Evolution of Katabatic Flow on the Island of Hawaii on 10 August 1990

Abstract

The data collected on 10 August 1990 from the Hawaiian Rainband Project were analyzed to study the evolution of katabatic flow. Results from this study show that for a relatively dry case thermal forcing is important to account for the onset, evolution, depth, and offshore extension of the katabatic flow on the windward side of the island of Hawaii where the mean winds are weak because of island blocking. During the evening transition, the initiation of katabatic flow on the windward slopes was mainly driven by the diurnal heating cycle. At the windward coast, the wind shift was caused by the arrival of the drainage front from the windward lowlands. After the arrival of the drainage front at Hilo, slopewise instability was generated because of rain showers along the coast and diminishing orographic clouds and showers on the slopes. Along the coast, because of the reduced radiative cooling due to cloud cover and vertical mixing associated with rains, a warm period was observed at Hilo. On the slopes, the katabatic flow dissipated the orographic clouds and produced strong surface radiative cooling. As a result, the surface air on the lower slopes became potentially colder than the surface air along the coast and continued to move downward toward the coast. Furthermore, because of surface radiative cooling, the surface air at higher elevations moved to lower elevations along the isentropic surfaces but remained within the katabatic flow. With continued cold air advection down the slope under opposing trade winds, the nocturnal inversion was deeper along the coast than on the windward slopes. Throughout the night, the katabatic flow extended gradually over the ocean. A narrow updraft of ?0.5 m s?1 was observed along the leading edge of the offshore flow, which resembled a density current. The location of the convergence zone between the offshore flow and the incoming trade winds is related to the offshore extension of katabatic flow and is not solely determined by the upstream Froude number.