Characteristics of Near-Surface Winds and Thermal Profiles on the Windward Slopes of the Island of Hawaii

Abstract

The near-surface winds and thermal profiles on the windward side of the island of Hawaii are studied using the dataset from the Hawaiian Rainband Project (HaRP), with special attention to the early morning and late afternoon transition periods that separate the daytime upslope and nighttime downslope flow regimes. The effects of rain showers on the development of daytime upslope and nighttime downslope flow are also discussed. Before sunrise (?0600 HST), the temperature profiles are usually characterized by a nocturnal inversion about 50?150 m above the ground, with a strength of 1?4 K. The stable downslope flow layer usually extends above the nocturnal inversion with a nocturnal jet beneath the inversion. For rain cases, the nocturnal inversion and the nocturnal jet are weaker with a deeper stable downslope flow than clear cases because of rain evaporation aloft, reduced infrared radiation heat loss at the lowest levels, and vertical mixing associated with precipitation. After sunrise the nocturnal inversion weakens and eventually disappears as a result of surface heating. Three types of upslope flow onset are observed: 1) the upslope flow appears simultaneously within the downslope flow layer after this layer becomes well mixed and warmer than the environment; 2) slope flow develops near the surface because of surface heating, progressing upward; and 3) upslope flow onset occurs above the nocturnal inversion, progressing downward because of turbulence mixing after sunrise. In the early afternoon, a superadiabatic layer is observed in the lowest levels and disappears before sunset (?1900 HST). Normally, the surface temperature of the slope surface becomes colder than the environment about 1?2 h before sunset with the nocturnal inversion forming near the surface. About 30?45 min later, the downslope flow starts at the surface and progresses upward. It strengthens gradually during the night. Several cases during HaRP show a much earlier downslope flow onset and a deep downslope flow layer on the windward lowland because of precipitation. The early downslope flow onset on the lower slopes is caused by the surface virtual temperature on the lower slopes becoming colder than the environment at an earlier time because of the evaporative cooling of failing raindrops. The development of a deep downslope layer is caused by the evaporative cooling aloft. Occasionally, the downdraft outflow from continuous afternoon rain showers on the upper slopes can also contribute to the early downslope flow onset on the lower slopes. This type of downslope flow is localized in nature.