DIURNAL-VARIATION OF SURFACE AIR-FLOW AND RAINFALL FREQUENCIES ON THEISLAND OF HAWAII

The high-resolution Portable Automated Mesonet data from the Hawaiian Rainband Project are used to document the circulation over the entire island of Hawaii for the first time. It is shown that the surface airf low and rainfall occurrences over the island are strongly modulated by the diurnal heating cycle. Most areas over the island show daytime up slope and nighttime downslope components separated by morning and even ing transitions. On the windward slope, the onset of the downslope (up slope) wind during the evening (morning) transition starts on the slop es and progresses downward. The effects of island blocking are also ev ident. On the windward slopes of Mauna Kea and Mauna Loa, the mean win ds are weak (similar to 1 m s(-1)) due to flow deceleration. Flow spli tting occurs in the Hilo Bay area as the trade winds are forced to mov e around the island. On the lee side, the trade winds are absent. For stronger trade-wind days, the island blocking is more significant with a higher surface pressure (0.2-0.5 hPa) on the windward slopes and lo wer pressure in the lee side than weak trade-wind days. Along the wind ward coast, it is much easier for the land breeze to overcome weaker t rade winds. For weaker trade-wind days, the onset (cessation) of land breeze there is earlier (later) in the evening(morning). At night, the area of maximum rainfall frequency is over the windward lowland west of Hilo. Most of the nocturnal precipitation there starts in situ. The effects of orographic lifting aloft are enhanced by the nighttime con vergence west of Hilo due to the interaction between the katabatic-lan d-breeze flow and the trade winds. In the early morning, the rainfall frequency has a maximum along the windward coast due to the inland dri fting of the frequently observed rainbands offshore.