The Effects of Precipitation on the Surface Temperature and Airflow over the Island of Hawaii

Abstract

The effects of precipitation on the surface temperature and airflow over the island of Hawaii, which are not considered in previous studies, are presented. It is found that clouds and rains can modify the surface thermal fields and result in changes in the intensity of diurnal circulations and the timing of wind shifts from downslope (upslope) to upslope (downslope) flow at the surface in the early morning (late afternoon). The onset of upslope (downslope) flow at the surface is closely related to the virtual temperature anomalies on the slope surface from the adjacent environment for both the rain and dry cases. Prior to sunrise, the rain cases feature higher surface temperatures in contrast to the dry cases because of a more extensive cloud cover, which reduces the longwave radiation, and the precipitating downdrafts, which bring the warmer air above the nocturnal inversion to the surface. Hence, at the surface a weaker (stronger) downslope flow is observed for the rain (dry) cases, which is consistent with warmer (colder) temperatures on the slope surface. After sunrise, because of reduced insulation by clouds, evaporative cooling of raindrops, and slower heating of the wet surface, the rain cases have a slower surface temperature increase than the dry cases. For the rain cases, the latest turning from downslope to upslope flow at the surface occurs in the Hilo area where the total rainfall is the largest. For the dry cases, the latest upslope flow onset is at the eastern tip of the island where the surface temperature remains colder than the environment after sunrise. In the afternoon, the extensive cloud cover, the evaporative cooling of rain showers, and moist soil conditions contribute to a lower surface temperature and result in the weaker upslope flow at the surface for the rain cases than for the dry cases. During the evening hours, the surface temperature decrease is slower for the rain cases than for the dry cases because of a reduction of longwave radiation heat loss due to a more extensive cloud cover. For the rain cases, the evaporative cooling and precipitation downdrafts cause a low surface temperature and the early downslope flow onset at the surface in the Hilo area, whereas on the upper slope, the orographic clouds reduce the outgoing longwave radiation and delay the turning from the upslope to downslope flow.