Prediction Experiment of Extremely Intense Rainstorm by a Very-Fine Mesh Primitive Equation Model

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Abstract
Forecast experiment is made using a 13-level 42km-mesh primitive equation model for the extremely intense rainstorm event which occurred over western Japan on 23 July 1982. The rainstorm was associated with a meso-α-scale depression and a low-level convergence line developed in a subtropical frontal zone, but not with an intense synoptic-scale trough. The large amount of precipitation (maximum-500mm) was concentrated in a narrow area (-100km square) and for a short time span (6-12 hours). Several aspects of the rainstorm including the concentration of the rainfall, development of the meso-α-scale depression and the convergence line are simulated in the model. However the predicted precipitation rate (-70mm/6 hour) is much smaller than that observed. Dynamical and thermodynamical processes involved in the occurrence of the rainstorm, such as the moisture convergence, changes in the vertical stability, diabatic heating, changes in vorticity and divergence fields are described on the basis of the experiment. The effect of the increase of the horizontal resolution in the model on the prediction of rainfall rate and the meso-α-scale circulation is also examined by comparing the model predictions with the different horizontal grid sizes (42km, 63km and 127km). The result indicates that both the precipitation rate and the total amount of precipitation are increased with the finer grid sizes. This is attributed to the increase in both magnitude and the degree of concentration of the moisture convergence in the lower layers. The result of a forecast experiment on another intense rainfall event over the western part of Japan in 22-23 July 1983 is also discussed.