On Engineering Hurricanes

In the last year there have been two papers that have proposed that seeding hurricanes with small hygroscopic particles, as opposed to conventional giant hygroscopic particle seeding, could lead to the reduction in their intensity (Cotton et al., 2007; Rosenfeld et al., 2007). The Cotton et al. (2007) paper was based preliminary results of simulations of the impact of African dust on hurricane intensity (Zhang et al., 2007), which showed that dust acting as CCN influenced the storm development by inducing changes in the hydrometeor properties, modifying the storm diabatic heating distribution and thermodynamic structure, and ultimately influencing the storm intensity through complex dynamical responses. Some simulated storm intensities showed a monotonic decrease in storm intensity with increasing concentrations of CCN under certain configurations of the model but this trend was easily modified just by introducing slight variations in the GCCN profile. Thus, Zhang et al. (2007) concluded that the physical processes responsible for the impact of dust as nucleating aerosols on hurricane development need to be examined in the future under a wide range of environmental conditions.

Since then Henian Zhang has carried out more simulations that illustrate that the response is by no means simple. In some cases increasing CCN leads to a strengthening of hurricane intensity. Moreover, the results of introducing dust acting at CCN further in the lifecycle of the storm reveals that the response to CCN varies greatly depending on the stage of introduction of the aerosol. Thus this work illustrates that even using simple, rather idealized simulations the response of a hurricane to aerosol can be quite nonlinear. This makes the potential modification of hurricanes to small-particle hygroscopic seeding even more challenging than envisioned by Cotton et al. (2007) and Rosenfeld et al. (2007). Nonetheless we urge that this topic should be investigated much more extensively and in further detail.