Improving tropical cyclone forecasting skill: Investigating the impact of aerosols on the intensity of an idealized tropical cyclone

Observations and modeling studies have demonstrated that aerosols interacting with tropical cyclones can impact the intensity of these storms. While the effects brought on by the interaction with aerosols tend to be less significant compared to the dynamics of the storm, they nonetheless appear to have a great enough impact to account for storm intensity prediction errors. Accurate prediction of a tropical storm's intensity prior to landfall would be of great value to those managing public safety. The desire for improving the accuracy of storm intensity forecasting leads to the need for understanding the mechanisms, related to storm intensity, that take place when aerosols are introduced into the storm region.

Recent research has shown in general that storm intensity is diminished when cloud condensation nuclei (CCN) are injected into the lower levels of a tropical cyclone. These studies found that the introduction of CCN at lower levels in the outer rainbands modifies the cloud droplet size distribution (DSD) to one that contains a higher number of smaller droplets in a narrower range of diameters. This results in the suppression of collision and coalescence making more small droplets available to be lofted above the freezing level where they become supercooled and freeze, thus releasing latent heat that enhances updrafts. The amplified updrafts eventually result in greater precipitation with associated downdrafts that form low-level cold pools in the rainband region. These cold pools tend to dampen the energy source (warm, moist low level air) to the core of the storm resulting in the weakening of the storm intensity.

Despite the general weakening of storm intensity when aerosols are injected into a tropical cyclone, the degree to which the intensity is dampened does not always appear to follow a simple monotonic response with the concentration amount of CCN. The non-monotonic response of storm intensity to CCN concentration levels suggests that the interplay of aerosols, microphysics and dynamics of the storm are still not well understood. We propose that there are competing aerosol-microphysics-dynamics processes at play that cause the non-monotonic response seen in some simulations. One example is that if significant amounts of giant CCN (GCCN) are injected into the lower levels during the process outlined above, the presence of these GCCN could result in changing the DSD to one that is bimodal with a large amount of small droplets and a small amount of large droplets. This DSD would tend to favor collision and coalescence thus counteracting the situation described above when only CCN is considered. The surface winds of a tropical cyclone generating sea spray are a likely source of such GCCN.

This work consists of idealized tropical cyclone simulations, utilizing the Regional Atmospheric Modeling System (RAMS), where varying concentration amounts of CCN are introduced at the periphery of the storm. In addition to the CCN source, a GCCN source (built into RAMS) that is based on a sea spray model is utilized to allow for a more realistic distribution of GCCN across the extent of the storm. Initial results do show the non-monotonic response of storm intensity to the amount of CCN concentration, and it is expected that further investigation will reveal various aerosol-microphysics-dynamics mechanisms that are behind the non-monotonic nature of the storm responses.