Effects of terrain heights and sizes on island-scale circulations and rainfall for the island of Hawaii during HaRP

Islands in Hawaii have different sizes and terrain heights with notable differences in climate and weather. In this study, the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) land surface model (LSM) is used to conduct numerical simulations for the island of Hawaii with different model terrain heights and sizes during the diurnal cycle for the Hawaiian Rainband Project (HaRP) period. In addition to island blocking and orographic lifting, terrain heights also affect the land surface thermal forcing throughout the diurnal cycle by the variations of orographic clouds during the day and by the variations of the longwave radiation heat loss at night. The simulated rainfall distributions and amounts throughout the diurnal cycle are closely related to rising motions caused by nonlinear interactions among island blocking, orographic lifting, and land surface processes. Besides the terrain/mountain height, island size is another factor that affects rainfall production and distribution in Hawaii, because island size affects orographic lifting, surface forcing, and the advection time scale for an air parcel to reach the mountaintop. The heavy rainfall maximum on the mountaintop of the island of Kauai is due to its suitable height and size. This study confirms that the evening rainfall maximum along the western Kona leeside coast is caused by the convergence between the westerly return flow and the offshore flow. For a lower model terrain, the westerly return flow is weaker; as a result, there are smaller evening rainfall amounts.