SAR Observation and Numerical Simulation of Mountain Lee Waves Near Kuril Islands Forced by an Extratropical Cyclone

Several groups of atmospheric gravity waves (AGWs) were observed on a Sentinel-1A synthetic aperture radar (SAR) image acquired near the Kuril Islands in the Northwest Pacific Ocean on June 1, 2015 during the passage of an extratropical cyclone (ETC). These waves occurred on the lee side of the mountains located on the islands. Both diverging and transverse waves with wavelengths ranging between 20 and 30 km are shown as alternating bright-dark patterns in the SAR image. For the diverging waves, there exists a prominent asymmetry in the wave motions of the two arms. The Moderate Resolution Imaging Spectroradiometer and Landsat 7 Enhanced Thematic Mapper Plus images acquired 5-7 h prior to the Sentinel-1A pass also contain the same groups of AGWs. The mesoscale Weather Research and Forecasting model simulation confirms that the AGWs are lee waves triggered by the airflow over the islands. AGWs are aligned perpendicular to the wind direction and locked on the lee side of the islands. The life span of the waves is about two days, consistent with that of the ETC over the region. The numerical model also successfully reproduces the main characteristics of the lee waves. Simulation results demonstrate that the variation in the wave parameters (i.e., wavelength, amplitude, orientation, wedge angle of the diverging wave, and vertical propagation characteristic) and the wave asymmetry of the diverging wave are mainly caused by the wind and stratification changes. The smaller amplitude of the diverging wave seems to be associated with a smaller Froude number.