Temperature sensitivity to vertical resolutions over tropics in KIM

   Korean Integrated Model (KIM), a global weather prediction model, plans to extend into a variable resolution system covering the short to medium forecast range. To improve forecast skill, we intend to increase vertical resolution from 91 to 137 layers in KIM, which has been updated with an enhanced horizontal resolution of 8 km. The vertical coordinates of 137 layers (L137) followed those of the European Centre for Intermediate Forecasting (ECMWF) Integrated Forecasting System (IFS),  with reduced vertical grid spacing throughout the troposphere and stratosphere compared to the 91 layers (L91). Here, we focus on understanding the sensitivity of performance to vertical resolution in the medium-range forecast. The overall conclusion for increased vertical resolution is that it shows mainly neutral in the northern and southern hemispheres, but also some impacts in the tropics, against the IFS analysis. One of the notable impacts of changes in vertical resolution is a decrease of temperature in the lower and upper troposphere, which results in a considerable degradation of the upper troposphere over the tropics. This is because the physical processes rely on vertical grid spacing, which induces radiative cooling while increasing cloud hydrometeors. The refinement of the vertical grid increased the planetary boundary layer (PBL) height, which enhanced vertical mixing results in cooling/moistening above the PBL and warming/drying within the PBL. This increases the amount of cloud water content, influencing radiative fluxes and contributing to the lower troposphere cooling. The refined vertical resolution increased the amount of cloud ice content remained in cumulus convection, wherever weak or robust in its occurrence. This contributes to the cooling in the upper troposphere over the tropics. Increasing vertical resolution is needed in improvement of physical processes.