Effect of Variable Resolution System Based on Stretched Global Grid in KIM

   As science and technology develop, the operation of global numerical weather prediction models at high-resolution is considered to demonstrate more accurate forecast performance. However, when using the high-resolution numerical forecast model, the size of the grid becomes smaller, the number of grids increases by the square of the horizontal resolution change rate, and a relatively small time-step size must be applied according to the Courant-Friedrichs-Lewy condition. For these reasons, the overall amount of calculation increases and the limits of computer resources are encountered.

   Now, in this study, we intend to use a variable resolution system based on a stretched global grid in Korean Integrated Model (KIM) to see the effect of high-resolution numerical forecasts at least in the region of interest without increasing the number of grids due to resolution changes. This system uses the Schmidt transform to expect a high-resolution effect using fine grids for the area of interest depending on the contraction/relaxation magnification, and shows a low-resolution effect using coarse grids for the other side of the Earth. In other words, in a variable resolution system, only the size of grid in each region changes depending on the contraction/relaxation ratio without structural modification of the dynamical core in a uniform grid system, and the number of grids is the same as the number of uniform grids. For the real case, we will use the variable resolution system to show the effect of the high-resolution in the region of interest (surrounding of the Korea Peninsula) by adjusting the contraction/relaxation scale. As a primary result, this study aims to analyze the numerical results of forecast performance and computational efficiency in variable grid systems compared to uniform grid system in KIM.