Since 1986, a simple computation model for a nuclear accident has been operating in the emergency information center of Japan Agency for Science and Technology. It was developed by introducing the variation method for wind and a random walk particle model for diffusion in 50–100 km scale. Furthermore, we developed a new model with dynamic equations and a diffusion equation to predict more accurately the wind and diffusion, including local thermal convection, as an improvement on the present system.In the new model, the momentum equation and the continuity equation are solved numerically in non-hydrostatic and incompressible conditions, using a finite difference technique. Then, the equation of thermal energy preservation is solved for potential temperature in the predicted wind field of every time step. Pressure is given as a deviation from the average value of each level by a numerical solution of a parabolic equation introduced from the momentum and continuity equations. The diffusion of nuclear pollutants is computed numerically by the finite difference method in the predicted wind field, using diffusion coefficients obtained from the predictive dynamic equations.These computations were verified with meteorological surveys and gas tracer diffusion experiments over flat land, along a sea shore and over a mountainous area. As the result of new model development, horizontal circulations and vertical convections can be computed in any mesh size from several tens of meters to several kilometers, while small vertical convections less than 1 km or so cannot be represented with the former hydrostatic circulation models.