Trade Wind Rainfall atop Mount Waialeale, Kauai

Abstract

Very large coast to mountain rainfall gradients have been observed in the trade winds and winter monsoons. Since the surface moist layer is usually capped by a subsidence inversion near 2 km, the rain is ?warm.? On the top of a mountainous island, which is generally below the inversion, such as Kauai, trade wind rainfall can be very great and the coast to top rainfall gradient very large. Autographic rainfall measurements at the top of Mount Waialeale (1598 m MSL, one of the wettest spots on the earth) on Kauai together with surface and upper-air measurements made at Lihue, 20 km to the southeast, and weather satellite images confirm and expand on earlier descriptions of the nature of mountain rainfall in the trade winds. Significant rain results from moderate or fresh trade winds being lifted up the eastern escarpment of Waialeale, but only when a band or area of cumulus extends upwind of the mountain. Small wind shear in the vertical and a sharp upper limit to the moist layer reduce entrainment and facilitate growth of cloud droplets. At the mountaintop rain is usually light or moderate, with drops smaller than 2 mm, but persisting long enough to produce large accumulations. Along the windward coast, drops usually evaporate before reaching the ground. Divergence and upward motion east of an upper-tropospheric trough barely affect the moist trade wind layer. Cloud lines associated with shear line extensions of cold fronts or with dying tropical cyclones to the south account for much of the rain though short-lived mesoscale cloud systems are also important. Thunderstorms are very rare with surface flow from a trade wind direction. The wind then curves cyclonically on the northwest sides of sharp troughs or small cyclones. Upper-tropospheric southwesterlies usually prevail. The nocturnal rainfall maximum at Waialeale probably stems largely from radiational cooling at the top of the moist layer causing clouds over and upwind of the island to increase. Other trade wind islands of about the same size and height as Kauai, but with no mountaintop rain gauges, probably also have large coast to mountaintop rainfall gradients.