Relative Importance of Orography and Planetary Boundary Layer Schemes in Forecasting Cold-Air Damming in the Kanto Region

Cold-air damming (CAD) in the Kanto region is an important forecast target because of its association with heavy snowfall and heavy rainfall that can cause substantial societal impacts. In this study, we conducted sensitivity experiments using the Japan Meteorological Agency (JMA) non-hydrostatic model (JMA-NHM) to investigate factors affecting CAD forecast in 30 cases for which the JMA operational Mesoscale Model (MSM) exhibited large forecast errors, especially a warm bias in the low-level cold-air field. We combined two representations of orography with different effective resolutions and two planetary boundary layer (PBL) schemes to evaluate their relative importance in CAD cold-air forecast. The 1-km-equivalent, more detailed orography tended to strengthen the damming of low-level cold air compared with the 5-km-equivalent orography. Differences between PBL schemes had impacts comparable to or larger than those of orographic refinement, through differences in heat and momentum diffusion over the warm ocean. Moreover, the PBL impacts emerged earlier than the orographic impacts. These results indicate that, toward future high-resolution CAD forecasting, not only orographic representation but also the PBL scheme requires attention. The impacts of these settings on actual forecast accuracy, beyond the relative sensitivities examined here, remain to be evaluated.