WRRCTR No.112 Diurnal Variation in Rainfall and Cloudiness

Abstract

Maps of diurnal rainfall patterns based on hourly rainfall records are presented for the six major Hawaiian islands. The resulting rainfall distributions demonstrate the complexity of the interaction between trade winds and large islands. Distributions for windward stations on the low islands have early morning maxima common to the tradewind rainfall over open oceans. Mesoscale circulations and moisture extraction by windward mountain barriers modify the oceanic pattern, particularly on the high islands. Patterns on Hawaii shift from nocturnal maxima on windward coasts to pronounced daytime maxima on the leeward coasts. At higher windward elevations on Mauna Loa and Mauna Ke'a, the pattern shows afternoon peaks as contrasted to nocturnal maxima at windward coasts, such as at Hilo. First harmonic R^2 approached .90 at windward stations on small islands and in regions of strong mesoscale circulations. For the largest island, Hawai'i, R^2 exceeded .60 for all stations examined. Hourly mean cloudiness maps for a two-week period in July 1976 are presented for Hawai'i Island based on SMS-2 geostationary satellite images. Tests verify the subjective analysis procedure, particularly when applied to the visible images. Although rainfall over most of the island was below normal, rainfall frequencies were not, indicating that observed cloudiness was typical of summer conditions. Stations were geographically grouped. Windward coastal stations have small diurnal cloudiness variations but produced nocturnal rainfall maxima. Higher elevation windward stations have larger cloudiness variation but smaller rainfall variation, while leeward stations have large and pronounced variations in both rainfall frequency and cloudiness, with maxima occurring in the late afternoon.