Effects of increasing horizontal and vertical resolution in Korean Integrated Model (KIM)

Since the dynamical core of Korean Integrated Model (KIM) was developed in the 1st phase (2011~2019) of KIAPS, we have been aiming to develop a variable resolution prediction system covering short to medium range in the 2nd phase (2020.9~2026). As a first step towards moving to km-scale resolution, we have increased the model resolution from 12 km to 8 km horizontally and 91 to 137 layers vertically. For increasing the resolution horizontally, dynamics core configurations and terrain elevation data were newly set up. For vertically, vertical coordinates of 137 layers followed that of the European Center for Intermediate Forecasting (ECMWF) Integrated Forecasting System (IFS), which has been increased vertical resolution throughout the troposphere and stratosphere comparing to 91 layers.
This study discusses the forecast impact of high-resolution KIM in terms of objective verification scores against observations and analyses. The overall conclusion for horizontal high-resolution is that it shows slightly positive in southern hemisphere and mainly neutral for northern hemisphere, but also some negative in tropics. One of distinguished results is increasing horizontal resolution leads to cooling in the temperature in the lower and upper troposphere. The cooling in the lower tropospheric over the tropics seems to come from smaller time step that has to be for smaller dx, which results in enlarged low cloud formation and thus more radiative cooling. In case of the upper troposphere, the cooling results from outgoing long-wave radiative cooling by decreasing hydrometeors in physical response to smaller grid spacing. The increase of vertical resolution had an effect of neutral to slight positive in northern hemisphere but showed significant degradation in tropics. To achieve the consistency and improvement for high-resolution model, it is necessary to understand the physical processes related to time step and horizontal and vertical grid spacing.

 

Acknowledgement
One of the authors, S-J Choi, wishes to acknowledge this study was supported by 2023 New Professor Support Program of Natural Science Research Institute in Gangneung-Wonju National University).