The island of Maui, Hawaii is dominated by two large mountains connected by a flat isthmus of land known as the Central Valley, which is heavily influenced by the dispersion of air pollutants produced by biomass burning of sugar cane fields. A locally known circulation called the Maui Vortex acts to trap pollutants over the Central Valley region resulting in increased health and economic concerns.

The fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) coupled with the Noah Land Surface Model (LSM) was used to simulate island-scale airflow and the Maui Vortex during summer trade-wind conditions (1 July-31 August 2005). Numerical simulations were performed using an updated version of the LSM (i.e. landuse, soil type, and vegetation fraction) which made an impact on the ability of the model to simulate the diurnal airflow and weather pattern. Airflow and the Maui Vortex were examined under normal (~7 m s^-1) trade-wind conditions throughout the diurnal cycle with orographic blocking, flow deceleration, and flow splitting being main factors in vortex formation. Model statistics show that the MM5/LSM does well at simulating diurnal pattern of 10-m wind speeds and 2-m temperatures.

In this study, we focused much of our attention on the peak heating and cooling regimes of the diurnal cycle, 1400 and 0500 Hawaiian Standard Time (HST) respectively. At 1400 HST, the MM5/LSM simulateed the Maui Vortex at the surface with weak anabatic/upslope flow over the lee side slope of Haleakala. At 0500 HST, the low-level airflow in the Central Valley is governed by katabatic flow from windward West Maui and the lee of Haleakala creating a weak cyclonic circulation. Above the surface, the MM5/LSM simulated dual counter-rotating vortices in the lee of Haleakala which has not been simulated by previous studies using numerical simulations.