Diurnal and Semidiurnal Oscillations of Sea-effect Cloud Bands over the Northeastern Part of the Sea of Japan: A Numerical Study

Abstract

During wintertime cold-air outbreaks (CAOs) from Siberia, wind-parallel sea-effect cloud bands over the northeastern part of the Sea of Japan (NE-SOJ) often exhibit oscillations with a prominent semidiurnal cycle, causing periodic snowfall where they make landfall on the western coast of Hokkaido Island, Japan. To elucidate the mechanisms underlying these oscillations, we analyzed realistic simulations of CAO events using the Weather Research and Forecasting (WRF) Model. Furthermore, using a linear reduced-gravity shallow-water (RGSW) model, we examined the response of an idealized boundary-layer flow to temporally periodic thermal and momentum forcings representing the effects of diurnally varying boundary-layer turbulence over the windward land. The power spectra of the simulated boundary-layer wind speeds over the NE-SOJ exhibited clear peaks at approximately 24 and 12 h. The disturbances with 24- and 12-h periods identified by harmonic analysis were compared with the flow responses obtained from the RGSW model. The results show that both the diurnal variations in heating and friction within the boundary layer over the windward land are important for generating these disturbances. Both disturbances can be interpreted as superpositions of geostrophically balanced disturbances induced by heating and near-inertial waves induced by friction. It is also shown that the topography of the windward Sikhote-Alin mountain region enhances the relative contribution of the 12-h-period disturbance to the diurnal variation over the NE-SOJ, thereby making the semidiurnal oscillations of the cloud bands more pronounced.