Japan Spectral Model (JSM) was developed for operational forecast use in JMA. The model is a 19-level spectral limited-area model with a horizontal resolution of 40km. The purpose of the model is to predict meso-*-scale phenomena and fine structures of orographically induced disturbances.The model predicts quite well the mesoscale structure within a synoptic scale disturbance and the evolution of a mesoscale cloud system associated with a polar low. The model also shows sufficient ability to predict severe rainfalls in the Baiu season.Forecasts of precipitation show a good performance in spring even in the early hours of the forecast time. However, the scores for the summer season are worse than those for spring. This may be due to the cumulus parameterization scheme used in JSM. Slow spin up of the model is a serious problem in the summer season.
Forecast experiments are performed from various initial states using an operational mesoscale model of JMA in order to improve the spin-up of precipitation forecasts. The initial conditions are designed to investigate the impact of FA cycle, diabatic NNMI and moisture initialization.The diabatic effects in the NNMI are treated in two ways; diabatic heating rates calculated by the model physical processes, and those estimated from observed rain rates with an assumed parabolic heating profile.The examined case includes a mesoscale convective cloud system. The operational forecast is characterized by very slow spin-up of precipitation and position errors after late build-up of precipitation. In the forecast experiments, use of the FA cycle was effective for improving the description of the mesoscale features such as a low-level vorticity maxima in the initial state. As a result, position errors of the rain area in the late hours were reduced, while the model still showed slow spin-up of precipitation. The diabatic NNMI with the model physical processes did not introduce large changes of the divergent motion due to weak diabatic heating calculated in the NNMI. The NNMI using observed rain rates, in contrast, produced strong divergent motion and associated updrafts corresponding to the cloud system in the initial state. However, diabatic heating in the forecast is still weak, and the divergent motion in the initial state dissipated rapidly. In the case where all three methods, i. e., the FA cycle, the NNMI using observed rain rates, and the moisture initialization were combined, the model produced realistic precipitation from the first hour of the forecast, and the effects persisted for a long time. It was found that smooth transition from initialization to forecast is important for the rapid spin-up of precipitation.
Initial condition dependence of the statistical properties, especially the energy decay law, of Burgers’ turbulence is investigated using the modified zero-fourth cumulant approximation. It is shown that the power index of the energy decay varies from −0.67 to 1.2 on the initial conditions.
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