A Case Study of Trade-Wind Rainbands and Their Interaction with the Island-Induced Airflow

Abstract

A case study of trade-wind rainbands observed on 22 August 1990 during the Hawaiian Rainband Project is presented. It shows that the interaction between the morning rainbands and the island-induced airflow is important for the evolution of the rainbands. In the early morning of 22 August, there are two convective periods, 0400?0600 and 0700?0900 HST (Hawaii standard time), on the windward side of the island of Hawaii. For both periods, preexisting rain cells are observed in the trade-wind flow at least 40 km upstream of the island and move westward toward the island. At night and in the early morning, the offshore flow opposes the trade winds resulting in a convergent region over the area immediately upstream of the island. As the first group of rain cells (0400?0600 HST) moves toward the island, the low-level convergent airflow provides a favorable kinematic background for the enhancement of the coming rain cells. These rain cells merge in the convergent zone and become a well-defined rainband. However, after the first rainband meets the offshore flow, the cool air feeds into the lowest levels of the rainband. This is an unfavorable thermal condition for the rainband and is thus partly responsible for the decay of the first rainband over the windward lowlands. After the arrival of the first rainband, the depth of the offshore flow at Hilo increases from about 250 m to over 500 m. Its horizontal extent also extends from approximately 10 km to more than 20 km offshore. The second group of rain cells (0700?0900 HST) also becomes a well-defined rainband as it moves over the convergent zone. Interacting with a deep and extensive offshore flow resulting from precipitation effects from the first rainband, the rain cells associated with the second rainband are much deeper and stronger than the first rainband. The second rainband moves toward the island during the morning transition, during which the offshore flow retreats and onshore flow begins. After the onset of the onshore flow, the low-level airflow in the Hilo Bay region diverges and splits around the island. This provides an unfavorable dynamic condition for the maintenance of the rainband. Therefore, the second rainband weakens. It dissipates and only reaches the eastern tip of the island.