipitation, P, minus evaporation, E), are investigated based on SST-forced ensemble dynamical seasonal atmospheric prediction using the Japan Meteorological Agency global model. High temporal correlations between the observed P—E and the model-ensemble average prediction of P—E are found in the central part of the USA for boreal winter and spring, and in eastern Brazil for boreal spring and summer, while a negative correlation is found in India for all seasons. The correlation in Siberia is low in all seasons, and that in Japan is high only in boreal winter. The P—E potential predictability is highest in the tropics and lower in the extra-tropics. High variance ratios exceeding 0.4 are confined to limited land areas. In addition to river basins where seasonal discharge predictions using statistical models were possible in previous works, the Congo, Mekong and Columbia rivers, were also found to be promising with high-potential predictability.

Seasonal prediction of the South Pacific Convergence Zone in the austral wet season

The position and orientation of the South Pacific Convergence Zone (SPCZ), modulated by the El Nino-Southern Oscillation (ENSO), determine many of the potentially predictable interannual variations in rainfall in the South Pacific region. In this study, the predictability of the SPCZ in austral summer is assessed using two coupled (ocean-atmosphere) global circulation model (CGCM)-based seasonal prediction systems: the Japan Meteorological Agencys Meteorological Research Institute Coupled Ocean-Atmosphere General Circulation Model (JMA/MRI-CGCM) and the Australian Bureau of Meteorologys Predictive Ocean-Atmosphere Model for Australia (POAMA-M24). Forecasts of austral summer rainfall, initialized in November are assessed over the period 1980–2010. The climatology of CGCM precipitation in the SPCZ region compares favorably to rainfall analyses over subsets of years characterizing different phases of ENSO. While the CGCMs display biases in the mean SPCZ latitudes, they reproduce interannual variability in austral summer SPCZ position indices for forecasts out to 4u2009months, with temporal correlations greater than 0.6. The summer latitude of the western branch of the SPCZ is predictable with correlations of the order of 0.6 for forecasts initialized as early as September, while the correlation for the eastern branch only exceeds 0.6 for forecasts initialized in November. Encouragingly, the models are able to simulate the large displacement of the SPCZ during zonal SPCZ years 1982–1983, 1991–1992, and 1997–1998.

Aggregation criteria for surface heat balances in a heterogeneous area based on a linear model

Abstract The present study treats the aggregation of the heterogeneity of the surface heat fluxes. The aggregation criteria proposed here are used to evaluate the aggregation error and to calculate the maximum values of the statistics of the distributing parameters with a given tolerance error. The intrapatch scale distribution of the surface flux bulk transfer coefficient can be aggregated within a tolerance error of 5 W/m 2 using the mean parameter method (MPM) in all cases, but the evaporation efficiency cannot be aggregated using MPM in some cases. However, aggregation is possible even when the moment method of second-order (MM2) is applied. The interpatch scale heterogeneity at an area constituted of different land cover types can be aggregated with good accuracy using MM2. This result reveals that aggregation using the dominant land cover method (DLM) and MPM could result in a significant error of more than 40 W/m 2 .