Conditions Favorable for the Occurrence of Trapped Mountain Lee Waves Downstream of Oʻahu

The purpose of this study is to determine the synoptic conditions necessary for trapped lee wave development over Oʻahu. Environmental conditions related to the development of three trapped lee wave events (27 January, 2010—TRAP1; 24 January, 2003—TRAP2; 25 August, 1977) are analyzed using soundings, charts, satellite images. The onset, development and dissipation of TRAP1 and TRAP2 events are successfully simulated by the high-resolution WRF model, as can be suggested by the evolution of w-component in different wave phases (Fig 1). A strong pre-frontal southwesterly flow is the typical synoptic setting for the occurrence of trapped mountain waves in winter, whereas in summer the presence of an upper-level disturbance with easterly winds aloft is a necessary prerequisite. The 1-km resoluted WRF model simulations indicate the common factors of trapped mountain wave occurrence: 1) the presence of a well-defined inversion above the ridge tops; 2) abundant low-level moisture; 3) strong low-level winds with Froude Number (Fr) > 1 impinging on the mountain ranges; and (4) wind shear with increasing wind speed with respect to height through the inversion. The trapped lee wave occurrences requires a vertical wind profile without critical level. If a critical level exists between 500 hPa to 300 hPa, and wind decreases with height in the low level, a downslope wind storm or mountain wave may occur instead of trapped lee waves. Sensitivity tests for the TRAP1 case are performed with reduced relative humidity (RH). With lower RH, trapped lee waves have smaller amplitudes and shorter wavelengths suggesting a latent heat release feedback to the environmental flow.