Investigating Sensitivity to Saharan Dust During Hurricane Helene (2006) Using an Adjoint

As Atlantic tropical cyclones develop from waves propagating across Africa they can interact with Saharan dust through radiative processes. There is long standing debate over whether dust inhibits or advances intensification through radiation, or whether the affect is negligible. In addition to the intensification of the storm the dust has also been argued to affect the track. The extent to which the dust can impact the storm is examined using the adjoint of NASA's GEOS-5 forecast model. With the addition of linearized radiation and GOCART physics it is possible to compute sensitivity to dust in the initial conditions using the adjoint. Using hurricane Helene as an example, two cases are considered, the early development, and later when dust potentially affected the track. Helene is chosen as it formed in a particularly dusty environment. Adjoint sensitivities are converted to perturbations and new perturbed model forecasts are made. This optimally measures the explicit effects of dust on the behavior of Helene. The adjoint assumes linearity, limiting the integration length. However, it is shown that the storm strength can be both increased and decreased by dust perturbations depending on the perturbation location relative to the storm's position, even at relatively short time scales. The perturbations can also impact the location of the system. The forecast model can be run in a linear `advection only' mode that allows for sensitivity to sources to be determined for much longer integration lengths. Using this technique the sources which have biggest impact on both the strength and track of Helene are highlighted.