On the Dynamics of Hawaiian Cloud Bands. Part III: Local Aspects

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  • 1 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

This paper extends our earlier discussion of the flow past the island of Hawaii and the accompanying cloud band to smaller-scale effects occurring on the scale of the Hilo Bay region. The evolution of cloud bands forming upwind of the island on 1 August 1985 is studied using a high-resolution numerical model and available field observations. The current work provides further evidence in support of the view that the phenomenon of Hawaiian cloud bands is closely linked to the dynamics of strongly stratified flows past three-dimensional obstacles. In particular, results are presented that document the cloud interaction with a secondary, vertically propagating gravity wave and the formation of horizontally oriented vortices in the lower upwind flow—two characteristic features encountered in studies of idealized low Froude number flows. Quantification of the effects due to nocturnal thermal forcing is attempted, and it is shown that cooling along the volcano slope doubles the depth of the dynamically induced downslope flow as well as its maximum wind speed, whereas it has a little effect upon the position of the mesoscale convergence line and coinciding leading edge of the downslope current. Downslope surges of cold air from the volcano slope are shown to temporarily enhance the depth and strength of the downslope flow, leading to invigorated cloud development at the leading edge of the current. Analysis of the Hawaiian Rainband Project (HaRP) sounding data relates cloud bands to the theory of squall lines and suggests that the trade wind environment upstream of the island is favorable to the formation of cloud bands consisting of isolated cells advected by the local cloud-layer winds.

Abstract

This paper extends our earlier discussion of the flow past the island of Hawaii and the accompanying cloud band to smaller-scale effects occurring on the scale of the Hilo Bay region. The evolution of cloud bands forming upwind of the island on 1 August 1985 is studied using a high-resolution numerical model and available field observations. The current work provides further evidence in support of the view that the phenomenon of Hawaiian cloud bands is closely linked to the dynamics of strongly stratified flows past three-dimensional obstacles. In particular, results are presented that document the cloud interaction with a secondary, vertically propagating gravity wave and the formation of horizontally oriented vortices in the lower upwind flow—two characteristic features encountered in studies of idealized low Froude number flows. Quantification of the effects due to nocturnal thermal forcing is attempted, and it is shown that cooling along the volcano slope doubles the depth of the dynamically induced downslope flow as well as its maximum wind speed, whereas it has a little effect upon the position of the mesoscale convergence line and coinciding leading edge of the downslope current. Downslope surges of cold air from the volcano slope are shown to temporarily enhance the depth and strength of the downslope flow, leading to invigorated cloud development at the leading edge of the current. Analysis of the Hawaiian Rainband Project (HaRP) sounding data relates cloud bands to the theory of squall lines and suggests that the trade wind environment upstream of the island is favorable to the formation of cloud bands consisting of isolated cells advected by the local cloud-layer winds.

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