Feasibility study of the mitigation of the intensity of tropical cyclones by CCN seeding in the outer rainband region: A HAMP project

In the 1960's there was great interest in exploring the possibility of mitigating the intensity of a tropical cyclone which culminated in project STORMFURY. The approach taken was to seed the storm with ice nuclei (silver iodide) near the eyewall, but outside the eyewall, with the intent of enhancing the convection in that region due to enhanced freezing with its associated release of latent heat. The enhanced convection would then cause an eyewall replacement cycle to take place resulting in a larger diameter eyewall with corresponding slower wind speeds. Ultimately, the project failed due to the lack of enough supercooled liquid water in the region of interest to get this process to occur.

The devastating 2005 United State hurricane season sparked renewed interest in developing methods to mitigate storm intensity. Recently some new strategies (e.g., Rosenfeld et al., 2007; Cotton et al., 2007) have emerged which focus on seeding with condensation nuclei in the outer rainband region. The idea here is that an increase of CCN in the outer rainbands would cause reduced collision and coalescence, resulting in more supercooled liquid water to be transported aloft which then freezes and enhances convection via enhanced latent heat of freezing. The intensified convection would condense more water ultimately enhancing precipitation in the outer rainbands. Enhanced evaporative cooling from the increased precipitation in the outer rainbands would produce stronger and more widespread areal cold pools which block the flow of energy into the storm core, ultimately inhibiting the intensification of the TC.

The strategy of seeding with CCN in the outer rainband region, if successful, would overcome the primary downfall of STORMFURY by providing the necessary supply of supercooled liquid water. In order to study the feasibility of such an approach, this work is looking at the effects of broadcast seeding at the periphery of a developing tropical cyclone. The intent of this study is to identify patterns in the storm response to seeding that would hopefully lead to tactics that could employ much more precise (and smaller scale) seeding procedures. A companion study by Carrio and Cotton investigates the storm responses to specific prescribed seeding trajectories, mainly in the outer rainband region.