On the dynamics of Hawaiian cloud bands: island forcing

The authors focus on basic island-scale forcing mechanisms for the formation and evolution of a band cloud typically present upwind of the island of Hawaii. By means of numerical experiments and verification of the results against observations and laboratory experiments reported in the literature, it is shown that the band cloud is a complex three-dimensional phenomenon which is inseparable from the airflow around the island. In particular, it is demonstrated that the event needs to be analyzed in terms of the basic fluid dynamics of strongly stratified flow past a three-dimensional obstacle. The band cloud is found to arise primarily from the dynamic interaction of the trade winds with the island. The upwind surface flow forms a separation line with an associated stagnation point. A low-level convergence zone forms along this line, resulting in an updraft line. If the updrafts are strong enough, a band cloud forms. Formation and characteristics of such a system are mostly controlled by the environmental stability and strength of the trade wind. A simple criterion for the occurrence of a strong band cloud is offered in terms of the height of the island, trade-wind speed, environmental stability, and the lifted condensation and/or free convection level. A series of controlled experiments addresses questions on the role of the thermal forcing in the formation and evolution of the band cloud. In particular, it is shown that the band cloud is not primarily related to the diurnal cycle (as was anticipated in the literature), but that the diurnal effects are relatively weak modulations of the primary effects of a strongly stratified fluid flow past the island. The possibility of vortex shedding in the lee of the island and its implications for the band cloud are also discussed.