Investigating Sensitivity to Saharan Dust in Atlantic Hurricane Formation Using the GEOS-5 Adjoint Model

As tropical cyclones develop from waves propagating from the coast of Africa they can interact with Saharan dust. There is long standing debate over whether dust inhibits or advances the development, or whether the affect is negligible. Dust can absorb incoming solar radiation, warming the air locally but cooling the air below. As a result, the energy sources upstream of the developing tropical cyclone are modified. The adjoint of NASA's Goddard Earth Observing System version 5 (GEOS-5) is extended to include dust and the interaction between dust and meteorology and vice-versa. Using an aerosol capable adjoint model allows a quantitative measure of how much dust can impact the system in these different ways. It is shown that a largely positive sensitivity to dust occurs and that the storm becomes more organized and stronger when more dust is present. This is explicitly demonstrated by making perturbations to dust in the nonlinear version of GEOS-5 that are based on the sensitivity structures. Results show that sensitivity to dust is generally smaller than for other variables but it is argued that the perturbation approach likely under-represents sensitivity to dust. The adjoint model is extended to give the sensitivity to dust sources. This allows the sources of dust which go on to have the biggest impact on the developing hurricanes to be identified.