Title:Investigating sensitivity to dust in tropical cyclone formation using the GEOS-5 adjoint model

Author: Daniel Holdaway (GESTAR, NASA/GSFC)

As tropical cyclones develop from easterly waves coming off the coast of Africa they interact with dust from the Sahara desert. There is a long standing debate over whether this dust inhibits or advances the developing storm, and how much influence it has. From one point of view dust can surround the storm, absorb incoming solar radiation, cool the air below and inhibit growth of the storm. Alternatively dust may interact with clouds through micro-physical processes, for example by causing more moisture to condense, increasing the strength. The first of these points of view is investigated using the adjoint of NASA's Goddard Earth Observing System version 5 (GEOS-5). This global model has recently been advanced to include the linearisation of the convection scheme, the cloud scheme, the long-wave and short-wave radiation schemes and the dust model. As a result it is possible to investigate the sensitivity to dust through its impact on the radiation. The investigation focuses on the hurricane season of 2006, which coincided with an extensive observation campaign and significant Saharan dust outbreak. The relatively calm season that year was widely attributed to this dust outbreak. It is shown that the adjoint can provide insight into the sensitivity to aerosols and reveals a relatively low sensitivity to dust compared to the thermodynamic variables in tropical cyclone formation. It is demonstrated that if excess dust also results in drier air then it can significantly reduce the cyclone intensity through the sensitivity to moisture.

GMAO Head: Steven Pawson Global Modeling and Assimilation Office NASA Goddard Space Flight Center

Curator: Nikki Privé Last Updated: Feb 9 2015