Conditions Favorable for the Occurrence of Trapped Mountain Lee Waves Downstream of O'ahu

Abstract

The purpose of this study is to determine the synoptic conditions necessary for trapped lee wave development over Oʻahu. The study also shows that the high-resolution mesoscale numerical models could possibly provide valuable numerical guidance for the onset, development and dissipation of trapped lee wave events in Hawaiʻi. The occurrence of trapped lee wave clouds is possible in Hawaiʻi, especially downstream of the island of Oʻahu. The Koʻolau and Waiʻanae mountain ranges of Oʻahu are oriented NW-SE. The pre-frontal southwesterly wind in winter has a large wind component perpendicular to these mountain ranges. With the presence of an inversion aloft, trapped lee waves may occur during the wintertime. In the summer time, northeast trade wind is persistent and trapped lee wave event is relatively rare. In this study, environmental conditions related to the development of three trapped lee wave events (27 January, 2010—TRAP1; 24 January—TRAP2, 2003; 26 January, 2014—TRAP3, 25 August, 1977) are analyzed using soundings, charts, satellite images. The available model input datasets for the summer case in 25 August, 1977 is ERA-40 in the resolution of 2.5⁰×2.5⁰, which is not enough to simulate high-resolution model in 1-km. With input analysis datasets in the resolution of 1.25⁰×1.25⁰, the three wintertime events are successfully predicted by the high-resolution WRF model. Among them, TRAP1 and TRAP2 are initialized using the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR), and TRAP3 is initialized with NCEP Climate Forecast System Version 2 (CFSv2) output.Results from a WRF model simulation with a horizontal 1-km grid indicate that the trapped lee waves are most significant just beneath the inversion. There are several common factors involved in the occurrence of these trapped mountain wave events: 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. A strong pre-frontal southwesterly flow is the typical synoptic setting for the occurrence of trapped mountain waves in winter, whereas in the summer months the presence of an upper-level disturbance with easterly winds aloft is a necessary prerequisite. The vertical wind profile is the key element to determine whether trapped lee waves or downslope winds form. None of the trapped lee wave events in winter or summer have a 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 27 January, 2010 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.