Influences of Topography and Land Use on the Track and Structure of Hurricane Douglas (2020) During its Passage Near Hawaii

On 26 July 2020, Hurricane Douglas approached the Hawaiian islands. The storm ended up moving northwestward just north of the islands, but in the days leading up to the storm’s close passage, forecast track models indicated a significant risk of a direct hurricane or tropical storm landfall in Hawaii. Previous studies show islands with significant topography can impose meaningful deviations in a tropical cyclone’s path. It is therefore of interest to determine whether Hawaii’s topographical features had an impact on Douglas’s ultimate path.

In this study, a static nested configuration of the WRF model is used to reproduce a Douglas-like storm and a trajectory matching the storm’s path. The model is initialized at 1200 UTC 24 July 2020, and the Douglas simulation is executed through 29 July, well after the storm has moved past Hawaii. To elucidate the impacts of Hawaii’s terrain on Douglas’s track, a variety of sensitivity experiments are conducted. The control simulation uses the YSU boundary layer scheme and WSM6 microphysics. Sensitivity experiments include subtracting out the islands completely, replacing the island topography with flat terrain at sea-level that retains the existing land cover, and finally performing an experiment that includes Hawaii’s orography but uses oceanic land cover everywhere. A small ensemble is then created by repeating these simulations with the MYJ boundary layer scheme and again with Thompson microphysics. As a result, there are 12 total numerical experiments.

This presentation will focus on the physical mechanisms underlying Douglas’s northward deflection around the Hawaiian islands. The sensitivity experiments show that, consistent with previous studies, the topography itself deflects Douglas northward as it approaches from the east. Moreover, the islands have an impact on the overall structure of the storm by introducing axial asymmetries. What is perhaps more surprising is the effect of land cover on the storm track. The experiments show even flat land shifts Douglas’s track northward. Because Douglas passes to the north of Hawaii during the day, heating from insolation produces convective circulations that modify the storm track. Thus, there is an intricate combination of factors associated with the Hawaiian islands that can influence a tropical cyclone’s track and structure.